Repair Briefs - Guitar Amplifiers, Band/Stage Gear, A to M

The following are repair briefs for various 
equipment.The infomation is directed
to technically competant repair engineers.Generic terms have 
been used to make this info less model specific,eg terms like 
replace transistor Q123 have not been used.
The equipment is Guitar Amplifiers, Audio Amplifers, Stage Gear, CDs and Radios, with some cross-over 
eg fault in radio section of a tuner-amp would be listed in radio section . 
I would be interested in finding 
any other repair listings on the internet structured as i have 
done ie intended to be less model specific. For convenience using search-engines, 
use keyword divdevrep to target these files.

There is no point in contacting me about any of the following, the 
repair job may have been done 15 years ago .
I cannot clarify or enlarge on any of the following.
In the following V ac means RMS DVM AC volts 
unless stated pk-pk.

Should the location of this file change please use the keyword "divdev7" in a search engine to find it again

Band Amps and other stage gear

AER Compact 60, made 2000 German made very expensive 60W "practise amp" but well liked for its features. The owner's cat , not a tomcat fortunately, decided to piss on the cabinet. Piss went down inside the Neutrik XLR/0.25 inch socket and also corroded the gain pot so it stiffened up too much to turn properly. Easy enough to desolder the pot, clean and ream out the aluminium corrosion. The neutrik was a different matter, could use the XLR input but not the 1/4 inch, horrible blue-green mess of corrossion inside after desoldering and prizing off the rear. Cleaned out the worst with meths but could only return to normal use by removing the grounding contact that lodges in the rear closure, it would not cleanly make or break and as had been used for months without , in effect, the grounding due to corrosion. As a pin for pin replacement not available in the UK. pinning arangement (I've labelled the A,B,C as not on overlay) A B C 2 3 1 SS R S G B,C,1,R,SS,S were all grounded with no plug 2 to ground 1/4 inch S=gnd, A=tip,R=ring no plug R to gnd Also fixed the rather crass foam cover in place with strips of double-sided carpet tape. Technical notes Large Rs 2 x 82, 5W TDA7294, 7815,7915, 2x 7805 33078 opamps, LF353 SSM2142, 470R, SSM2018,LM393,TL072 3x W24257aj-12, HC02A, HC123A,HCT05, AD SD 2115, CS4220 TL072,HCT14 and in all the amp just one tranny K30A incidently "diode" DVM test readings for a good TDA7294V relative to the heatsink 1,3,5,7,9,13,15 .6,.6,-,.55,.6,-,.48,1.2 2,4,6,8,10,12,14 .58,.58,.55,0,.6,-,.49 Akai AMX10 mixer amp, 1999 Dropped then nothing. Surprisingly the latched connector between IEC socket and ps had disconnected, large toroid filter broken away from pcb barely held on by two wires and a crystal problem on the mixer board. ps and amp uses K1181,2x K1179, .15,.22,47K,LT1509 6x 1k, 4x W38NB20, 47k,3x 1M 7915,7815,24M05,7812,78M24, 4x .47,22k 18,10,18,470 ribbon to mixer 5,0,0,0,0,0,5,48,-15,0,15 Ashdown ABM500 bass amp, 1997 Farty noises Had been seriously dropped in a flight case and a number of problems, now sorted so it now works but there is a fairly low level background hum problem with no input. Attending to all the pots as most were worn or suspect in some way and some solderings at the heatsink. schematic from Ashdown Festoon bulb replaced with a physically larger 12 V bulb so mount needed extra fixing in place, o/c voltage supply, has dropper, at the bulb mount 17V ac. Some other data secondaries 13.5V; 62,0,62 ; 17,0,17 ; 270V (16 ohm DC ) main ps , +/- 84V Thermal sw n/c. 1V dc over meter reads "2". At 22 deg C , the heatsink thermistor , in circuit, measured 70 ohms giving only 9.6V at the fans, parallel with 68R gave 11V and 28R // gave 12.3V. To remove the preamp board, remove pot and switch knobs and push in switches and slacken off the transformer to give space. Shift the other way to remove the rear board. See tips files for coachbolt removal problem. All inputs and outputs and pots (exept the pot casing) are electrically isolated from chassis, on schema and in reality. Just the mains inlet earth bond point to the chassis. The pa has a nominal "grounding" point of // 100nF + 10K, 1/3W R and the pre has the same to chassis and the signal line "ground" between them. Another sort of problem with this faux ground arangement. I thought it odd how long it took to drain the 450V caps and the main reservoir caps before I previously dismantled all the boards. You have to connect the low ohm resistor to faux ground rather than through 20 say + 10k//10K to real ground. So about 5K between real ground and nominal ground. Disconnecting the signal lead, the pa o/p is clean. Driving into 8 ohms , with shorted inputs then at max volume setting then 0.7V on DVM ac range, dropping to 0.002 V for min vol. It is more intrusive than I would normally expect but I don't have any figures for what is normal. shorting the preamp 10K then the hum goes. The input 1/4 ins shorting contacts, with no input jack, are ok Putting 1K across the pre amp 10K then the hum level drops to a much more acceptable 0.1V. What are the safety issues with this sort of arrangement ? They are not marked on the schema as fuseable resitors nor on board marked ! in a triangle. I've probably found the source of generic hum noise problems in Ashdown amps with this faux ground structure. The problem pot marked with a "B" meant it was linear and not log so a replacement easily found. Taking the original apart there was no broken track or rivet /paxolin failure, stressing/bending paxolin and measuring etc showed nothing untoward. But I did notice the track was offset, ie not coaxial to the paxolin base. The noise intrusion was most prominent at about 2/3 and at that point due to the off centre track and slight ovalling of the outer pot casing the track/wiper at this point was very close to the casing. For the replacement pot I bent back the four tangs that hold the middle casing/housing section of the pot to the bushing plate, introduced four bits of insulation, closed back on them and glued in place. So the casing is now isolated from the spindle and bush which is still properly grounded to the chassis. Now normal hum level with shorted input and maximum vol/gain settings varying from .04V clean to 0.07V ac on full grind setting into 8 ohms. So factor of 10 or better improvement just from isolating that one pot casing and presumably more centrally aligned pot tracks perhaps to lesser effect. The 2 x 10K faux to ground resistors still as the original design. One of those annoying self-made, in a sense, problems also associated with the faux earth. Trying the amp with the equalisation board free of the chassis was fine but mount it back in and horrible mains noises. To save messing about I was temprarily placing the board back in without the 3 spacers on the 3 pots. These pots have the casing to chassis earth but the 4 slider pots have the cases connected to faux earth. Without the spacers then the slider cases came too close to the chassis. The grid vltage on the ECC83 varies from 0 to -3.8V via the clean/grind pot I wish there was a proper techie/muso translator somewhere ,ie full translator techie-speak for techies and muso-speak for musos . What is grind ? in terms you can see on an oscilloscope for instance, not the usual muso speak. I want to know, in this instance, if you feed say a triangle wave in what comes out on full grind? Final loading check , 400Hz, giving 20W in 4 ohms, no rise or drop in Vac over test period. Heatsink with the 2 fans active gave 20 deg C rise over ambient of 22 in 12 minutes. Also into 4 ohms, no input , clean channel full vol & gain of 0.03V ac of mains hum over the load Ashdown ABM-500 EVO-II Head, 2002 Intermittant cracles and gain dropping, part into a set with normal usage. 2 temp sensing things , one vaguely touching , the other bent away from the heatsink. Perhaps every roadie should be put in a flight case and dropped down some stairs as a lesson. Anyway both heatsinks must have bent relative to the pa pcb, bending the leads , when dropped. Main spring held items stayed ok. How to anchor back more firmly than just white paste? The main TOP66 power devices etc have those slide on spring steel clips but the loosened TO92 tranny and fan thermistor are a long way down from the mounting slots, for anything like that. Any ideas ? - live heatsink btw, hence not rigidly held to body. Too crowded to drill/tap the heatsinks and the assembly of these sort of hook-clip arrangements is realy make once, dissamble/reassemble at your peril. This is a likely generic fault scenario for all such "live heatsink" amps that are only fixed to flexible polyester pcb board Double sided foam/tape tape was supposed to be keeping a duaghter board marked EB SUB ? (not on the Ashdown supplied schema, 4013 and 3 x 072) fixed to the preamp but dislodged in the same drop presumably, failed solder joint/s leading to crackles. I removed 2 nearby caps , 2 small holes ,to match cap pin spacing, in cut down lengths of cable-tie , and soldered back in, regooing and positioning the 2 shiftees. Volume was fading down after half hour of use, presumably because the NTC fan thermistor was staying at room temp and fan speed not upping with increasing heatsink temp. Unsupported daughterboard is now braced back to 2 chassis anchors NTC thermistor TH1 ,88 ohm cold 2.2K,4.7K, 1W, 4R7 vitreous 33R , 1W on prea board main bridge rect , one prong poor solder, supply pcb conn block for fan , bad solder. Ashdown Electric Blue 180, made 2004, 26Kg The amp cut out totally and when it had cooled down and he switched it on , it worked, but he noticed the fan was not turning so he switched it off again. Beware the heatsinks are not grounded and +61V on one and -61V on the other. The supply to the 12V fan is off the preamp board so had to remove from the front pannel. The owner had never had to tighten any knobs but one grub screw (brass) had been graunched at manufacture so had to drill that out first (see the tips files for technique). The 100 ohm dropper to the fan had resonated and failed inside the pcb hole, copper coloured fracture face so presumably work hardened copper failure. 150 ohm dropper in the display area had overheated to 156 ohm and giving apparent band colours of brown black black. Uses 2SA1668 , 2SC4382 2068, LB1443, 5x TL072, M51134FP 2x .33, 4.7, 4.7K large Rs Speaker DC resistance 5.6R Ashdown MAG 250 For factory retro-fitted fans. 4 faults in one just fitting a fan and power bleedoff For random fan stoppage - look for Instead of 47R 5W W/W placed in line with the fan (dangling R unsupported at one end) , 0.047R soldered-in so 21V across the 12V fan. Surprisingly thousands of hours of unproblematic use like that. Even without that , for any long term excessive wear on the bearing of fan , the reaction force pushes the impeller towards the mounting bracket, eventually touching it. You could see a slight trace of where one fan vane had rubbed the bracket and rubbed the plastic of that vane, ie clear of usual gunge. If meter cover stays in fascia cover to protect when working on prea. Replace with 47R, supported by tagstrip bolted to fan, and new fan with the centre section of the mounting bracket hacksawed out. Take negative lead via a wire to an added terminal set in the prea. Fourth mistake : the fan just circulates the air internally, no added baffles to duct air in from outside. Set the fan in a stiffened card bafle going from LH corner of front to centre divider of rear vent. So air heats up, softens fan's plastic vanes , vanes flex a bit, to the extent of touching the mount. Removed the the sw and checked for temp action and any vibration sensitivity and checked caps in the controller area. If the sw goes o/c in these Ashdowns the gain drops at the top end mainly, not cut out . 1KHz in and 1V ac over dummy load then if sw changes then o/p drops to 0.04V ac, less so for lower frequencies which seems upside down for amp protection. Still random intermittant gain drop and distortion. Beefed up return socket switch as per tips. The holes and pads for these are far too big for the pins so a good place to see crcked solder, but not this one, so far. R46 , 1/3W, resitor at the 1.6K bias R cold joint. Ashdown MAG 300, 2004 pcb marked MAG250 EB150 Blows mains fuses during low volume practise session. Survives long enough for the meter lamp to come on half a second. Nothing found obviously suspect in ps or pa probing around cold and inspection. Bad internal AC supply fuse holder contacts , arcing burn marks on the fuse barrels. Both are 5 amp rating but neither blown or showing stress on the fusewire. So much for so called "Top Cheers" transformers. Initially could take variac to 50V and then run away , now cannot even put the minimum of 5V on the primary , with nothing connected to the secondaries. So just coincidence of bad fuse holders on the 48V ac lines or directly leading to failure ? Could repeated breaking of secondary current induce very high voltage in the primary leading to shorted turns? Nothing visibly wrong through the tape wrap or smoke trails from inside. DC measurement on primary now about 1 ohm , unpowered, was about 6 ohm when received. The first winding was the low current 15-0-15V secondary. This was wound with doubled up .45mm wire in one go so reducing the number of shuttle passes to wind on. The 2 wires brought together to form the "0". Thought to myself, perhaps that is ok for low voltage winding as only 50V peak or so maximum for 2 thicknesses of lacquer to sustain. Then the 3 amp 48-0-48 wound as 2 separate layers of 1.06mm wire, not run as paired winding, fair enough. Anyone know why 3 runs of loosely packed turns per layer when one run of tighter turns per layer could have been done, so avoiding overlaps? Get down to the unruptured thermal fuse, tails of the primary and the .7mm wire primary and nothing obviously wrong in the way of hot spots. But blow-me-down a return to the paired up winding and ends joined to give 120-0-120V for 240 UK use. But of course now 2 thicknesses of lacquer having to sustain a normal maximum differential of about 350 volts. Not wound off yet but somewhere in the bulk of windings must be bridge/s. Thermal fuse marked 17 AM 033A5 M4 AB if anyone knows what that means as temperature. speaker 5.6R Difference on cold probing between drivers on both sides of the amp due to resistance 47 + 330R between B-E of C4387/ A1668 but not corresponding ones. If gingerly powering up this sort of amp don't have a speaker connected as amp is unbalanced below about +/-25V. Powering up on bench power supply and monitoring output line +/- 10V o/p 9.9V +/-15 o/p 12V +/-20V o/p 18V +/-25V o/p settles at 0V Uses +/-70V 2x 2SA1294 , 2SC3263 Output pot goes past the endstops, dismantled and reformed the outer case. Meter on this ashdown stuck at the top, needed desoldering off the pcb, measured 650R and read 80 percent on DVM voltage, unstick the tape before resoldering. Ashdown MAG 200, maybe 2000 or 2002 schematic available via email from Combo with 1 x 15 inch speaker. After hour of use it cut out. Next time it failed after 10 minutes. Lead-free solder "volcanoes" or electrolyte from the 10uF cap near the J112A N-D type FET caused solder failure on the cap and falsely triggering the protection circuit. In use voltages on the 112A were 0.2,0,-8V clicks over at about -1V As mosfet discharge the reservoir caps before dismantling if no speaker load. In working order the Source to ground of each power fet shows a forward "Vbe" one way or the other and only that terminal. To get to the front board leave the 4 nuts in place and remover the 2 captive nuts and the 2 small screws on the top. To replace put the sleeves into the front panel rather than on the pot shafts and locate the meter into recess before pushing in the pots and switches and sockets. The thermal switch T'-key 125-a15 code meaning 125 degree C, I took to 120 C and did not change state and not mechanically suspect. Also 3 of the pots were (needlessly) long shaft with nothing other than paxolin and nylon bearer around shaft keeping them in place. Could easily have used the normal shaft ones and extend the pins, so proper bush nuts to front panel could be used. Replaced with a panel mounted one , wired into the board. Meter had been pressed in and soon would have had solder joint broken. Made a hole through the pcb for a nylon bolt and nut glued to the component side of the board so could be tightened against the meter when aligned and fixed in the chassis. I do not know if this is correct setting but this VU meter was 0 on scale with 0.18V RMS 1KHz input from a 600 ohm source and input pot at maximum. I don't know what the illuminated meter looked like as original but I reckon my re-jig would be better, looks quite nifty, although not 'retro'.
