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
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.
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
incidently "diode" DVM test readings for a good TDA7294V
relative to the heatsink
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
ribbon to mixer
Ashdown ABM500 bass amp, 1997
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
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
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
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
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
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
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
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
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
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
if anyone knows what that means as temperature.
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
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'.
Because the cover protrudes 1mm or so it even looks effective
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
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
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
Mark all the 1/4 sockets before removing as their orientations
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
"X2 080" ,"X8 761" ,"TL 761" (not opto) ,"X8 082" ,"X6 081" ,"X6 082" ,
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
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 http://sound.westhost.com/highspeed.htm#a33
+-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
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
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
pa uses MJ15024 / MJ15025, 2x 2SC4382, 2x 2SA1668 , 2x 4580,
TL071, BA10393 , 4093
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
main bridge rect ac 102V, 138V dc
othe rbridge 18.8V ac
Carlsbro GRX7, 1996
Sometimes failed to work and sometimes dropped in power
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
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
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
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
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
.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 ?
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.
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.
then row of TIP41C,42C,42C,41C
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
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
All pa W/W measured in circuit at about 350 to 380 ohm.
5 x 4558 , 4049 + 4x 4558
240V primary 4.3R
Secondary red , in circuit, 32.5R
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
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.
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
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
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
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.
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
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
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
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
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
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
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
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
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
Tackling a functional replacement
Wire a chopped down to 10 pin turned pin socket to the pcb and glue in
Another chopped down socket , to plug into the first, wired to a piece of
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
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
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.
1.2K, 4.7K, 2x W/W 470, 4x 100K
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
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
240V primary 30R
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
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
Negative pulse trains at a repeat rate of about 38mS
,only outputting pulses with a change of switch , vol or continuous
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
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
Uses 4x MJ15003, 2x MFE15030, MJE15031, T072C, LM339
Motorola MPSU10 plus another of same low format maybe MPSU60
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
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
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
MM5871,MM5891 in National
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
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.
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
over 6.8K 24.3, -19.3
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
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
no mention of ac or dc so could be 200V dc, maximum dimensions about 8x10x4
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 (
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
Two of the 1970 741s had at least one I/P at
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
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
+/-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 ? ,
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.
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
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
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
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
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)
(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
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
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
Logo - C followed by 2 square wave pulses
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
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
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
mains primary , 240V, 52.5R
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
Uses 2 pairs of 5 pin SAP15NY and SAP15PY output devices
according to Sanken datasheet , in English,
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
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
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.
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
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
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 /
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
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
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
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
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
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
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
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.
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
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.
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
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
power interconnects to prea
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
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,
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
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
By the time I powered it up just some low level
crackling whatever vol setting and no signal
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
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
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
broken wire on one tail of one coil
35R and 205R
mains transformer secondaries
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
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
second in place and first empty
With both valves in place and DC at the 4 rectifier diodes
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
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
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,
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.
If this email address fails then replace onetel.com with fastmail.fm or
replace onetel.com with divdev.fsnet.co.uk part of the address and
remove the 9 .
Please make emails plain text only , no more than 5KByte or 500 words.
Anyone sending larger texts or attachments such as digital signatures, pictures etc will have
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blame the spammers. If you suspect problems emailing me then please try using
my fsnet.co.uk account.
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