 Ashdown Mag 200 meter mod
 Ashdown Mag 200 meter , side on
Because the cover protrudes 1mm or so it even looks effective skew-on. Put a 2K , 3W dropper in and a bright blue LED wired-in and moved to vertically under the meter movement when mounted in chassis and angled toward the ring section, so internal refraction , through the plastic, forms a blue ring around the ali cut out , especially after abraiding the coating off the aluminium in that recess, plus a bright spot under the meter that shows in bright ambient light and it also throws a light on the scale. Filed a small notch angled downward so the front periphery to the meter was still circular but allowed more light into the bottom area of meter cover, revealing more of the LED directly as an indicator for bright light conditions. 2 LEDs , under but angled in and across, would have been better as the static part of the movement shadows part of the light cast. Comment on return to the owner. "looks amazing! thanks for doing that, it really makes it stand out, cheers" The 20mm fuse on the preamp board is too close to the metal edge so covered with a cut-down piece of wide flatted-U file binding spine. As all lead-free solder redid all heatsinking joints on both boards.Main AC rails 50-0-50 and preamp 18-0-18. Ashdown Peacemaker P60 valve combo, 2003 Volume fluctuation and background rustle I discovered how to get into it, remove front knobs etc and remove the facia panel first. Lever out the mains switch enough to pull the fascia around it. Remove and replace amp with cab on its side. Meter unslots and tape it to the underside of the chassis. This amp runs very hot in the region of the EL34 output valves, almost to the point of charring the pcb, localised black rather than just brown discolouration, through the bulk. The valves are inverted and no ventillation path under or around the valve bases. There is plenty of space to mill holes in the pcb around the periphery, and avoiding traces, to allow some ventilltion through. Scraping back some of the pcb surface browning and into the bulk of the pcb material , the browning extends the depth of the material. Also some brown staining on the surface of nearby solder points, probably some vapour coming off the polyester and settling on the solder. No ventillation grill in the cabinet top either. I'd half considered a fan but there is no obvious place to put it and without cutting a hole in the steel chassis or the wood cabinet it would just be circulating the air, unless mounted outside the chassis , directed over the bottles. Could use the rectified heater supply as a less than 12V supply for a fan. Cut a 3 x 2 inch hole in the cab top , masked with grill. Positioning it asymmetrically off-centre in both senses so above the big bottles and clear of internal encumberances so a 2 inch fan could be fixed under the grill , later, if required. Bolted down some metal bar etc with the handle bolts to form a slideway, for 45 degree drill bit guide to slide and step along to cut 2 angled slots to bring the vent hole inside the cghhassis. Cut through the ends and tidied up and painted black. And the pcb perforations with dremmel and 1/8 inch ball mill Putting a thermometer in the quiescent vent flow, 40 degrees C above ambient, better out than in. On EL34 side of one AC coupling cap .25V ac dropping to about 0V and on the other side .25V upping to .3V on volume drop. Replaced that one nearest the heat build up and its match. The pair of 220K in blacked glass sleeving to -12V is to " stablize the bias until the tubes have warmed up enough for the self-bias to take over. It adds about -6 volts to the grids" soon after switch on. One HT cap had bad solder. Replaced weak input socket. Beware earth continuity is thrugh 1 only pcb standoff and beware of trappping loose wies against standoffs. EL34s use ac on heaters, 12AX7 s use +/- 12V DC in pairs 33K large dropper, bad solder points and overheating, stood off the board with heavy duty pins, also 2x 1K, 2x100R, 1K. 2 canted 072 because of large Cs. Mains 7R // 29.5R,.2/.3R,.3R o/p 113R bn, 103 bk,// be/gn .9R, Y/W .3R with o/p set to 8R Audition spring-line amp Excessive white noise. Germanium technology low power practise amp one time sold at Woolworths with detacheable 15W speaker.Replace the input side transistor 2SB440. Boss DD3 digital delay foot pedal, version 3B , 2003 ? Does not always latch on, momentary footswitch seems ok eg physical switchpoint, zero ohms on , the latching is via RC and taking an input to a 74HC00 low. Mark all the 1/4 sockets before removing as their orientations are critical. Original problem was probably (due to sooting/tiny etch mark) the electro (too long lead had touched the adjascent ground line so cut it back)at the 5V line of the SM 5V regulator marked "A E" , IC9 I don't like the indirect footswitch ground line via 2x 1/4 ins sockets and casing but nothing loose. placing varioyus Cs across the sw made no improvemnt. Desolder the small pcb off the switch so the metal casing can be removed for ease of working on after demounting the sockets and pot board. Did a subcircuit determination Using one wasted, as paired-up, input to one of the SM 74HC00 gates , cutting just one pcb trace and added an R,a C and a D constructed a 0.4s monostable, pulse stretcher so now latching properly. It does not seem to have affected the hold mode option either. Footswitch goes to ground to activate, with input lead is connected. via 100 ohm to pin 1 of 74CH00, linked to 2 (H via 100K), 3 to 4, 4 to 12 and 13 and red lead to hold option on mode sw, 5 (H via 100K), 6 via 10K to pin 62 of main IC, 8 to pin 59 of main IC, 9 to brown lead and hold option on mode sw, 10 to 11. Soldered 1M between pin 2 and 14, cut trace between 1 and 2, then .47uF SM cap from 2 to diode to pin 6 to conduct when pin 6 is low. Latching failures seem quite common with DD2 and DD3 according to the archives. The line to the led had been squashed against the power-in socket. The input socket switch only functions with battery power Boss ODB-3 bass overdrive Sometimes fails to work or stops on battery and sometimes on adaptor. The plating on the power socket metal parts was breaking up , presumably interfering with the switch contact as well. Bullet VM30 combo, GDR from 1990 But looks more 1970s . Pull+slide the covers off the handle to release and springline ICs marked "X2 080" ,"X8 761" ,"TL 761" (not opto) ,"X8 082" ,"X6 081" ,"X6 082" , V4007D B080D = TL080 B081D = TL081 B082D = TL082 B761D = TAA761 V4007D = CD4007UBE 2x KD607 , SD339 and complement SD34? SD335 , 336 on preamp , 2x330R , 120R An E-line size or varicap type package of a diode like SAY32 marked 32 VV or 32 YV, 32 volt zener ? The ones marked 15 are the zeners for setting the +/- preamp rails. Works for 1 second and then fades out. There was a burnt section of the ground track between the secondary ps side of the preamp and the signal side. The grounds of the 1/4 inch sockets were measuring 6V. Bridged temporary with 68 ohm and measured .6V so bridged with a fuse holder and 1.6 amp Probably due to operator "error" and have replaced with a fuse there. The siting of the ones marked 32 maybe for overvoltage sensing , but not normally in service. Probably those are 32V zener. There are 2 more of that package on the preamp. These labelled 15 X4 and next to the +/- pass transistors for the preamp +/- rails so probably 15 Volt. Springline 315R and 10R, if the 10R one is disconnected there is distortion to the output with reverb set to zero. Repaired now, but for the archives there is a safety issue with these amps. The earth bond wire to the amp chassis is fine but the bond to the front panel metal is very poor, just a brass rivet to the phenolic of the pcb, of a few ohms drop over the years. Needs redoing to the metalwork properly. Carlsbro BassBeaver, 20Kg Broken pin on input socket, desoldered but soldered wire to the remnant and soldered to the nearest component down-trace. While in there noticed a burnt 2W resistor that was o/c. This was a snubber at the speaker output consisting of 1uF,100V non-polarised electrolytic and this resistor. Remaking the remnants of the R it was about 3.9 ohm. Nothing on TDA7293 product pdf. Replaced with a 2.5W, 8.2 ohm with long solder joints along traces. As probably o/c for months obviously not too critical The R-C combination is a Zobel network "can be quite important to the correct operation of the amp. It's purpose is to neutralise the inductance of the speaker voice coil, with the intention of flattening its impedance curve at the HF end. However, without this network in place and doing its job, I have known output stages to burst into spontaneous ultrasonic or even RF oscillation. The values of R and C are calculated according to some esoteric formula that I'm sure real audio buffs could quote at you chapter and verse, but the general rule of thumb is that you start with a resistor of equivalent value to the voice coil's nominal impedance, and then calculate C from there. " - Arfa Daily also +-41V, uses TL071, 072 Resistors 150 ohm, 2x680 and the unknown 2W Carlsbro Cobra , 1986, 28Kg Someone threw something at this speaker in a Carlsbro Cobra combo. It works with distortion, like a weakened form of one of a pair of complimentary drivers working only. By digital probing (using my fingers) at about 4 o'clock position , unpowered, the rubbing resistance feel to movement of the cone disappears. Same if powered up, no distortion, to a power level where the effect of my fingers damping comes into play. A liquid that will contract on drying to paint radial stripe on cone and dry and test, repeated until problem goes plus one or two more stripes.? Or would freeing the rim or part of the rim of the cone by hot-air heationg and reseating all or part be better. If part then release 2 to 6 o'clock say and pull or 8 to 12 o'clock and push, my guess is pull would be better. The central dome was buckled in so attacked this first. Drilling a few 1mm holes and using a dental sickle brobe managed to pull it back into reasonable shape. Somehow that corrected things, just a small puncture in the cone periphery to patch over now. Uses 2x 3055, 2x1C03, 1C04, 2x 10R, 2x 0.24 , 150 ohm on phones line in the pa. TL071 s , 2x680R , RC4136 on pre-amps +-44V dropped to +-15V Someone had been in there before and managed to disable the reverb. The transmit side of the springline wire has no earth return, just a dummy pad on the pcb. That person had soldered the braid of that wire connected to the receive ground to the driver output. Tracing the RC4136 quad the spring line driver output is pin 3 o/pA via 100 ohm to springline and 220p+100K return to "A" i/p and via 100 ohm to i/p(D). Receive side of springline connected to pin 9 i/p(C). Carlsbro Eclipse 12, 1998, 12 ch mixer amp Drummer laid his snare drum brushes by the side of the amp and the wires flicked into the vent slots on the side and killed the amp except for front panel lamps. Replaced 2SA1668 pre driver, and FS7 fuse and added "crockery scourer" grill cloth inside the vents either side. Also for personal H&S cut off all the jagged corners on the sheet metal work with double action snips. pa uses MJ15024 / MJ15025, 2x 2SC4382, 2x 2SA1668 , 2x 4580, TL071, BA10393 , 4093 IC03, 7815,7915 digital board TIP132, 7805, 7815 discoloured 220R ?, measured 265R over each in cct. Disassembly, remove side panels, remove top and then divider panel 5.7V, -15, 15, 15/5/50V, 47R in cct, 2x 10R PL4 for LCD display panel PA works with all mixer panel disconnected PL7 2,2,2,2,2,2,7,7V PL3 0,15,13,13,2.1,0 PL4 6.5,6.5,6.5,6.5,1V PL8 2.2,2.2,2.2,7.2,7.2,7.2,2.2,2.2 PL1 -12.8,2.5,17.5,26 HT 68,0 main bridge rect ac 102V, 138V dc othe rbridge 18.8V ac Carlsbro GRX7, 1996 Sometimes failed to work and sometimes dropped in power and crackles. All the 1/4 inch sockets needed attention, front and rear. The signal pa/preamp interconnect lead needed attention The subsidiary +rail 156R dropper needed replacing. Uses 4x MJ15024, 2x SM2177A, 2x SM2178A, CBC182A, BC212 1C03 , 2x TL071 156R, 5x R33, 10R,2x 4R7 Mains primary 6.9 ohms uses +/-15V regulators on preamp Carlsbro Hornet 45 lead, 1984, 17Kg Distortion. Problems with output transistor, poor joint at base for some odd reason, heatsink hold-down bolts not tight enough. Soldering generally did not look very good - flux residue problem? - re-did all power handling ones. Uses BDW93C, BDW94C, RCA IC02, RCA 1436 Rs one measured in circuit as 156 and 2 not larger than 0.3ohm. Put a union in the speaker cable for ease of repair. 2 pairs of side screws at rear release the rear panel and front ones for front panel release, springgline fixed to centre board. Unusually the amp separates into 2 sections separately connected to the wooden casing and so electrically isolated as far as any earthing. The mains transformer is on the rear panel which has a proper earthing point from the IEC mains inlet. Power supply is plus and minus rails but the common is not connected to ground at all. The power and signal lead to/from the pre-amp / reverb is 4 lines : plus , minus , common , line level signal with no earth connection. The front panel is connected to the common , not to earth anywhere. There is much more noise immunity from general stray pick-up if I run a wire from the earthed back plate to the front so bridging common and ground but left as is. Interestingly this unit was yearly PAT tested and passing presumably earth bond and insulation test each time. If they had checked insulation resistance relative to the front panel , he assuming it was earthed, then could that have caused damage.? Carlsbro Marlin 6-150 m2 1987 Insurance write off -not working after being in a room with a serious fire not associated with this amp. Some minor radiant heat damage and smoke stained but failure probably generally relevant to other equipment in similar circumstances. Although there are no ventillation holes with these amps there was considerable smoke deposit on the inside of the amp. The concentration of smoke must have been so great it had penetrated through the 1/4 inch jack socket holes. All these sockets needed cleaning but the real reason for not working was smoke entering the mains switch and IEC integral fuse holder and insulating the electrical contacts. Power devices 2 off MJ802 ok . To gain access remove one case end face,slide out top and bottom pannels after marking and disconnecting springlines. Remove the rear heatsinks to get to the power TRs that unusually B and E fix into screw terminal blocks on the pcb. Carlsbro 8400 , 2003, 400W mixer amp that immolated itself. Remove the front panel first to work out which screws on the back to remove. Q319 is a high voltage TO92 ( totally burnt and erupted but 2 isolated lighter blobs on the remnant of TO92 face are in the position of the isolated printed 2N of other 2N5551s on the board) then the melted blob component goes from the base of this feedback transistor to the speaker output line via the relay and choke. But it was 2N5401, (from the obtained schematic) which did not have a match elesewhere. The board is also burnt so overlay for this blob also missing. Anyone ever come across a metal oxide resistor melted to a 3mm diameter blob like a small tantalum. Metallic grey blob that measures 40 to 150 ohm or so across diameters, was 1/3W reistor of 330 ohm. The thermal heatsink monitor tranny was also exploded but as the face was gooed to the heatsink, that face was spared from charring/shattering and that was 2N5551 also, the thermal switch is 105 deg C. The subframe inside the chipboard casing is held to the chipboard with 12 x 4mm bolts, into clinch nuts on the outside covered with the black vinyl. 2 had dropped out and because you carry this kit vertically but use horizontally, plenty of chances for 1 inch long steel screws to roll into the power amp. But only plain washers and not star or spring washers under the heads and no varnish /glue spots over the heads to stop any undoing. On reassembly all internal bolts were replaced with star washers and glue spots. Audience were treated to piles of smoke erupting from the back of the amp - Hendrix was more impressive. Plenty of other damage but identifiable components for the others. 3 .5R, as well as above items and even a 5mm wide , current reinforced with solder, track had burnt. Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major axes of .15mm and .8mm. Copper fusing current of the trace = 12 amps Lead+Tin elipse then 6 amps (not as much as I would have intuitively thought) Total 18 amps so 18/4 = 4.5 amps conventional fuse rating. Presumably the circular to sheet allowance would up this 4.5 amp figure , but by how much ? What sort of correction factor for thin sheet/non-circular heating then rupture allowance? I had a play with track width calculator and assuming it is still sort of valid at very high temps. Putting the melting point of copper of 1080 deg C then for 3.5mm strip of presumably 1 oz copper then the rupture current would be about 48 amps which seems reasonable. I knew it must be higher than 12 amps as that calculation was for round wire. By 1080 deg C we can forget about the solder run beefings. I think, assuming they survive ordinary power-ups a few times, I'll settle on a mains side 4 amp anti-surge , with a 5 amp A/S ready to hand spare and 2 off 10 amp quick-blow in the DC lines. Emailed schematics from Carlsbro. Oriental script on the mains transformer and on a schematic. Q319 was 2N5401 and the R was 330 ohm. I'd not thought about it but VxV/R heating with single rail of amp voltages of 65V to melt a 1/3W resistor, not charr or explode it, it must be about 300 ohms or 10 watts dissipation. Uses 47V-0-47V ac, 3x 2SC5200, 3x 2SA1943 2SA1306, 2SC3298,2x TIP41C, TIP42C 2K, 6x .5,5W 2x 47 Replaced output trannies with lower voltage, higher current but much cheaper TIP35C and 36C and 2SA1668 to 2SA968 and 2SC4382 for 2SC4159 On bench tried powering with half rails of +/-30V , too low for relay to click over and no load at all. TOP66 of shorter form than the originals so ground split some standard spring to heatsink retainers and ground a small notch in the end to engage with the mounting screw plus star washer and glue dab and sprung against the tranny body (with glue dabs) so using existing mounting holes. With these lower rails .3, 30, -.14 and .8, 30, .3 on "positive side" TO220 and TOP66 .8,30,-.3 on thermal feedback TO92 and .13,-30,-.07 and -.4, -30, .13 on "negative side" Full rail voltages not measured. +/-15 V on the separate TO220s With amp outside cab but in normal vertical position and 7V ac of 400Hz driven into 4 ohms then temperature of thermometer clipped to the top of the big heatsink stabilised to 27 degrees over ambient in 35 minutes. No need to remove all knobs etc to acces the preamp control section of pcb. The master volume slider pot "knob" was broken , owner used a match. Plenty of sideways room to keel the slider over to glua an extension to the side. A bit of black cable tie bent and glued to double up in a vice. A bit of a bend in this piece to roughly match the leaning pot. Desolder the 2 pins on one side to lean over and introduce back into the cover and resolder after positioning better. Carlsbro Marlin 6 150 IV, 1995 ? ps +/-44V prea 2x 1K pa 2x 0.3R (in circuit) 2x 10R,150R TO3 nearest signal IP line probably NPN Beware the effects switch at the rear can give this "fault" as well if falsely engaged. All pots "worn" to the extent that its harcdly possible to find a position on any of them to pass a signal to the amplifier , so effectively dead when the main vol control becomes extremely "worn" as probability of blindly finding 2 active spots in a pair of controls with 90 percent defective is unlikely and pretty useless if found anyway. This occured at a period of cold weather which may be contributory. The pots were not worn as such. Believed to be the original lubricant had hardened over the active pot surfaces. Renovated all 8 gain pots and treated the bass & treble pots with meths bathing , both as per tips files. Tx 9.7R//0.4R,0.4R Carlsbro Marlin 6-300 Mk 2 1986 Blowing fuses. TR7 TIP41C s/c all round,R (R21 ?)between C10 and VR1 burned but probably 1K,R29 burnt replaced with 100R, other damaged Rs R9 22K ?, R13 3.9K ? R14 1K ?,22K ? preset burnt out. 3 of the 4 2N3773 defunct. As an interim replaced power devices with 4 x 2N3055 while checking out biasing section before replacing with 2N3773s. This amp had been repaired at least once before so cannot be sure if correct components or voltages but from repaired amp :- Thermo feedback used a BC182B, unknown CBC640 small BC182 type body in 60V section of circuitry. Other devices RC4136N and TL072 . Driving 8 ohm load DC voltages reading from heat sink side on track side of board. 4 2N3773 0,61.5,.4;0,61.5,.4;-61.2,0,-60.9;-61.2,0,-60.9 then row of TIP41C,42C,42C,41C .4,61,.9,60.9;59.7,.9,60.2;0,-60.6,-.5;-60.6,-.4,-60.1 Carlsbro PM15, 2001 Dual purpose 200W PA head or a foldback monitor. Reported occasional loud bangs and pops without any signal input, otherwise no distortion. Of course I powered up and with any amount of banging of cabinet no induced problem. Took apart ( need to remove the 15 inch speaker first ) and powered up with twizzle stick and serious bangs and whistles can be induced by tapping in area of electrolytics C1 (4.7uF,63V )and C3 (47uF,63V) and 2 small signal MPSA92 all at input from pre-amp. C1 probably AC coupling from pre-amp and C3 local DC rail decoupler. Otherwise no dodgy looking fuses, spade terminals hotspots or suspected dry joints anywhere. Replaced the caps and resoldered the trannies, removed Cs seemed ok and one trannie pin/solder did not sweat properly , contaminant ? TR3, SM2178A voltages, -59,-0.6,-59.8. 2 possible dry joints on this TR3 so added insulated heatsink tied to the pcb with cable ties. 45V ac on both fuses 60V,-60V on bridge rectifier uses TO3 devices BUZ906D & BUZ901D power to preamp 15.6V,-15.6V dc 3 sets of 8 screws, machine bolts on speaker, washered screws on front grill. Carlsbro PM65-100 , 1991 stage monitor amp-speaker , Carlsboro No repair as problem was in external feed. In exploration a 100R resistor next to TR7 was charred but functional,replaced with 1W. Replaced S M marked pnp TO220 trannies 1C04 with TIP41C and npn 1C03 with TIP42C. ( for 1C03 use RCA1C03 and RCA1C04 for 1C04 to find data ) Supply rails +-44V TR3,7,6 voltages -43.9,-.52,-44.6 -.52,-44.1,0.1 1,44,.5V Carlsbro PM65-100 , 1990 stage monitor amp-speaker Loud buzz plus hum and no power LED on front. Someone had been inside and fiddled because they had moved the header to front board one place out. So + feed not connected. Still a buzz when put in correct position or disconnected so presumably original problem was on main board. Erroded "dry joint" on -ve pin of the bridge rectifier so remade all pin joints. TR3 marked S M and 1C03 ? ICo3 ? 1C03 ? IC03 ? replaced with TIP42C as seemed to have been running hot for no heatsink. Carlsbro Viper bass combo, 1990 , 25Kg Pops and bangs then drops out after 20 minutes, starting ok again when cold. Vibration induced failed dropper joints at the preamp 15 zener supply rails. Redid droppers tied together with silicone rubber sleeving and silicone rubber support pads and replaced associated caps has one had probably leaked. Redid solder joints on major leads on pa as 2 would soon be a bit suspect. One 1/4 socket broken, the other weak , replaced with stouter types , upside down and cros-wired and located in place over the intrusive switch for one. Rails +/- 44V , with back plate horizontal and feeding with 400 Hz and o/p of 6W continuous, then thermometer in the heatsink back bracket stabilised at 75 degrees over 20 degree ambient after 40 minutes. Uses 2 x 3055, 2x RCA 1C03, RCA 1C04 2x .24 , 22x 10, 1K 1/3 W 4x 4558, 3x RC4136, vectrol linear optocoupler 2x 470 ohm Carvin Pro Bass 100, PB100 Blown o/p , maybe started with poor solder, hot spot, small burns at solder pads, device Carvin marked 60-72940 , replaced with TDA7294V 15 pin device to 10 wire ribbon H2 15V,0,-15,sig to pa H5 1 sig in, 2 gnd, 3 stdby, 4+5 + supply, 6+7 - ps, 8 mute, 9+10 o/p Carvin Pro Bass 100, PB100 Any movement of the frequency pot for mid band lift created full DC bangs. Ultra-miniature pots that have next to no metal for the wipers. I assumed gooey grease in the pot was sufficient to lift the wiper from the track which in itself had no wear. Decided to replace the pot with sub miniature pot but as overall length less, padded out with a block hot-melted to it and board and extended connections. Uses 4558s apparently dated 1983, although Carvin WWW schema for version B dated 1995. Used black 4 lead opto-coupler Vactec/Vactrol LED -ve at notch corner, cell at the other end. Used 15 pin hybrid 100W,100V labelled Carvin 60-72940, 2 large Rs, 820 ohm,7815, 7915 The rattle noise on moving the amp is a spare fuse in the IEC socket. The amp to speaker link needs the elbow jack replacing with conventional, just enough room. Poor mechanical connection at the elbow means a resistance and voltage drop, heating the plastic disc that is all that forces a mechanical connection so progressively worsens. Carvin X50B tube amp, 1989 Told the amp was not working Amp seems to work perfectly well, being repaired for minor problem , broken wire at the return of the external effects loop so not working if externally connected. no succeptibility to microphony or tinging distortion. Testing the 2x EL34, gains are good and matched but after a minute on heated C/H insulation check, the resistance gradually decreases to 1M for one and 0.5M for the other. What problems will this lead to if continued use, and how quickly ? The amount of leakage apparently is no big deal for power tubes, if it gets below 100K then have a hum problem. I was wondering if there could be some sort of thermal runaway situation. Measured no more than 1mV rms hum over 4 ohm output for this one , level set for 2.25 W of 400Hz output, but disconnected and monitoring for shorted input. All pa W/W measured in circuit at about 350 to 380 ohm. 470K, 2W 5 x 4558 , 4049 + 4x 4558 240V primary 4.3R Secondary red , in circuit, 32.5R yellow/yellow 1.2R blue / gn 2.2R large gn wires heater supply H2 wire colours Bl,P, D Brn, R, Lt Brn, O, Lt P, Lt Blu, Bk,Y,Pk H4 P,P/W,O,W,Y,Brn,Bk,Bk,R,Gn H3 voltages 0,-15,13,0,13,0,0,0,13,0 470K , 10K Someone had been in there previously , maybe removed R arounfd the Master pot. Could not see why they would employ just 1p 1w of 2p2w switch and no use of the pot. Placed a 680K R where R13 on the overlay . Load test , thermometer wired to a 85mm pickle jar lid , balanced over the 2 bottles 4 ohm load, 400 Hz giving 3v ac over 4 ohm 15 minutes to reach 40 deg c over ambient and 0.06V ac increase Casio CZ 5000 1985 keyboard No function,LEDs or LCD display The only convenient 0V point for the TTL I could find was the -ve pin of the ps bridge rectifier. No 5V for the TTL. It looks as though the keyboard had been dropped as the large electrolytic on the ps had moved and consequently the track continuity to the 5V regulator part of the ps was broken under the capacitor pin,not obvious at al. With all 3 connectors unplugged and PA order on main pcb not the ps end which is different order. PA 0,15.3,-16,0,5.6,0,5.2,0,5.2 PC 0,0.03,5.2 PD 0,16,15.3,0,0 Casio WK 3000, 6 octave keyboard,2003 Intermittant loss of left speaker. Attended to all ps/pa solder points and conneections and desoldered headphone socket and remade. Open case upside down , from the rear first and then free the keys section. Work on it upside down with the keyboard section innermost. 3 pin TO126, device marked S2003 marked D for diode on overlay , only 2 of the 3 pins connected and diode and resistance test to the other 2 pins probably o/c so n/c ? All else functioning perfectly, but seemed odd using 2 of 3 pins. A nice stable 5V over the 2 pins in the ps area, so that a 5V, 10W (with heatsink) zener diode was my assessment. No heatsink used on this one but a useful TO126 format for say 10W zeners that are bolt-downable. Uses LA4636, 2x 2068,Sharp PQ1CG21H 3,1.2,0,5,14.8V NEC upD63200 dual DAC , pin 15 serial data in Al caps 2.2V, 2.7,5, 3.3V , SM tand 0.8V 2x wired caps on uPD65881GK062 gate array on underide of large board 4.9V Ami LP62S2048-70LLT 256x 8 SRAM ps to main processor board ribbon , starting red 5,0,5,.2,,01,2.9,0,5,14.7,5,1.6 (o/p signal line 1),1.6 (o/p signal line 2),0,0,2.7 D shaped recesses in case for washered screws On first powering up it plays dumb, display echos the key presses and midi functions normally but no sound out of the built in speakers unless you wake it up going into demo mode for a bit. Nothing wrong with mute/standby functions of the main amp and low level hiss from the speakers so not headphone bypass problem. Possibly on powering down "local off" setting for midi use is stored in the memory. Seems no access via the keybpoard/keypad/buttons etc to alter this, must be via an external pc/midi link. But just pressing "piano setting" resets a number of things , one of which is setting local to ON (if OFF), in the troubleshooting guide , if you have a silent keyboard. This Keyboard not used via MIDI But there must be some powerdown (or power up ) fault creating this random occurance (left unpowered overnight and powered up perfectly ok, despite deliberately switching off at the mains rather than powering down via the power on/off button). No obvious on-board battery , although overlay symbol and unpopulted section of the main board has this for some other model type. The corrupted memory is on 'song memory', keyboard ges silent after the owner pressses that button to recall previous recordings. Probably corrupted when writing to it when someone pulled the DC supply plug out of the back. Citronic SSL 1001 Sound-to-light unit No lamp function on any channel Fracture of a pin on 4025 due to corrosion at juncture of legs and solder.Other ICs looked similar so removed all and replaced with socketed new CMOS ICs.Note for personal safety when working on the logic circuitry disconnect from the mains and power the logic between Vss and Vdd from a 10V bench power supply. Crate GFX 65, 1999 Triple footswitch , no name , no ratings on switches. Impossible to use as a mind of its own which channel it wanted to be in. On resistance anywhere between 200 ohm and 100K on each of them. At least easy enough to get inside them, dissolve away the grease and deburr the rough edges and put back together again. uses .2, 3x .47 plus 4 1.5K, 3.3K,220, 2x10 5x 072 TIP42, TIP47 7x J175, 3x J112 primary 11.5R , secondary .5R, 60V ac Crate GFX 65, 2000 Green LED but no sound on Clean channel Someone had been inside with a power drill or something doing some "repair" and stripped up tracks and board. In particular the trace that goes past near the input socket that they were trying to "repair" With clean ch controls at 1/2 and 14mV, 400Hz, at input and DSP board diconnected IC1 pin 1 56mV , pin 7 360mV IC3 , pin 7 40mV TIP42/147 , 2x 047 ?, JE340 2x 270, 470, 0.2R 220, 1.5K, 3.3K, 10R 1/2W Remove 4 heatsink screws and 2 pcb screws and DSP board to release the preamp board IC3 pins 6 and 7 to clean level pot and IC1, pin 1 the trace to th eclean channel For testing needs a ground connection to both the PDCb ground points, PS and signal or nasty noises. Crate GLX65 combo of 2004 Uses miniature pots with clear plastic shaft and illuminator bulb inside the pot. Body 10x10x5 mm and even then space inside for a wire-ended bulb. The spindle made of clear plastic to conduct the light into a knob with red plastic inserts. Anyone know of an independent source of these. For future reference, as in for mechanically broken switch, all the pots are fine. Only 4 years of normal use and yet again flimsey push/push switches are falling apart. Replaced those with robust conventional toggle switches, enough room after bending the pins, wired across to the pcb, and only SPST use although they are DPDT switches. The mute switch is of stronger style. The compact LEDs illuminate the surrounds and the pot bulbs illuminate th epips in the knobs. 3 screws at rear release the prea section from the rest. Mark ribbons for orientation etc, to relese ribbons push the clusure strip. Reconnect cables before rejoining metalwork, double check you don't have 2 wires entering one socket, very easy with these lack of terminations. DSP fottswitch mono, solo/clean/rythym stereo 1/4 inch Tx 55R// .9R,.9R pa 0.2R, 2x 270, 2 x 0.47, 220 2W, 1.5K 3.3K 1W TIP142 TIP147, 072,JE340 prea , foam cylinders go around stemmed LEDs Crumar Roadrunner 2, 1979 , Italy, synthesiser Chorus effect not functioning. One of the 5 CA 3094AE not functioning but low level of output. Connected 6.8K between the chorus maximum level signal at one of the CA3094 and other end to the board output. The F 4727 was probably Fairchild 7 stage CMOS counter. ITT SAA1004 f divider. Mostek MK50240N probably = Thompson MK50240 top octave generator. If one key sounds but repeat depression does not then due to mis-alignment of contact spring. Un played the spring must touch the long contact rod common to all the keys to discharge or it will not play when depressed. Crumar Roadracer, RRC, 1978 One note per octave absent. AY-1-5050 Its a frequency divider , 7 stages , broken into a few isolated stages but in this use they are chained together so 1 4024 would do and perhaps 1 + 1/6 4049 for buffering. The trouble is the original uses 2 supplies -15V for outputs buffer supply and -27V for the logic. Buffered trannie input and running the CMOS 'upside down' between 0 and -15V ? Found an exact replacement so didn't bother fudging pinout in D.A.T.A 1982 digital along with AY-1-1320 and AY-1-0212T, MM5891 adjusted pinning may be useable there. Two keys were sticky. Not due to rusty pivot but under each key is a standoff carying a shaped rubber piece to stop keys swaying. Presumably going sticky , replaced each with a piece of 0.19 inch silicone sleeving minus cores. Perhaps squirting talcum powder under each key as maintainence may help. Gain access to underside of key by opening out the metal channel at the rear to release one pair of nibs on the white pivot plastic which allows to slide off the remaining 2 nibs. Slide the key to release from the hidden constraints but still stay captive by the thin and vulnerable switch contact. A nail wire staple hammered into the casing to keep the top switch plate away from the leftmost key. Custom Sound 2005, Mosfet amp, made 1988 Volume faded down after about 1/2 hour . No tranny/diode thermal monitoring of the heatsink and no FET for gain control in the pa or FET in the preamp , all 4558 opamps. No obvious over-heating or solder problems on any of the boards or flaky gain pots. Springline reverb throughput had failed due to corrosion on one of the springline phono connectors. This Accutronics springline was 200 ohm for in and out pickups. Dummy loaded (4 ohm and 6V ac) took the heatsink up to 70 degrees C and held that power level for 40 minutes with little variation in preamp output, pa o/p or DC rails so a matter of the usual treatments for the numerous IDC interconnects, sprung IC sockets and 1/4 inputs, despite no response to twizzle stick and reported fade. Dealt with all interconnects (stagger-marginally-bent the pins and cleaned to give better holding), IC /IC sockets and beefed up all 1/4 socket connections as 5 channel all in a row, by running silicone rubber cord the length over the sockets with Hama/Perler/Pearler beads 2 over each contact, fed onto pired up cord as in threading a sewing needle. Pairs of holes drilled through the pcb to take wire loops and a Hama on each to tension down the cord between each socket. Then hot-melt at the ends the down-pulls and the twisted ends of the wired tensioners. The mains fuse cap was exposed to a screwdriver so replaced. 2x 2SJ50 and 2x 2SK135 bf470, 2x BF469,2x BF423, 1K, 2x 1K, 7x 4558 on preamp Custom Sound Colt 10 combo amp Intermitent o/p like a bad i/p socket. Dry joint on resistor at i/p. Another dozen or so leads had holes on one side of the solder joint. It looked as though the lead cutter plate after assembly was blunt and pushed all leads over before cutting and it had taken 10 years for this stressing to open up as dry joints. David Eden "The Metro" Bass Amp,600W , 2000, 50 Kg Cut out in use then came back. Next time of use, failed to give output but owner noticed clip light in pre-amp functioned as normal Now I have it, I cannot induce it to fail. Likely suspects the discrete wire IDC connectors for power and signal interconnects, unsupported wire-wounds on end, mains thermal switch. I did not like the front panel switch in line with the speaker line. In standby instead of cutting/shorting the main amp input you switch out the speaker. IDC interconnects on the speaker lines and power lines seem a bit irregular to me but as cut out rather than distortion those power lines presumably ok. Speaker units check out ok. The function of the 12V, 10 W "festoon " bulb in the variable crossover in the cab is a fuse. Speaker switch rated 3amp and thats for AC, could easily be the problem, let alone potentially "fatal" to the amp. The cab wiring and pair of 4 ohm speakers in series seem fine. It would not take much fumbling in poor light, drunkeness, confussion or whatever to flip that switch in full use , its not recessed and in the back of the cab. The other 2 paralleled outlets are not switched, but I may hardwire/solder back to the pa. Found another nasty under the ps board but don't think it is the main problem. This pair of wires to the speaker switch had one of the wires squashed between a large W/W ceramic cased dropper under the ps and the chassis. Cut some high temperature silicone cable sheathing into a spiral, wrapped around the replacement wires and put cable ties to each end to. Original wires melted through but as it cuts the earth to the speaker, presumably no problem as such and wire not broken, just failure to sometimes switch out the speaker. But there is smoke blackening or something grimy in that area but it could be a small electrolytic parallel to the fan cooked as it is directly over the large droppers and leaked electrolyte but all rather nasty. The aluminium of the chassis directly under this particular dropper has a strange flecked grey corrosion or something that probably more likely due to electrolyte rather than vapours off the charred PVC insulation. The corrosion i've photoed here The overexposed T area is the mains transformer and the groove marked "<" was created by the squashed/trapped cable or exposed wire core probably vibrating. From "<" to T only about 10mm so just 5 mm or so free gap to run these 2 speaker wires between mains torroid and high temp wire-wound dropper with no added high temperature sleeving. The grey flecking is what I'm assuming is electrolyte corrosion effect rather than burnt PVC product. If this fan lytic over the fan supply dropper leaked to short in use then dropper would generate more heat as well so compounding as well as shorting out the fan to the main amp heatsink. The owner had used the speaker switch in the past, during sound checks. The protection circuit is activating before the 60 degree centigrade fan switch is cutting in, under high load. The fan switch temperature , testing in isolation , is about right. The big triac on the mains, crowbar operation to blow the mains fuse if too much mains power drawn presumably, 60K to neon for UK and about 22 ohm in line between triac and switch. I've photoed the tracks and components and determined the schematic of this power amp. Hard-wired the 2 lines from amp to speaker outlet , there were 12 chances of a break in that route, to and return. 3 pairs of IDC and pin connections in each line. Using dummy load the thermal protection circuit was activating before the fan thermal switch. Monitoring the line from pa to ps with the LED thats on if all is well. The voltage changes as heat builds up from about -13.5V down to about 12.7V when it switches to about -0.3V , LED goes out and pa is killed. It was the LED on the ps that was failing. This LED passes current to the start-up hold-off circuit as well as turning off PA if the fan voltage fails genuinly or falsely. This LED also on over the high power droppers, replaced this LED off the board as well. If I'd thoutght about it I would have added a second board off the ps , bolted to the chassis with these 2 offending high power droppers on it away from the ps board. Bad design slinging them under the ps with active components over (heat rises). Added a 40 degree C switch in parallel to the existing Asahi US-602S , 60 C one but resets at about 25 degrees C. If one switch fails there is still the other. Replaced the MPS8599 with a BC212 rotated 180 degrees to pin match , in case the original was suspoect due to excessive heating if the fan had been stopped. Before pushing into the heatsink, check temperature controlled function of reducing gain to zero by a metal funnel leakily connected to a low setting of hot air gun and piece of heatshrink tubing on the small end of funnel. ps main voltages +/-78V dc. fan supply w/w dropper 75 R , 100R for preamp pa uses 10K, 2.7K, 3.9K, 4.7, .39, 150R 2SC3858, 2SA1643, 2SA1494, 2SC4327, MPSA43, MPSA92, J112A, MPS8599 Heat output measurements on 400Hz continuous sine i/p. Into 4 ohm load with room temp 19 degrees C, for 25W o/p in load, fan outlet temp 25 degree C for 52W , 30 degree C at fan outlet Dunlop Cry Baby GCB 95 ,wah wah,2005 From a wah-wah pedal only 2 years old so I don't see the point of replacing with more of the same, as these symptoms are very common. Poor bypass function and now total failure to switch between on and off Both 1.8mm x (11 down to 10mm swaged )stainless steel rivets ground off to separate the halves,push-push latching switch Unless anyone knows differently then I suspect the poor bypass contact is due to the interior packed with non-conductive grease. Carling, Mexico is the make of this one , single pole c/o, silver plated contacts and dumb-bell that can just about be seen in the centre of the top image,then a locating cup, then compression spring, then brass cup that seats the end of the swinging arm marked L in the next images. The dumbbell jumps the 2 contacts at the top of the image and runs along the continuous lower one, enlarged in 4th image, 5th is the swing arm magnified. some background on Carling switches Featured Article :: A comparison of boost pedals :: by Ken Nossardi I think I agree with the comment in the quote about machine tool wear. I've not removed the rivet that holds the ratchet mechanism - maybe shown on a patent site somewhere. I suspect that ratchet mechanism, produced by worn dies, is just enough to bring that swing lever over to just over half-way when tested off the production line. With, now, no reserve for in-use wear. Cleaned the silicone grease out as not used for mains switching here, otherwise 1A 250V / 3A 125V rating. Why silver plating and then non-conductive grease in low current/ low voltage use ? There must be a "ball point pen" type latch action in the bush part so the down action pushes the L arm one way then the other on the next down stroke, with a click of its action on each upstroke. plenty of spring in the ompression spring between dumbell and brass cup, so no lack of contact closure pressure , leading to the bypass problem Presumably due to wear the underside of the button eventually bottoms against the end of the threaded part marked V, when L is only half way across. It needs to go a bit further than half for L to swing across, via the dumbell/cup spring sub-component action , to the other side. Slightly tightened a small Jubilee clip around the threaded bush, as a guide, and hacksawed off about 2mm at the V position, not apparent in these images, so can be quite neat. Stuffed more plain silicone grease in the ratchet section No matter, in this case (not mains use), may replace the rivets with Spanish windlass "E string" wire over 2 small soldertag rings and locked in place. Anyone know of a source of such small diameter /long rivets for other occassions? Of course just fixing the lack of switching does not require removing the rivets. In the end just used 2 small eyelets in the metal to locate under the back nut , 2 cable ties around and one across to remake. If there is non-conductive grease in other , otherwise 240V rated switches, then that could be a generic problem with all switch makes, used for low V, low A purposes. Electro Voice M/C 150 microphone Dead mike.Broken wire at the switch.To access unscrew the dome,desolder the wires to the active part. Remove XLR housing and then the 2 screws holding the switch to the body. Came bouncing back a few months later. It was now obvious the problem with these is the transverse pin bends. If the gust is held in place while the end of the pin is ground off and extracted and a replacement one made , then that would be the solution. Elka organ /AY1-5051 replacement From usenet discussion, placed here for clarity AY1-5051 Fixed Modulus Divider chip? It was made by General Instruments (later Arizona Microchip) and is a PMOS technology. a couple of salvaged 1978 , AY1 5050 in front of me with pinning, no knowledge if working order. With a DVM diode test and -ve to ground pin one measures .71 to Vgg and .66 to Vdd pin other is .8 and .74V for isolated dividers with no internal linking and a few readings referred to pin 1 as -ve to pin labelled input .88 and its o/p .78 and some others .95 i/p and .86 o/p ,97 and .85V o/p those with internal linking are nearer the o/p readings, up and down + probe to pin 1 ground then no forward drop reading to any other pin , max 2V IIRC on Fluke 77 Early Maplin in UK catalogues had pinning etc for a number of AY1- organ use series Tackling a functional replacement Wire a chopped down to 10 pin turned pin socket to the pcb and glue in place. Another chopped down socket , to plug into the first, wired to a piece of matrix board. Try an "upside down" ,positive ground (is that possible?) CMOS divider between Vdd and ground and voltage divided input signal to one of the CMOS inputs and and try just one output , level shifted with transistor and a couple of Rs , from Vgg, to drive any high level tone generators or whatever comes after. Power supply. You have to use the -27 volt VGG since VDD can be modulated. So a 12 volt or thereabouts zener, and a series resistor should be ok to get a -12 volt supply. Which will feed some CD4000 family chips, running upside down. Their positive power supply being the PMOS ground. Typical drain for the old chip is 3 milliamps, that should be plenty for a handful of CMOS logic running at audio frequencies. For each input, an input resistor, diode clamps to the CMOS power positive (gnd) and negative supply, and a schmitt trigger buffer or inverter. CD40106? Then use either D latches (4013) or JK ff's (4027) for the dividers. Easier to get the short divider chains than the CD4024 if you don't know how the external circuit is wired. If which edge is the trigger matters, you may needed to chain one of the spare 40106 inverters to get the right polarity to feed the flip flop. For each output, a small p channel MOSFET, source to (PMOS) ground, with a 6.2k ohm drain resistor to VDD. (Or maybe a jellybean PNP with a base resistor. Depends on the sound that results, perhaps). You might need a capacitor or RC filter across the output to slow down the edges.- Mark Zenier Fender AB763 Super Reverb, 1966, 32Kg with added auto-transformer Loud farty noise , intermittently. Trouble was most of the time it was working fine HT voltages were 248V and 460V on main caps and 458V, 444V and 403V on B,C,D and 5V ac of hum on the main HT. USA 110V mains transformer was 1.6 ohm DC. 4 combined speakers was 1.4 ohm DC. When it farted then 192V and 363V on main caps , close to 0V ac on the earthy cap and 90V ac on the hotter one. Replaced with 2x 200V, 330uF in series, in line and protective insulation at the ends as longer than the original. -65V on negative supply. Also the 2 12AT7 were leaky so replaced. The original USA mains cable was perished and one of the "bright" switches needed replacing, requiring making a captive nut rather than tapped hole in switch mounting plate. So cured the main problems but the owner seems to live with what to me seems an excessive amount of stray mains interference, without any signal leads and input contacts shorted to ground. Blanked off the 110V outlet on the rear. This amp is 110V running off a 240/110 V autotransformer, neutral common, with the chassis having an earth to mains earth (UK). Other than probably something to do with the awkward cross-linking of the 2 sections of 2 7025, of normal and vibrato channels . It is possible to minimise the noise using one "normal" signal input and nothing in the "vibrato" inputs but the vibrato volume set on about 3 of the scale, not 0. Is there a recognised fix/amelioration of this effect. ? Removed the 47nF,600V "ground switch" capacitor and also tried a 55-0-55 isolation transformer feeding the 110 V transformer but no noise reduction. The only solution seems to be for a "clean channel" only amp by pulling valves 2,3,4,5 dealing with vibrato and reverb. Fender Champ, 1979 Probably model 5C1, uses 12AX7,6V6,5Y3 Probably 2 main valves dropped out and someone replaced the wrong one to each socket. Replaced with new and rigged up valve retainers as the chassis is inverted. Some light gauge expanded aluminium mesh that is used with resin for car body hole filling. Originally thought of deforming between convex and concave domes but could not find anything suitable. Settled for cutting some discs , hooking springs to the mesh and bent solder tags fixed to the chassis screws. Then pinched and flattened four "corners" to form a cap, looks reasonable. As a neater refinement, if a next time, I would upturn the edges and bend back on itself to form a neat rim and find some suitable dome shaped moulds to deform between. Some Transformer DCRs , forgot to note which was which 420, 278, 25 ohm (mains primary) Fender de Luxe, PR 246, 2004, 18Kg Only used , not abused, domestically, ie not bumped around ina van at any time. Mechanically broken dome end of GT badged Sovtek 5881, neat circular crack. Black deposit around a jet out of the envelope matching a black deposit on the metal "protection" cage screwed to the cab back panel. Gripper function around valve base was fine. Not obviously valve/cage touching, but close, I repositioned this cage 1/4 inch lower down and bent the horizontal part of the cage downwards a bit to give about 3/4 inch clearance from the bottles when no flexure of the back board. This model should have the plastic/wood composite back board braced back to the front as it has 2 large openings that seriously weaken it, but the speaker is in the way. Or a vertical bar from top to bottom to prevent the panel flexing inwards if pushed up against something. Large Rs 1.2K, 4.7K, 2x W/W 470, 4x 100K Fender HRD Reported fault/cur added here as aide memoire making a horrible buzz on the 'drive' & 'more drive' channels, as well as letting some sound through on the clean channel when the volume is at zero. Replaced 22uF caps - C33, C35 or C36 Fender DeLuxe Hot Rod ,PR246, 2002 Intermittant distortion The polished stainless steel control panel has the legends only silk screen printed on. No more durable than the white markings on valves/tubes. On/Off and standby legends already worn off. Removed this thin cover panel and fitted some 0.3mm thick celluloid over this panel. Form a right angle over a thin straight edge first and tightly bend returns behind each long side and cut holes through with a scalpel. R67 , at footswitch input, according to schematic should be 2W was 1200R, 1W , uprated to 3W Speaker 6.9R 240V primary 30R in cct P11 to p12 44r, p2-p6 7.2R, p5-p10 16R, p15-p16 4.6R o/p primary 80/107R to p18, Brn /Blue Bright switch only operates on clean , no LED, channel confusingly, it would seem. Reverb tank in circuit 55R, 206R Through AC voltages with 1mV rather than 4mV agreed near enough with the schematic as did cathode DC, NB use dummy load not the built in speaker, not mentioned on the factory schematic. Poor switch contact on the "auto" 4ohm / 8 ohm switch on the more complex wired speaker socket so poor feed to the normal 8 ohm socket also poor switch to ground if plug is removed. Beefed up as per Switchcraft in tips file. Replaced the valves. Fender DeLuxe, Hot Rod No reverb Failed coil on the output pickup of the springline reverb tank. Heat glue spots, unhook active spring from anchor and release gently for both springs and hold against the tray with tape. Remove 4 suspension springs. Grind out the brass rivet and later replace with brass screw and nut through the remnant of the replaced rivet. No need to remove the other rivet to release the soft iron pickup frame and thence the coil. Wound 2800 tuens of 45 SWG. NB safer to leave the rear metal gaurd attached to the back cover and so avoid putting long bolts in there instead of the 2 short ones. Fender Frontman 15G , 2002 ? practise amp Loud buzz and internal fuse blowing if replaced. TDA2050 supply rail to output 3 ohms, replaced. Grey insulator looked a bit suspect so replaced with mica but otherwise speaker seemed ok and no other reason for failure. Uses BA4560, 2x TL072, J111 FET 2x 470, 120, 2x .68, 2x 220 Fender PR466 Cyber foot controller,2001 No response to the pedal labelled continuous controller 14 switch and 2 pedal, midi output for amp. Used 9V dc supply, measured at only .2 amps so perhaps 300mA if all LEDs lit. Took apart to discover that the unit was outputting varying MIDI signals on all of the controls. The output pcb needs the retaining screws anti-rotation gluing. Negative pulse trains at a repeat rate of about 38mS ,only outputting pulses with a change of switch , vol or continuous pedal movement Fault on the "continuous" controller probably a case of RTFM From a Fender Cyber amp manual The unit is not responding to any MIDI continuous controller messages from external devices... Make sure the continuous controller numbers matches the value in the UTILITY menu or is one of the predefined numbers listed in the appendices. MIDI board just has 7805 , 2x LM358 PIC16C711 with 057127 v 1.0 firmware version mirocontroller and a 74HC00 for midi buffer etc. Other board , not seen, but probably just 4x TTL multiplexers , 3x 7s LED drivers and the display and no effects ICs. At least while in there dealt with the horrendous squeal from the foot pedals. Seems that for neutral hold, ie stays where the foot angle was last , relies on a steel sleeve inside then the moving and mount-steel plates, either side, and a through-bolt with nylock nut supplying enough pressure to make resistance to movement, as axle. But it is steel plate turning against steel plate on either side of the pivot - urgh ! Cut 4 washers from 1mm PTFE sheet and introduced between the grating surfaces after opening out the foot plate arms a bit. Fender PR559 Rumble 100, 2003 Broken guitar input 1/4 inch socket. Fender have used 1/4 inch switched sockets intended for domestic amp headphone use. Plastic bushes would be ok there but not where guitarists in full flight are likely to trip over or yank the guitar lead. Replaced with a beefier standard metal bushed, switching 1/4 socket fixed to front pannel and wired tip,pin,gnd,switch into pcb Opening- remove front grill, 2 side screws, handle and 4 top screws. Slide forwards enough to release the speaker/gixmo LEDs lead and then remove amp rearwards. One screw on 15 inch driver , near the tweeter, epoxied and maybe spiked anchor had grunged thread to hinder access. To invert the driver 180 degrees, for more balanced usage. Hook out epoxy with a dart point and pack out gap while unscrewing to avoid releasing the anchor, retap before reuse. Remove screws at rear of Ali, only, to remove the preamp board. Use thin card to keep the ali foil (beware sharp adges) in place while sliding the amp into carcase. Uses c3263, a1294, c4793, A1659A TL072,CA3080,BA4560 4 0.15R, 470R,47,75R 270,47R on preamp Fender PRO 185, 1988, 28Kg Getting excessively hot on the top over an hour total loss of volume, and no bass at any time. The 2 power rail zeners for the preamp had failed with age so over an hour heated up and rose from 16V so eventually exceeding the rating of all 8 of the same batch of Motorola T072C dual opamps. 2x 1N5353B 16V,5W replaced each with 3x5.1V large 3.5amp zeners in series and all T072C replaced with TL072C. Running with the speaker the large .22 ohm speaker line dropper fell off the board. This had been repaired before apparently but whoever had replaced with no allowance from the brittling of the work-hardened leads . Replaced with stout pins to the R and anti-vibration silicone rubber pads. Uses 4x MJ15003, 2x MFE15030, MJE15031, T072C, LM339 Motorola MPSU10 plus another of same low format maybe MPSU60 marked PSU60 2x270R, .22, .1 large thermistor 2.2 R cold 2.2K,3.3K,2.2K,1.8K,2x 47, 10R Thermal sw N/C 248 deg F ? = 120C ? The J111 NJD FET is 20 ohm and for the main supplies +/- 49V some voltages with no load on o/p Q11 -.64,1.1,-1.2 Q15 1.1,49,.56 / Q16 -1.2,-49,-.6 R177 -14.2, -15.6 W12 -15.6, W13 -14.1, W14 14.8 W4 -14.5, W3 ? +/-? 14.5, W5 -15.6 W8, W10, W9 -15.6 40 minute dummy load test (after 30 minutes the temperature stopped climbing) metal encased thermometer laying on the heatsink settled to 68 deg C. 4 ohm dummy resistive load with 1KHz sine source and amp gain adjusted to give 9V rms on DVM capable of 1KHz monitoring , about 20 watts, into the load. After about 30 minutes although the power out was still dropping it was very much slower than from cold. V rms over the load dropped 10, percent so equivalenmt power out dropped 20 percent. I don't know if this is too much or normal but I would want the output to drop over an hour rather than rise. Anyway it was satisfactory to the owner. Fender Pro 185, 1989 A later case of same problems as the one above. In particular the large R that is common between all 4 Re and the black thermistor or whatever (cold 2.5 ohms ) in the mains line. With all controls anti-clockwise bangs. To avoid mangling the ribbon interconnect, wrap the cables to the springline around the subboard locked onto the main board. The zeners were not causing a problem (yet) but added a "M vane" heatsink to each of the two zeners that caused problems in the other one. Some strips of shimstock bent to form an M shape and held by removing the diode and settling the centre of the M underneath and a snug fit around most of barrel with thermal compound and heat resistant glued to board with dimensions/position so that if it should move then would not cause shorting problems. Also raised the 2 droppers off the board with varelco pins. The 2 paralleled speakers measured 3.4 ohm DC. Did the following with all 4 of the largest Rs and perhaps they will survive better than the original coppery leads failing. I'm assuming the oscillation mode is transverse to the resistor axis rather than axially rocking. If laid against the pcb then the ceramic "feet" act as leverage when rocking and if elevated off the board then extra momentum also amplify the rupture force at the point where the leads go through the board. This is my latest attempt. I have hundreds of these hermaphroditic Varelco, Elco, Edac (DERA company, over-shelflife disposal) like in pic (in the bag ) I could find no other pics on the net. The flat part is the mating part so they can mate like 2 fingers of one hand rotated 90 degrees and mating twith 2 fingers of your other hand, so no male/female specificity. A pair of these gold plated brass contacts with one of the 2 fingers cut off with snips . Matching side with side so when crimped and soldered to the resistor leads , facing opposite directions, they are, locked against rocking, against the board. It will still be a weak point at or in the pcb hole but will they be more resistant to fracture than the usual resistor leads metal ? On opening this amp there was a smell of hoese manure. Compare with JBL amp on the other repair file. Different company , different function of amp , but both made in the USA. Trying to investigate this with the help of an organic chemist it may be Hexanoic acid or Caproic acid but is it a biological breakdown, age deterioration, overheating or burning of shellac or phenolic insulation. Anyone any input on the chemistry or any othe rexamples ? General Instrument AY-1-5050P replacement Pinning in 1982 Digital D.A.T.A and similar logic types for organ use MM5554,MM5555 MM5556,MM5559,MM5823,MM5824,MM5832,MM5833,MM5837, MM5871,MM5891 in National 1977 MOS/LSI Geni Electronics RL3 075A Disco Strobe No flashing but with no room light could see a glow in the connector of the zenon bulb crackling and burning smell and smoke. Connector made of bakelite looking like a Bulgin connector. Sequence probably poor contact between socket and pins leading to arcing between the 2 pins carying the 400 Vand heating leading to carbonising of the bakelite.between these 2 pins. Removed bulb and powered up and no arcing. With a small ball mill and dremmel milled out area of charring leaving an air gap hole thru the bakelite between the 400V pins. This problem may have been long-term the problem was no oscillator function. All 4 outputs of the 4093 were stuck high. For bench testing power up with 11V across the 12V zener diode. Hacker AL 42 (second channel to a GP 42 ) 1968 Used as a guitar practise amp. Severe buzz 10 minutes after switch on. Badly worn track on the vol pot. Dismantled and renovated. Hartke HA1200 Broken input socket soldering due to tripped over lead. Had to reinforce the tracks on the pcb but as another one missed decided to replace the CH2 socket with an upside down chassis one wired in, just in case . Held needle to pin 3 of the 4025 injected signal but not at the jack input. Remove 4 outer bolts inside heatsink and 2 on pcb to remove rearwards from casing. Uses 3x 2SC5197, 3x 2SB688A (C/E "diode" checks 0.53V on all) 7815,7915,4x 2058, 2x 072, 2SA1659A, 2SC4370A 6x .47, 2x 6.8K, 270 thermistor 347R in circuit, NTC Vs on non signal connectors right to left 48,0,48 V ac 18.5,0,18.5 ac 0.24 over 6.8K 24.3, -19.3 ditto -19.3,-64 270 .55,-.35 one 10R -24.1,-24.3V Dummy load test of 400 Hz giving 8V ac in 8 ohms Stabilised at plus 23 C over ambient in 25 minutes , thermometer resting over fins, 8V ac dropped to 7.8V To remove or replace the amp without dislodging the captive nuts or tearing the metalised card screen it is necessary to use 2 steel foot rules or similar and release the 2 top corner protectors or hack back. And pierced gaffer tape over each of the captive nuts as belt and braces. Hartke HA3500 , 350W, 2004 Not being overdriven just failed in mid song and replacement mains fuses keps blowing. Mains transformer 2.5R primary and 2x 0.6R main power secondary. The 100n , 200V polyester cap had failed to dead short, replaced with 630V one as plenty of space with a bit of rewiring into the vacant second bridge area. The only marking is 104J 200N no mention of ac or dc so could be 200V dc, maximum dimensions about 8x10x4 mm. I expected the problem to be near the pins which are about half way along the foils, staggered for the pin spacing. Problem was buried in the assembly about 3/4 into the centre, a number of sputtered/fused layers but no smoke emanation. There was localised micro ruffling of the foil in that area whether cause or effect of localised heating , I don't know, but the ruffling extended the width of the foil, not just the small "spot weld" area. 3.15A mains fuse ok on this amp, not torroid. Thermal sw n/o. uses 2068, 5532, 4558, SSM2018, M5227, 072 7025 valve (don't know where the HT is derrived) TA731? (obscured), as in 4x complement side of 4x 2SC5200, 2SC4370A, 150R,15K , 10R PS uses 22, 3x 10, 56R 125V ac over main bridge, +/- 84V other ac secondaries 6.7V, 11, 24,0,24 Load test with no top cover so no fan cooling ( directs inward). 4.5V ac over 4 ohm , with thermometer laid over the bodies of the top power transistors. Took 35 mins to stabilise at 65 deg C over ambient down only 5mV or so HH ( H & H ) Echo unit (Tape loop), 1970 No echo Two of the 1970 741s had at least one I/P at rail level HH Electric cabs The fuse holders use a cap/closure that is larger diameter 10.8mm and coarser thread than usual 20mm fuse caps HH L50 50 series, 1986 During break in light use , internal fuses blew. Also ch1/ch2 select is fine with added footswitch but temperamental on its own, due to dirty contacts on the 1/4 inch socket. With speaker (3.8R) soldered in about 5 and 8 ohm across the main DC rail caps. Both TIP132 and TIP137 power darlingtons were low ohmic "C/E" Replaced with added heatsinks as no venting and signs of long term overheating, cream and dusty thermal grease under the transistors and slight local pcb discoloration. Added 1 TO3 type heatsink on the mounting bolt of each tranny. 1 3/4 x 1 3/4 x 1 inch , 20 vane type , projected 1/4 inch out the rear of cabinet. 2 more retaining bolts and bent middle sets of vanes to touch for more robustness as well. Don't know what previous load test results would be but this for thermometer over the 2 heatsinks and inbuilt speaker. 80 Hz producing 0.1V ac on line out and 3.86V ac over speaker, cabinet surrounded in wadding. Stabilised at 16 deg C over ambient in 15 minutes and dropped 0.05V ac. Uses 150R, 2 x <>0.3R, RCA 1C03-C, RC4136, 2x 4558, 2x330R Without speaker "C/E" measured "diode" test .5 and .47V "C/E" on TIPs Still no speaker and no signal, 24V ac on each fuse +/-33V on main interconnect lead. 1C03 measured -39,-3.9,-31.6V HH 100W monitor, 1975 Excessive hum and reported , but not induced, low level noise problem after an hour. Remove the 2 machine screws at rear panel and 2 on the side of the control panel to remove the electronics. To get to speaker , presumably prize off the front grill frame. Hum due to lack of screening between toroid and preamp. Just placing a hand in the gap would significantly reduce the hum. Even just a finger in line between core centre and IC1 would make a noticeable reduction. Fixed a mu-metal screen between which helped but had to screw an aluminiul plate over and down one side of the Transformer / insulated from the back plate, to near enough cancel the hum. FT5415 seemed too hot even with heatsink and replaced with 2N5415. Reduced the I/P resistor on high I/P from 330K down to 30K. Voltages at ZD1 and 2, +-16V Uses 2 x .33 , .1 , 15 ohm wirewounds 2 x 2N3773, 40872, 40871, FT5415. HH MA100S 5 channel ,stage amp, 1975 On penultimate use distortion at high volume. Then last use, distortion like power-down distortion, at any level and tinny, no bass sound. The TIP29C diode test Vbe and Vbc was about .8V and discoloured printing . The TIP30C was about .55V so replaced both with TIP41C and 42C. Other Ts 2x 2N3773HG,BC204,BC207,2N3440,FT5415 and 741CS op amp. Power rails 46,-46V Repaired handle a la tips files. No bass was pot corrosion problem. To gain access to that area remove knobs and perspex. Desolder the nasty phosphorescent panel powered directly off the mains. Amp can be set for 110V mains but this panel needs at least 200V to light up it seems. Undo the pre-amp retaining nuts and then there is enough space when this board is propped up to pull each pot out to rennovate. Mark each pot 1 to 17 as rather vague wiring loom. Considered replacing the luminescent panel that was flaked and partially working. Could try computer case type electoluminescent panel or even EL string looped around each pot shaft. The best solution would probably be a pair of cold cathode tubes and inverter off 12V dropped from main supply. HH MXA 100, 1986 Reported as master vol control pot broken as intermittent breaking of output on touching Actually corroded/grimy bypass switch on the send o/p 33,33V ac , to +-44V dc uses 4558,071,RC4136, 2x 3055, RCA 1C03 2 off, 1Co4 2x 690 ohm measured in circuit 2x <>.1R, 2x 10R, 150R HH Performer amp ,150W, 1980 The effects plug-in of delay and phaser not working. The -15V rail missing due to an overhheated mess at the ps and a blown resistor. Replaced each of the 2 2W, 30V zeners with 5 seriesed 6V, 1W and the much larger 330R W/W. Can't be sure the other zener droppers were 3.3K as charred MO 1/2W but replaced with much bigger 3.3K. The pedal din connector needed replacing , beware of the bridge internal connection touching the din closure screw or the function plays up. Beware exposed springline is behind ch1. To gain access remove front 4 and rear 6 screws. Remove front panel and rotate to pass back through the casing and remove both front and back. Hold , for working on, with 4 tapped rods no more than 9 inches long so one panel can be reversed to work on. +/-58V, +/-30V rails On the plug in uses 3x HH badged CC100D (bucket brigade ?) 2741 M7815 and 7915C and Vactec VTL5C LED/LDR optocoupler. 2 bulbs were 16 or 18V ,.4A main boards use 2 bulbs 24V,1W Large Rs 2 x 330, .1, 2 x 330, 180 with diode 2R2 and 10R. ch2 620, 680 and ch1 1.5K MosFet HH HFN and HFP probably replaceable with 2SK135 and 2SJ50 2N5415s, MPSA42 and 2741 on ps Hohner Stereo 50, 1978/1988? Loss of external channel Uses 2 pairs of TDA2030. Bad solder associated with one pair of 2030. Added a switched 1/4 inch socket in the line of the integral speaker so both speakers could be used powered from another amp , as insurance. Hughes & Kettner Attax 80, 1993 Worn pots, long bushes , so reconditioned existing ones. Elbow speaker plug needed attention (see tips files) Uses 072,082,081,1538,4053,NE5532, MJ4032, $035 Rs 2x680, 2x 1k2, R15, 2x R.22 ? , 180,470 Jem Fogger smoke generator Failure to start warmup cycle Break in the lead between body of m/c and hand remote,this lead was 6 core ,5 only used ,so paralleled the sixth to the errant wire. JSH Encore guitar Physically broken vol pot and because no anti-rotation nibs on any of the pots they can rotate and have pulled and broken all the wires except a very twisted ground wire. Replaced the grounding wires between each pot with some rigid copper strip to stop chance of rotation as bush nuts are only tightened against the fascia, triple lamina plastic sheet ,used for sign making with a pantograph engraver, i forget the trade name. Unless they're (Wind/Reverse Polarity )=RW/RP to reduce hum, which is pretty standard for most Strat pickup sets these days. Hold the magnets up to each other to check the polarity (top of one PU to top of the other PU). If they're RW/RP, then the two that repell eachother go in the bridge and neck positions, and the one that pulls towards the other two goes in the center position. If all the pickups repell all the other pickups, then they're not RW/RP. Electrically all 3 measure the same about 5.03 K ohm and 3.26 Henries each. All 3 pots are 500K and one 47n cap in the filters section to ground and common of both tone pots. Output about 400mV pk-pk, one string. Resistance check for single pu , turning vol , from 0, to mid-500K, to 5K. The problem was finding a short shafted pot of 500K, the one I found to replace had a central detent which I cut off. Switch positions 1 pu farthest from neck, 2 mid pu, 3 neck end pu 4+5 o/p, 6 n/c,7 to lower tone pot wiper, 8 to upper tone pot wiper Only one or two large coils in each pickup. The exception being the hex pickups used in guitars that drive synths or midi controllers. In those, there are 12 coils one humbucking pair for each string. Typical pickups are either "single coil" or humbucking. Single coils are simply a few thousand turns of 42ish AWG guage wire around the magnet structure, often individual pole magnets or ferrous poles with a magnet on the bottom. The humbucker is made up of two coils counter wound and magnetically opposite built into one body and wired in series. A pair of opposite-wound pickups would tend to cancel a magnetic hum field in the transverse sense, parallel to the neck, as well. Kay Sound Fashion Gold State Amp Totally dead amplifier Severe corrosion on the two board mounted 20mm fuse-holders leading to break of contact.Replaced with new 20mm chassis type holders. This amp is the same as Sound City SC30,2N4033 is an adequate replacement for the SF118 and BFY51 for SF128. Kay 50L, Kay Sound Fashion,1983 2 KD606 power trannie DC voltages 42,20.9,21.3: 20.9,0,0.33 Kef Kefkube 100 equaliser PS input marked as 23V AC but needs 23-0-23 V ac as LM317 and LM337 are set for +20V,-20V Korg PSS 50 programmable stereo sequencer Failure to latch change of pitch selected on the keyboard and also 2 adjascent keys giving the same pitch change. Battery corrosion in the battery compartment had caused corrosive vapour to attack the PCB tracks and 3 of the front pannel click switches so even the 2 internal through connections in the switches were corrupted.The click switches have an unusual length of stem so a matter of cutting the 4 retaining pips on the old switches and the replacement switches and swapping over stems and gluing,also beware of turning over the top panel with the top PCB unscrewed as all the plastic switch inserts are not captive and will fall out.Also an intermittant open circuit 1N4148 type diode on the switch pannel was causing malfunction and cross-coupling between the number switch pannel and the pitch switch pannel. Kustom KGA10FX practise amp Intermittant loss of all including power on LED Loose spade connectors and poor solder on a W/W Transformer marked 22.8V ac, 250V/5A/130 degree C thermal fuse , measured 2x 11.7V , no signal Uses 7805, 3x 4558, PTC PT2399, TDA2040, 0.1R. V on 330Rs 14.7, 10.2 and -10.3,-14.8 1R 5V, 47R 13.7,14.7V Kustom KPM6160A mixer amp from 1997 intermittant crackles and sometimes loss of sound, waggling the interboard ribbon leads produces these symptoms. The .156in spacing Harwin/Molex/ Amp/Tyco connectors for power are fine , 2 sets of crimp pairs , one crimped around the conductors and one crimped around the sleeving. But the .1 ins spacing ones have single crimp pairs which are crimped around unstripped insulation and relying on the returned ends piercing through the insulation into the conductors. I pulled the wires out, very easily and cleanly, cut the ends off , solder blobs on the pins and the recut wires and soldered together. Some "hot-melt string" melted over the joins for some mechanical bonding to the shroud. I looked closer to the crimps and the returned ends are curled over where they pierced the insulation. The interconnect in the mixer section , .1 ins spacing was properly crimped, so just the ones between mixer and ps and ps to amp were bodged. Instead of 2 sets of crimp points - one for the sleeving and one for the conductors. These ones had one pair and instead of stripping back the insulation and crimping to the wires, they had not stripped back and crimped hoping the returned ends of the crimp tangs would make contact with the strands. Often seen with those small crocodile connector, jump leads, that you get in packs of 10 , 2 of each colour. I long since learnt with them is, when you buy them, the first thing to do is you pull back each croc and solder the wire to the croc. The heatsink insulating pads had shrunk, probably excessive heat, leaving a crumpling on the uncompressed areas, but all in working order as a pa. Replaced with mica washers. Previously took 50 minutes to stabilise at 33 deg C above ambient pumping 9Vac of 400Hz continuous sine into 4 ohms. Dropping by .35V ac over 30 minutes due to temp sensor presumably. With mica replacements, down to 30 minutes and you could keep your fingers on the body of the trannies, too hot previously. The pads may have chemically failed , but even then , mica does not degrade over 10 years. One of the 12V regulators had been broken/dealt with by a previous repairer , all 3 need protecting as very exposed. Uses TA7317P and NE5532 1.5K, 2x 220 on the ps 2SA1943, 2SC5200 x 2 , LM35DZ temp sensor (LN33 on the overlay) 2SA1837, 2SC4793 (All o/p devices 110 ohm B-E measurement) 2x double.22, 2x 10, 2x 47K , 10K, 2x 100 on pa R21,R22 = 10R overheating marginally , should use 1W Belton BSN2EB2E1T springline (210 and 62 ohm) Main power ps connector -48,48,0,.24 (it was 24 deg C room temp), 0,0 16,-15V to mixer Laney CD850S, 2002 2x 200W mixer amp Problem in protection (falsely triggering occassionally), both amps dropping out. Of course I have not been able to induce activation. Remove 4 feet screws and 3 screws from each side to remove the full metal case interior. 2 separate power amps otherwise apparently the same , both carying output DC protect and overheat protect output relay drive ccts. But only one normally open thermal switch going to one amp only so only dropping out that relay and amp only with no interconnect to the other amp. More likely a problem with the thermal sw or 12V supply to both protection cct and the fan , rather than a fan problem as it did occur once at sound check and once at switch on. The fan does stop when this happens so unlikely a send/return bypass sw problem, the monitor outputs still continue to work so not a mains failure problem. There is also a thin low current , signal level earth wire from the ps that had come adrift loose in the case , broken at some pcb solder point presumably, somewhere indeterminate from the schema. Yet again bits of hardware, 2 bolts that hold the heatsink to chassis loose and wandering around inside the case, but probably just incidental to the main problem. I checked the switch in a can of water and it goes s/c at 86 degrees C and reverts to normaly closed at 71 deg C and no amount of knocking will disrupt it. Logo - C followed by 2 square wave pulses KSD301 K85 probably (smudged) So the 85 means 85 deg C and perhaps K means NC Bit low for a 2x 200W amp even if fan assisted cooling but as nc operation means its immune to false triggering from contact corrosion I will leave that as is. The unusual yellow items that look like disc ceramic caps in the ps are fuses. I added a 90 deg n/o thermal sw to the empty socket of the other channel bolted to the underside of the heatsink. Just a signal diode connecting the existing thermal sw to the same pin on the other amp , empty socket, would have coupled both together but I added a higher temp switch instead to allow half-cock continued but still protected use on one amp should the other fail. The rest of the protection is to protect from DC at the output, functional in both amps. I accidently activated this by powering up with one of the wires from the main bolt down bridge rectifier disconnected. The stray low current wire would seem to bridge back one side of the ps to the other , on the same board, when they are both connected to main reservoir cap common via large traces anyway - there was a lone socket pin in the housing with matching frayed wire taking power to one amp , the other end was with the power connector for the other amp. I also replaced the 12V fan, with a 0.1A ( original unnecessarily overworked one 0.2A) as worn and wobbly and more to the point, fully cut the casing open to give 5 sq ins vent area at the fan vent and covered with a wire grill. There was just 14 small slots with a combined area of only 1.5 sq inches, let alone vortexing, so overloading the 3 inch diameter fan with through ducted area of about 6 sq ins and so losing cooling efficiency. Replaced the 12V regulator just in case. For the protect ccts, when in protect mode the voltge across the base resistor of the relay driver pair was 0.02V and .64V at the base and 0.76 V on the 12K resistor In normal operation 3.6V across base resistor, 1.26V on base . Long term problem in one amp leaving one Re o/c so on that power tranny the B-E "resistance" was 109 ohm instead of 94 ohm measurement on all the other 11 trannies. Running lower amp (right) only and 8V , 400Hz (16W ) into 4 ohm load with thermometer on the heatsink. Stabilised to plus 17 deg C over anmmbient (with this .1amp fan) after 20 minutes and .02V ac drop in output. Star washers and high temp glue on all internal bolts and hotmelt glue on the connectors, especially the small ones associated with fan and protect ccts as little friction keeps them in. Laney CK165 , probably 2003 combo Buzz reported as pitched at a G an octave and a fourth below middle so 100Hz. Both ps caps had oscillated to the point of breaking the glue bonds. One had broken the trace and the other had an internal fault. As could not find a sizewise replacement and as very cramped anyway plus would have to tie them together to (maybe) avoid a repeat. Replaced each with a pair of 10,000uF, 35V with 12K balancing Rs, bolted to the chassis between prea and pa. Uses SAP15 PO and NO, +/-49V 2x 560R, 3x 3.3K, 2x 10R, 2x 0.22R ribbon 15,-15,0,sig,0,49V No name elbow speaker 1/4 jack had a loose barrel. Undo th e2 screws , find a large nut that the tip will sit in comfortably and squash , firmly held from slipping, between tip and the central connection to squash the barrel back to the flange. Laney LC15R, 2002 Captive nut loose in the chassis blowing fuses etc Blew one of the HT diodes and mains fuse Some DVM resistance readings speaker 5.7R mains primary , 240V, 52.5R secondary 206/19/19/188R heater secondary 0.2R without valves o/p transformer 104, 114R/ 0.5R Laney R4, 2003 Bass Amp combo, 32 Kg, distorted breaking/muffled output, the distinctive sound of complementary output failure of one waveform component, the positive or the negative going one. First thing to say is anyone finding one of these, put some fine mesh under the top grill which lies directly over the power amp. Kids like posting coins into slots, even TVs of 40 years ago had such mesh under the ventilation slots. Perhaps block off totally and add a vent or 2 at the sides as there is no sides or top to the amp chassis. A small piece of copper wire had dropped in this one touching a couple of MPSA92, and half blown the amp. Uses 2 pairs of 5 pin SAP15NY and SAP15PY output devices according to Sanken datasheet , in English, h1-o03ea0-TR.pdf 150W, 15A, 160V bipolar with temp compensation. There is another major flaw with the production of this amp. The 4, 0.22 ohm ,emitter resistors are on end and one end soldered right down to the board. Not failed soldering so 5W must be the right rating but vibration is causing the resistor lead to progressively fail at the juncture of the component side of the pcb and the flaring , over the lead, of the ceramic covering of the resistor. Replaced each with a pair of 0.1 ohm with a pair of ceramic beads flat side against PCB and temperature resistant glue holding in place. As well as a mesh under the grill , cut an aluminium sheet to form a sloping cover over the amp (so you can tilt up the whole amp to slide in and out of the wooden housing). Extended the 2 long stand-offs , leaving a 38mm gap to the aluminium , and made brackets to fix to the heatsink screws. Covered the leads to the thermal switch. All nuts and bolts glued in place. Laney R4, 2002 Came in for replacing one of the input jack sockets. Used a chassis mount,gold plated, wired tag one upside down and cross-wired in. 2 of the output emitter resistors were neatly loose, one for the N and one for the P sections so output halved if he'd realised it. Used 4 pairs of .1R and soldered eyelets to beef up the standoffs and plaited some silicone sleeving to make a rope to tie all the twisted lead joined Rs together tied down to the board. Laney R4 , 2003 No output Failure of 2 transverse set emitter Rs so one pair of devices still operational, not blown due to overloading. Bad solder on one of the main dropper Rs Laney RB6, 2005 or 2006 Farty noises, reported but not found on arrival or induced. Poor central pin wiper contact inside the "enhance" pot so intermittantly no ground connection. Due to the bent pair of wipers not making good contact with the conductive track. Uses prea ,CA3080 , 5x 072 (SMD)m 2x 680R PA, 2x 10R, 2x 0.22, 2 x SAP16 3 standoff + nuts on the pa board. Beware of misplaced LEDs before refitting. Pull the amp casing rearwards to give enough room to disconnect the speaker connector. Luxtrack Tootsy 2 lighting sequencer No function after being dropped. The mains transformer for the logic is pcb mount type so shock loading had snapped one of the primary wires near the mounting pin.This equipment failed earth-bond safety test. Only the top panel is bonded to mains earth and as it is black anodised (hence insulated)and other panels are not separately bonded so added bonding. Also safety-wise in use the angle Ali heatsink is not firmly held and could flop into touching the chassis. Cover this area of chassis with suitable insulating sheet material. It is too easy just bending the Ali heatsink to get at the drivers area - don't as the pins of the triacs will fracture with age hardening / vibration. Undo each screw mount to remove h/s. For a tighter job remove each triac and push thru pcb pins soldered into place and solder lengths of triac legs to these pins. Mackie CR1604-VLZ 16 channel mixer , 1996 No tape insert inputs and had been used on 240V . Tied the IEC line socket to the mixer IEC chassis plug with a label with large letters saying 120V, The other end of that lead is US plug for the 240V to 120V auto transformer. Unusually, setup for rack mounting so the "back-plate" has upside down and reversed legends.
 Mackie CR1604-VLZ
Back plate has loads of active on it SM 4560 and trannies, the 20 way ribbon probably relates to the tape/in + others. The steel plate is held by 2 torx screws from the inside so you have to remove all the bush-nuts and XLR screws. ps has 2 x 5.6R, 2 x TIP29C, 317 and 337 set for +-15V via 130R. ps outs -15,0,15,-15,0,15,47 and floating 18V on the spade connectors presumably dropping to 12V on load for the lamp. One +-15 for the 4560s and one for the 2601 bar-graph, zener has 48V across it. Replaced the 317 as had overheated. Problem with the 20 way ribbon cable. Once again a piece of expensive kit that does not have shrouded indirect "edge connector" headers so easy to place the socket out of register with the header, especially one line connected to wrong line and one line unconnected. Ribbon leads' red is not a consistent identifier, Red on 40 ways is pin 40,red on 20 and 26 way is pin 1 , where marked on board. pins 18 and 20 of 20 way via 120 ohm to Main Outs 24 ,26 on 26 way via 120R to CR Outs 13,16 on 20 way to main fader top points Middle 40 way +15V on 38,39 +37 40 Way nearest Tape in/outs -15V on 2,4,3 remaining odds tend to be grounds Uses approx 15 off 4560 dual op-amps on rear panel and about 75 on main board plus 2 SIL 4560 for headphone amp and 8 2901 quad comparators for bar graph. The 8 pole 2 way relay marked on the user manual block diagram for SOLO LEDs and signal routing is probably numerous 4560 as gates. Using phones socket outside of chassis needs earth connection. Push RL buttons on each channel to pass feed through to main mixer. To reassemble rear section. Fix the pcb to the steel plate and then connect that to the rear plate. Feed cables through the plate with the long slot and screw through the ali of the ps into the fixing boss on the middle of that plate. Then introduce the 2 parts together and screw the power entry plate to the backplate only at that stage. Machine screws for the Al to Al casing and tapered screws for the XLR sockets. Mackie SRM450, year 2000 Horn output but no bass output. Had intermittently broken output and then one time of powering up, nothing Acces the bass speaker through the front grill not by separating the main body halves. Cutting away the cone the break was at the point where the round form of the voice coil was joined/ deformed to make the flat ribbon to go axial to join up with the braids. Looks like a design flaw the copper wire is pressed to form the axial tails so a stress point there. Also the coarse grill on the magnet end would allow degrading foam particles to pump in and out, really needs a pair of large hemisphere grills one inside the other with fine filter between properly. VC dimensions of the Mackie M1263W 2x 90 turns covering 15mm length, 63.5mm ID of core, thickness with 2x coilwire , 0.9mm Device listings in order power IRFP9140, B817,B940a,C1567 D1264a,D1047,IRFP150N 677F070,B817,B940A,C1567, D1264A,D1047,MJE295,MJE3055T signal c3788,A1478,JE340,JE350 loads of 4560, 1x 4566, LM339M,1x 2068 2x .22, 2x .12, 2x 2.2, 2x20, 2x .22 Mackie SRM450, year 2004 No abuse but failure of horn tweater First problem how to part the 2 main sections of the cab to get to the horn. P/N 0008093 Heated screwdriver as per tips file Drilled out the screw holes a bit for easier assembly/ next disassembly , (will add star washers under heads) Caproic smell came off - what do Americans use as glue or filler for construction plastic ? I only smell it with USA equipment. These Mackie screws, 17 of them, (2 slightly shorter either side of the bass driver) , in my opinion , are bordering on the threshold of shearing without such heating, they deliberately used undersized holes probably. So the second of 2 Mackie SRM450 to have mechanically broken tail at the voice coil. 25 turns of about 0.07 mm silver wire over about 1.75 mm . Previous , different unit the one above in this file, bass driver coil broken where the coil wire is deformed to flat for the tail. Not the slightest trace of overheating on that one. This one, horn , nearly at this tail juncture , the break marked B below, half turn vroken away from the resin core, and again so trace of overheating anywhere. 4 engineering bolts hold the diaphragm to the magnet but it is set in plastic frame, although engineering plastic, presumably can deform enough to catch something. As there is a milled out recess R already there I intend trying to take the + and - tails through the same side, not diametric. Grinding a slot in the aluminium ring to take some 3x3 plaited 46 swg wire as tail. Replacement driver 130 USD , diaphragm 70 USD so worth trying as presumably swap USD for GBP and add some for here. Very little diaphragm movement so should not foul anything. Cone thing presumably just stuck to the metal, fell away on removing the horn part. So it may be an effect of work hardening , relative increase in the effect of imperfections/micro fractures or some other metallurgical effect. Mackie speaker voice coil failures due to this flattening/ribboning process to make the tails to the outside world. Previous failure at the juncture of round to flat (0.07mm round to about 0.02 x 0.2mm) so at the peak stress point. This one along the length of the ribbon section, but the whole 50mm or so run was brittleised and disintegrated on touch, not the slightest sign of overheating on the remaining 25 turns of round wire. broken end marked B on this pic Cannot expore the metallurgy as that curve of "wire" as totally disintegrated to dust. Thought I could solder to remnant and lead in and lead out on the same point, not diametric opposite. But required grinding a slot in the aluminium disc. Doing it in situ with the burr action the wrong way and scudded into the voice coil. Had to replace with an APT horn 85W (6.2R), as much cheaper than Mackie and those are proven suspect anyway. Mounted the APT80 bodged with spacer and washers etc as holes don't line up with Mackie horn section. Added 1 12V, 21W bulb in series as a bit under-rated Threadlock on the horn bolts so heat well before undoing. Bass speaker , yellow leads / horn blue. Tx 5.1R // 0.5,0.5 / 1.0,1.0R 2x 0.22R, 2x 20R, 2x .22 5W 2 x .22 , 2x 2.2K Manacor miniature 8 channel mixer. No output on either output There is no reverse 9V protection so it is easy to change battery with unit on ,touching battery terminals the wrong way and knocking out both LA3210 SIL op-amps so replaced also add 1N4001 in supply line to on-off switch. Marshall AVT 150H, 2002 both tda7293 blown (neg to 0) due to o/p connected to another amp o/p . blown mains fuse mains primary 6R, sec (Yellow) 1.3R, (blue) .8R 2x 2R2, 2x 68R (in circuit), 2x22K .22, 2x 10R, 7805, NJM072, 4056, 2x 4011 one 68R solder joint soon to be iffy +/-49V, +/-24V, +/-15V 24V fans, 145V at D112, ECC83 A1,A2 81V Dummy load test with both fans disconnected and aluminium "heatsink" unbolted from chassis and thermometer laying on top with alongside the TDAs 3mm Al 270x 50mm plus about 270x100 mm under of the rest of the folded sheet. 3V ac into 4R, 400Hz, 20 minutes to reach stability of 45 deg C over ambient and drop to 2.95 V ac Marshall AVT 275 Replacement and befing up of input socket Added filter cloth , glued into top vent, to stop any small biits of wire, staples falling directly into the amp The overdriven / bad bearing noise on the fans is usual for these. Marshall AVT 2000, year 2005 Yanked guitar lead and weakened socket, replaced and beefed up as per tips. Removed the top plastic grill and glued in some pan scourer filter cloth as I don't like being able to see main power devices (staples etc ingress) .22 10W, .22 5W, 2 x 10R, 2x 22K 1W, R209, 211 4.7K 4x 3K3 speakers 6.4R CCOP5, 7915, primary 5.8R, secondaries 2x .3, 2x .5R Marshall 1987X from 2006 and probably other recent models. Traditional chassis mounted pots ie with loose wiring, not loomed, not pcb mount ones. To save cutting small holes in the front fascia, to take the pot ant-rotation lugs, they've cut them off and then not even secured the pots with star washers under the bush nuts or fascia. So even in careful useage the nuts work loose in a couple of years which is fine inside an electric quitar , just a rat's nest of knotted signal wires, no elf 'n safety issue there. But not for valve amps - needs a modification to stop each of the pots rotating and then bare connections touching. The 1mm tinned copper wire earth strap soldered to and across each pot is not enough to stop, wire breaking and the pots rotating. If in an electric guitar I replace the wire with flat copper strip, soldered to each pot case, the sort of enamelled high current wire used in some power transformers, with the enamelling stripped off. In these sorts of cases, not wishing to add to any electrical hazard, 1/8 plastic rod , bridging the back of each pot, and then paired 1/8 inch cable ties around each pot and the rod. Make sure the tie is lodged overe the aluminium fixing points and not the paxolin. These amps have a printed plastic finnish covering over the fascia metal. With a thread lock chemical approach the plastic can still compress under the bush nuts ,microscopically, plus metal creep and soon the pots will turn if the knob is rotated to either endstop. The anti-rotation lugs are there for chassis mount ring tagged and wired pots precisely to stop any chance of rotation - up until the point is reached when the the pot has fallen off , that is. If the wiring was loomed and laced tight it would be a reasonably satisfactory alternative . "Repair" job, that awkward type of one where no fault found. I can understand lager chillers or lighting triacs causing clicks and bangs and noises off but blowing fuses on 2 Marshall valve amps at the same time and no other pub mains problem showing itself. This 1987X kept blowing 500mA HT fuses after the initial failure. Not doing so now its in front of me. Mains fuse always ok , just the HT one repeatedly failing after 4 seconds or so. At the same time as this HT one failed the mains fuse failed on another Marshall connected to the same supply - that one just required a change of mains fuse and is still ok. Could a fault causing failure of HT fuse on one amp cause the failure of mains fuse on another amp , with no pub wiring problem at all, even problem local to the power ring/spur to the stage. Owner didn't notice anything else wrong in the room at the gig and confronting the landlord he said he'd had no problems. Checked the valves and transformers , caps etc and everything seems hunky dory , no wavering of HT of about 440V on dummy load. I looks as though it will have to be replacement 500V, twin 50uF electrolytics although no suspicious heating or bulges /weeping, unless an indisputable reason can be found. Fuses blew to black interior, ie not soft blown. SS diodes in this one. Amp is in pristine condition and owner never goes above 3 . 5 off 500mA (T) fuses blew black before but no problem while here According to Regulation 27 of ESQC Regs 2002, UK mains should be 230V +10% - 6%, which gives a range of 216V to 253V. I variaced up to 250V , switching on and off a few times, giving HT1 of 464 , nicely stable with amp under load. A thorough inspection of all components only revealed slightly loose nuts holding the iron laminations on the filter choke, which I cannot see as a problem. Otherwise inside and out it looks brand new, only some of the large binding post (lead-free of course everywhere ) solder joints looked grey but otherwise sound. On the assumption that the one of the 2x 22nF, 400V DC decouplers to the o/p valves could have failed and self-healed, I replaced those. Marshall 4140 valve amp, 1975 Been in a shed for years and no known history so treading cautiously. Measuring the DC resistance of each side to centre tap of this Marshall amp, shows only about 15 ohm each way for o/p matcher. Amp is 100W o/p using 4 EL34 , two paralleled anodes going to each side of this impedance matcher. Output resistance of about 2.5 and 4.5 for 4 and 8 ohm settings seems fine. I put a 1KHz AC LCR meter on the coils and it comes out to 32 Henries for each half. Amp is 1975 , from electrolytics, and uses Si rectifiers, choke is (in circuit) 105 ohms DC. Charred/burnt 1.5K grid resistors and blown HT fuses. All valves ( all marked Marshall) checked out good on Avo CT160 - I'd forgotten how problematic , with high current valves, to get the initial zero on the meter before rotating the SET mA/V. I always power up kit left idle for a long term with a variac + current meter + thermal trip. Perceived wisdom in such circumstances to power up valve amps with full speaker load on output with all valves in place and to power up transistor amps intitially without load. On the slow variac power up, I usually power up to about 80 per cent mains with no valves in and then add the valves and then go low to 100 percent Replaced the burnt stuff and its back working. Unfortunately after about 10 minutes hum makes an appearance and after half an hour becomes excessive. I stuck a piece of 20 to 35 degree C thermochromic paper to each of the 4 can electrolytics and one is heating up. Its normal for each of the 4 EL34 to have a blue glow observable through some of the holes in the internal metal-work as well as normal orange heater glow. Replaced that dual cap and runs cool and no hum after 10 minutes. But after an hour a hum develops, intrusive but not as bad as before and at the same time a serious glow forms on the zinc coloured metalwork of one pair of the EL34s. All 4 EL34s tested good , otherwise just 2 off 1.5K resistors to the grids of each EL34, correct values cold. The push and the pull from the preamp are AC coupled, replaced all 4 22nF, 400V caps in that area. Not a problem with the DC blocking caps at final push-pull separator stage. Replaced and still hum after 3/4 hour. No DC on the ganged volume pots. Monitoring the negative bias for the output EL34s. 10 minutes in, the -ps voltage at smoothing cap is -52V. The voltage to the (schematic marked) B pair is -39.2V before going to 1.5K then g1 of each EL34 For C pair -39.2V also 25 minutes in B = -37.8, C=-34 45 minutes in B= -36.9 , C=-32.5 hum is getting quite noticable and -ps rail is still -52 Increasing bias pot from its original of 8K to max of 22K brings the -ps down to -53.5 now B=-39 and C=-33 less hum but still going more positive over time and hum increasing. I stopped before the B pair started glowing like before. I was expecting from these voltage readings that it would be the C pair that would start complaining. Switching off the amp, not just to standby, for a couple of minutes , brought things back to original cold situation and another 3/4 hour presumably before hum gets too much. One thing that concerns me is each of these 1.5K g1 resistors has one end to the valve base pin and the other floating in space , not soldered to an insulated pin just the wire connecting through Tip from "Arfa Daily" "However, before rushing out and buying new ones, you can start by removing both the C valves ( these are the ones that glow ultimately - yes ? ), then removing one of the ( likely ) OK B valves and putting it in the C side. The amp will run quite happily with just one valve in each side ( it's a trick that I teach owners to allow them to finish a gig if they have a serious valve failure ). You will then be running it with two valves that have performed OK when they were both in the B side, so if it now works ok, next put them into the two unoccupied sockets instead. If it still continues to work ok, then the chances are that it is a valve problem. If it doesn't, then it must be a bias issue. This assumes of course, that it's nothing to do with the output tranny, which could suffer a partial insulation breakdown, resulting in shorted turns, when it has been running a while. Again, this could be checked by swapping the winding ends betwen anode pairs, and seeing if the bad behaviour swaps sides." Swapped pairs of valves with much less hum noise and better sustained negative bias but assumed a new set of valves would be the answer or having to switch off for a couple of minutes each hour of use. Marshall 6101, 1992 combo, 29Kg Crackles and bangs except at low volume and tinging output with fingernail scraping across the turned metal covers of the knobs.. How to remove the digital board to get to the valve bases hidden under it. The Switchcraft XLR socket is introduced from the outside and soldered to the board with the track side of the board inaccessible. A cunning latch in the XLR socket that releases the internal part that is soldered separatable from the XLR housing. With a jeweller's screwdriver turn the slot at the end of the socket housing to release the latch. No need to undo the 2 mounting screws. Shame about the "dp" pots used as they were weel soldered in , double sided, requiring "soldering" first before desoldering. Poor swage contacts between pot tracks and pins. Removed and squashed in with parallel jaw pliers, treating all vol and gain pots but perhaps should treat all of them. 5.6K 5881 grid resistors were charred but functional as was large 10K on preamp board , replaced with same wattage replacements. As were the 47Rs in the +/-15V ps but replaced with 1W. So problems on all boards. Remove the exposed screw heads to release the boards , hopefully leaving the nylon parts held by the nylock nuts. One ECC83, second from non-transformer end replaced as C/H leakage of 4M, giving tinging. Number 4 was also on way out at 10M. Punched out 5mm diameter bits of cycle inner-tube rubber with a paper punch to pack out the each knob spline recess to raise away from the fascia plate. Speaker 5.4 ohm DC All LEDs lit - release the speaker/line switch No output - use outlet nearest mains inlet. The earth point at the XLR is required for normal function. No ch1 if conn1 lead is disconnected Dummy load test, set for 4 ohm, 2.5V ac 400 Hz cont. sine in load. Ali plate 3.25x7.25 ins resting over 2 large O rings on the 4 6L6 and thermometer on that. Temp stabilised at plus 46 deg C after 25 minutes. Cutting HT then drops to plus 15 deg C, over, on heaters only. Marshall 8100, 100V, 1994 No reverb, broken coil, 7.2m of 0.07mm wire, 31R and 200R for the other one. o/p coil is grounded only. Decided to slightly bend the brass retaining bracket to release the iron lamina but difficult sliding back in (very soft and thin iron) so drilling out rivets is probably best. Solder the tails to the phono sockets with hotmelt linking the sleeving to the tags for strength. Mains transformer 17.5R, .4R, .4R HT transf 5.6K, 21.5R Output T64,T65 darlingtons are 1.7V & 1.2V diode checks. Marshall AVT50 , 2007 No more than 2 years old and failed springing / closure of input socket and because of leadfree solder all sockets had poor soildering that would have failed in a couple more years and dropper Rs that would have failed sooner than that. Transformer disconnected, 20R// 0.6,0.6R 2x270, .22, 10R 0.5W, 270 7W HT monitor point on R44 nearest C17 Marshal DSL50 (JCM 2000) 2002 Came in as suspect low channel A throughput as compared to channel B, but this would seem to be normal. Valves tested fine, no switch problems or any other problems found. For equal output the channel B knobs set about half the number setting of channel A. To release valve pcb only, unclip yellow wire,release 6 screws around upper metalwork to release the long pcb standoff screws. R23 270R soldering bad on rear pannel board , supplies the ICs rails with its fellow 270R Marshall JCM 2000, 1998 ? (also see DSL50) Latching switch types ok for VCRs etc, with tight clearances but not here. 2 DPDT worn , leading to problems unless you hold the knob in or out. There is about 2mm of play between the knobs and the holes in the metal of the front/rear panels. Replace those 2 plus 2 more with same 3.5mm pin spacing switches but ALPS make. The 2 off 4PDT switches maybe more resilient to off axis loading so will leave as is , unless anyone knows better. yellow stem "angle with an F" logo, found 4 ALPS marked ones, blue stem replacements and some slightly larger diameter knobs. Required slightly opening out the "gold" fascia holes but not the underlying panel and unfortunately red in colour. Permanent black felt marker over them , cooked in with hot air and repeated again sorted out that aesthetic problem. The easy fudge would be peripheral cut a 5mm LED clip or 8mm with a section removed fitted in the gap , but enlarging the fascia hole a bit, and gluing-in the clips internally. The other problem is the black knobs seem to be pushed on the shafts, hot, rather than gummy glued on , so do not come off easily. To remove knobs squash diagonally in a flat jawed vice. They always seem to split , once, a disguised seam, looks more likely ? Definitely mark all cables and wires before disconnecting from the boards. springline 198R/25R 2x 4.7K,1K, 2x 10K, 2x 4.7K 0.5W, 2x 1K, 3x 270R, 3x 100, 2x 1R rear 100R, 1K mains tx , no valves p 6.5R // purp .1, /blu 1.5,1.5 / o,r 23 o/p R.O .2 / gn bk .4/w 79 pur 40 bk Marshall JTM "1962" from 1993 Working order but for bad pot, but some things are not pukkah. Reconned existing pots The main carying handles, presumably original as no other holes, are just held with woodscrews into thin carcass wood, splintered away internally, as no pilot holes drilled, - replace with nuts/washers and bolts? The screws that hold the main lifting handles have very jagged exposed head-slots as though the windy driver( or bad human) slipped on driving each one, so replace with bolts for that reason alone. Owner is in the habit of going around with switch cleaner for the jack sockets so I will beef up the contact closing force on all those. So did the rubber cord beefing up of all 7x 1/4 inch sockets as described in the tips files. Bad soldering on one wire of the input sockets Finally there is the ridiculous system of casters on these sort of Marshall cabs. It is quite possible for all 4 casters to end up pointing inwards so the slightest of tugs on a guitar lead will pull the cab over. A 9 inch deep cab is then relying on just a 4 inch wide wobbly footprint but still 26 inches high. Especially if used on a carpet. Jacking up the innermost edge of each caster. So the self weight should bias the casters to pointing outwards when at rest , for maximum stability. For normal in line carriage then the change of orientation , via the cant, would have to lift the cab and hopefully that self weight at rest would return the casters to outwards position. To turn inwards would mean itself, unassisted, increasing the height of the cab at that corner so unlikely. The canted casters work well. They are not the original , part recessed ones, presumbly busted off years ago and replaced with standard hardware store ones. Canted casters Propping up on one edge with doubled up rubber feet used for kit, so 4 to each caster. Uncompressed feet stand off height 13mm . Original gap between base of cab and floor of 69mm and still that with feet pointing outwards and 83mm if both pointing inwards. The action to get all 4 feet pointing outwards could not be easier. Run the cab on all 4 casters in the longways sense of the cab , go 4 inches further than required and pull back 4 inches. The transporting action is worse, in the wayward supermarket trolley sense, a mind of its own, so perhaps only one rubber foot per standoff rather than 2 or transport using 2 casters and one of the main lifting handles rather than the top movement handle. Another advantage is the casters then end up an inch or so , each side, further out for an even more secure "wheel base". Uses J174 fet, 2x 160R, 2x 10k, 2x 470R. Parallel 190 ohm speakers , 6R, DC. Dummy load test with 3.25x7.25 ins plate resting directly over the main bottle clamp rings then temp took 35 minutes to go 44 degrees C over ambient with 400 Hz, 4V ac into 8 ohm , dropping 0.5V ac over 35 min. Marshall MF350, Mode Four, 2003 Amp works on its own but no crunch channel via footswitches. Failure of "D" connector pin, fine for intermittant pc use but not repeated making/unmaking. 15 D pinning Bk common, Br OD1,-,R,O,Y,Gn,Bl Vi,Gy,W,-,-,Bk OD2, Bn Rev Hot melt over LED mounts and bare common switch wires near metal switch bodies 4 momentary and 2 latching, and bare joint floating in space. Replaced the solder bucket plug and swathed in hotmelt to stop any pins moving rearwards, and checked the amp one which is pcb mount. Remove top section of rear panel, then top front panel from inside, then 4 screws under and unstick the chassis. Loads of dust in the gap between each pair of heatsinks. Uses 4x tda7293, 3x 0.1R, green (I/C) 100R, 2x 10R, 2x 22K, 2x 2.2R near bridge rec. transformer 2.1R // .9,.9 / .2..2R all I/C 15,-15,5V regs power interconnects to prea 0,1.7,5,4.8,0 .1,.1,5,5,0 to valves -24,-24,15,0,0,-15 , 107V on link6 from stepup D array. Bent inwards the flanges of the interconnect cable D sockets for better screwdown. Marshall MG10CD, 2003 Intermittant cutting out - fuse holder problem Uses TDA2030 , TL072,BA4560 0.1 and 220 large Rs 26V ac , +-16V Marshall MG15CDR, 2004 Volume intermittently going to maximum Due to the bush nut for the input socket falling off any plug pushed in strained the pcb with result that ground side connection to the volume pot failed. Also a plastic cover over the springline suspension plate had fallen off and laying over the springs , needed putting back and held in place with cable ties wrapped around. Uses LM1875T , small w/w is 0.1R Marshall MG30DFX, 2004 Intermittently drop in volume. The spring clip had failed at a bend that retained the 7805 to the heatsink shared with the TDA2050. Replaced 7805 just in case. At some point the heatsnk had seriously overheated as the hot formulation hot-melt glue that bound adjascent cap to the board had remelted and spread under the heatsink. speaker 3.6 ohm dc Input switch weak so reinforced with silone cord and Hama. the line out socket has the ground line switched and this was dodgy from grime. With 400Hz in giving 2.5V ac into 4 ohm dummy then heatink gained and settled at plus 46 degrees C in 20 minutes and dropped 0.05V Marshall MG 50 DFX repeated loud bangs on switch on Only about 2 years old, solid state. Series of loud bangs , about 2 a second, for a variable number of seconds but about 10 or 20 seconds calms down to normal. Only at switch on from cold, off and on , minutes or even an hour later , there is no bangs. I originally thought it was due to broken input socket so the input not shorting with no guitar lead connected, was open circuit, meant the input was unterminated. Replaced the socket with a more robust one with functioning switches but because powering up quite soon after initial powering, with me, this symptom did not appear on soak testing. Returned it to owner and of course a few days later and they powering up and bang,bang..... It certainly sounds more like DC surges. Its annoying having to wait so long for electrolytes or whatever to reform, degrade or "plates" deform to be too close. With these amps its possible to disconnect all lines to the output decice and associated comps. After leaving overnight and powering up, this morning, monitoring the line out there was nothing wrong at power up, so preamp opamps probably OK. I Let the DC rails fall to <02V after switching off, Reconnected the output stage and switched on and bang/bang.... Suggests a problem at the output. Replace the small 47uF,63V secondary DC rail electrolytics on this output board and left a few hours before powering up again. I cannot see what chemical ? problem there could be inside the TDA7293 that would take hours to reform. One of those very enigmatic faults. The 3 caps are on main +- rails and the bootstrap facility Changed all 3 caps and now a continuous high level buzz for always , no normal function however long amp is switched off. Put a higher ohmage speaker on output for a bit of leeway. Monitoring the stand-by line there was high amplitude oscillation during the buzzing. Cutting that connection allowing standby control i/p to go low then muted the o/p, as should be. Taking it high via a 3V battery and normal service resumed but whether the original fault is still there plus some feedback low-f oscillating loop involving the stand-by control ? Ther's no oscillation on the downstream side of the cut standby connection. Put 47uF on the standby line and it gradually ramps up to about 6V which is presumably as it should but no oscilllation. Tracing the circuitry back down the standby line and hopefully find a duff cap there. It seems as though there was a very small soot jet on the pcb tracing back to the collector of the little MPSA06 buffering the switch-on hold-off of the standby control. Otherwise cold "diode" checked out ok. Uprated that and associated 6.8V zener and 50V, 220uF cap. Marshall MG 50DFX, 2004, 17Kg Nasty noises due to failure of input switches in 1/4 socket, lost spring action to close. Replaced with a metal barelled socket upside down, wired in. The earthing ring removed , hole punched larger and used on the replacement as a complex grounding scheme. Hot melted into place against the board after placement in the front panel, for anti-rotation reason. Uses TDA7293 with fan, 7815,7915,7805 resistors 10, 0.1, 2x4K7 Marshall TSL100 reported fault same as switch problem on JCM 2000 reported here Marshall VS65R, 1998 the last time it was used was miked to a house pa as very low level output. By the time I powered it up just some low level crackling whatever vol setting and no signal throughput. More a question of what solder points not o/c. one pin of the bridge rectifier, input 1/4 inch socket signal line, all the large W/W , 1 1W, one axial cap. Beefed up all the solder joints with added "washers" of perforated zinc sheet - If i'd some copper or brass wire mesh I'd have used that. Remade all similar component jpoints. Accutronics sprinline i/p 60R, o/p 220R 10 way connector, p10 = 8V 5 way , p4=-44V,p5 44V preamp rails +/- 15.6V T4 -43.4,-1.1,-44V Marshall "Valvestate" VS65R, 1998 Reported as a nasty farty noise. Yet again an amplifier that does not seem to know that it will spend its working life on top of a large speaker. One tiny drape of hot-melt supposed to stop the main ps caps waggling around - tied them together with a cable tie , separated by a large nylon nut and hot-melted into place and extra around bases. Beefed up the solder points on the caps and W/W Rs with wire and extra solder. Also the pre-amp caps treated similarly. The pcb mount blade connectors will pull out on removing the crimped wires to extact boards. AC transformer supplies 33,0,33 / 300V / 13.6V O/P devices badged Marshall T65 and its compliment T64A (curiously one suffixed A only), Probably T64 = BDV64C, T65 = BDV65C eg Part Number = BDV65C Description = Darlington, Power Darlington T65 NPN silicon darlington power transistor. Complementary epitaxial base transistors in monolithic darlington circuit for audio output stages and general amplifier and switching applications by Magnatec Inc TIP31C, MJF122 (something like an oil film, only under the central spine of the retaining clip that touched this Motorola device, don't know what that means, original oil on sprung clip ?) , 2x R33/5W + R33/7W and 2x 330 on preamp also uses ECC83, 5201A, LM348, TL071, 072BDE Marshall "Valvestate" VS65R, 1998 no reverb and then no audio speaker 6.5R broken wire on one tail of one coil 35R and 205R mains transformer secondaries 13,225,30,0,30 For checking pa , conn1 , pin 6 or 7 .1V signal uses 2x LM348, 74HC4316,,072, 4x 5201A mitsubishi, 4558,2x 180R +/-12V reg, 071 pa 10R, 1W, 1.5K, 5x 0.33R, 4.7R .3W, TIP31C Simulating a guitar pickup. Find a 48 volt coil relay. The one I use is 3.6K and 5.6H. If you've no tuning fork to twang close to it then a set of steel feeler gauges. Ranging from 6 to 10 thou for bass notes to 4 thou pinched close to the tip for high notes. Matrix SR 4000 Guitar tuner Nothing after being dropped. All or most LEDs come on and stay on until switched off. Replaced the 10M crystal. Matrix SR 4000 auto chromatic guitar tuner, 1993? Owner had connected an external 9V ps reversed and knocked out the 78L05 as no reverse protection. Replaced V Reg and cut trace to switch to put a small 150mA diode in line I was intrigued by another 3 pin TO92 device marked 8054 HN 3C31 Presumably working order - just wondered what it did. Probably 8054HN not 8054 datecode , by Seiko, activation voltage 3.8V to 4.2V, battery monitor. One line is connected to INVERSE READ of the 80C51 micro , I didn't trace any other lines. Seems odd having this sophistication to hold the micro from erroneous but not fatal operation but no diode protection against battery or ext ps reversal, especially in a product destined for musos. Mesa Boogie V Twin , 1997 This preamp must have been used in heavy rain, or something like , with the previous owner. Had to drill out one of the casing screws to get inside. Present owner has not had problems for the couple of years in his hands. Vibration of drilling seemed to cure the fault so replaced internals showing water staining Working order , after vibrating , and could not reintroduce the problem, but reported as a mains hum, no output signal , but functioning LEDs. Valves check out on tester fine, no change in any hum swapping them around. No problems on any switches, pots or sockets. Soldering looks fine, No noise on the HT line. Note insulated washer on power inlet socket and beware of bending the red LED on refitting of case. Worn legends are Bypass, Clean, Blues, Solo Gain,Bass, Middle, Treble, Presence Monitoring on phones output some mains buzz at most on minimum of gain pot and least on maximum. No effect on buzz , altering the other pots, was due to having amp out of its case. It transpired owner had used a 12V DC supply when the proper 12V ac unit had cracked at lead outlet. Mesa transformer 1.5R / 250R 12V, 1A. Beefed up with a halved plastic knob glued around, made good wiring, and over a pair cable ties. DC will power the SS and control but 12V ac needed for the internal step up transformer for valves HT Mesa Boogie V twin amp foot pedal , 1998 The bypass and solo LED are just where someone's thumb would go when picking up the unit or a misplaced foot. Ten yearold plastic bexel and glue around LED fails and gets pushed inside. Silver finish brass screw cup/eyelet from ironmongery. Use the wrong way up. With a punch knock the raised bit around the hole, flat. Grind open the hole to take a standard LED bezel. Instead of the ring at rear use a cable tie and glue all together on rear. The cup spreads any thumb / foot loading. The bypass mode only works in normal foot pedal fashion with "to guitar amp" output. Uses 7812, 7812, 7805 2x 4N33, 74HC04, 4x 072, LM386 2x vactrol LED/LDR Step up transformer measures in circuit 28.5 and 4000 ohm With first 12AX7 in place and vacant second socket DC measured to case 243,0,0,14.5,0,243,0,0,0 second in place and first empty 254,248,0,14.4,0,248,0,0,0 With both valves in place and DC at the 4 rectifier diodes 236,95,102V Confusingly, unless not original knobs, they go on 180 degrees from the natural flat positions. Mesa Boogie V twin amp foot pedal , 1998 Bad power inlet socket. Flimsey piece of deformed plastic under the swaged surround of the bush. Interfering with the outer contact which is then gripped by the plastic and no electrical contact after that. Cut away this plastic plus deformed plastic edges of the barrel that are close against this contact so it can move freely. Beefed up by cutting back a piece of small cable tie, recessed over the contact, to pad out a bit and hot-melted onto the pcb. Then 2 small "O" rings over to increase the spring/holding action. Also plastic problems with Mesa , the transformer for UK use, has very week plastic "earth" pin. Midiman, M-Audio Omni i/o audio processor for A/D, 2000 Owner plugged in a new microphone that blew one channel. Pre-amp uses a Burr Brown INA103 very low noise instrumentation op-amp. In this M-Audio Omni i/o preamp and an outline design application in the Burr Brown book show much the same circuitry the 48Volt phantom supply to the mike is protected by 6.8K limiter resistors. But to block the 48V DC to the op-amp there is a 10uF/100V electrolytic in each line directly to the inv & non-inv i/p of the op-amp and only 2K to ground. If , as seems in this case, a balanced line microphone with a short to ground is connected to such a system then the +48V / 0V across the elecrolytic will instaneously go to 0V / -48V with -48V directly connected to the op-amp i/p powered from +-15V rails and according to the databook can be taken to only +-12V. Added 2 back to back 6V,1W zeners plus 10R to ground for each of the 2 inputs to each expensive IC. For the blown one replaced with an OP27 very low noise opamp glued over a 16 turned pin socket , with mods, first original pin number and second is OP27 1 to 3 via 68R 7 to 3 via 3K 8 to 4 9 to 7 10 to 6 also on original (turned pin socket) only 6 to 16 10 to 15 via 3K 13 to 14 via 56R Functional replacement for the INA103KP including the -20dB cut. Unit also used NJM4560 SIP amps and a 7555 for 48V generation. J2 DC voltages with all pots set low 1.6,1.6,0,0,0,0,15,0,-15,0,0,0,-13 lines 2 and 3 are the signal lines for the amplified 2 XLR inputs Monaxor MPX 3400E , 2005 ? digital echo mixer Breaking output connector problem A lot of solder points on the rear board were suspect although not seemingli dropped.uses 7805m the spring is for earth connection to front panel Diverse Devices,Southampton,England
Telephone number - the same number as it has been since 1988 but email is now the preferred method of contact so number deliberately not placed here.
I devote time each day to replying to emails.
(obscure/obsolete components,second hand test equipment, schematics etc) Postal: 66 Ivy Rd, St Denys, Southampton, Hampshire, England , SO17 2JN There is no point in contacting me about any of the above, the repair job may have been done 15 years ago . I cannot clarify or enlarge on any of the above.

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