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Marshall G100R-CD blown

Started by phatt, August 28, 2018, 04:32:20 AM

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Hi All, I need some help :-\ with a Marshall G100R CD head.
Owner purchased it Sc/hand, worked ok but now keeps blowing internal 1A fuse.

So I setup my good ol 60Watt lamp limiter and no DC Voltage at psu.
AC at secondary is ok 32-0-32VAC (no load).
I Removed links to the 2 power transistors and I got the two 45Volt rails back.
So likely power units are boofed.
They are stamped with *Marshall T64 and T65*. So I'm wondering what would be the equivalent?

I've dug up a schematic which states the power units as 2SC 3519 (NPN) & 2SA 1386 (PNP)
both are 15A/160V/130W, They are TO218 devices.

Now to save this chap some expense I've got 2 pairs of 2SC5200 & 2SA1493 (15A/230V/150W) sitting here doing nothing,, 
But they are TO264 type which will fit by drilling new mount holes in the heatsink.
I'm assuming they may work but I'm a bit iffy :-X

I've attached the PDF of what looks like the G100R CD Schematic (Found it on DIY Electronics)
but the power stage is not the same so I'm a little unsure of how this circuit works.

I'm ok with the basic power stages but the schematic is not well drawn and has some mistakes and junction points are not clear.
Rectifier is drawn wrong for a start. :duh

Also TR14 & TR15 (which would be the Voltage stage of a normal diff amp) are not present on this PCB so I'm wondering if there is another schemo floating around somewhere. I'm a little lost as to how this works  xP
This amp states Made in India.

*EDit; Sorry TR14&15 are predrive for output Tr's NOT the VAS :duh

Another point worth note, this amp has been repaired before.
R112 (330R 7W) has been replaced. (Obvious as it's been mounted under the PCB)
This is the Drop resistor for the -15V rail.
But upon inspection it has failed again.  (I did not realize this at first ???)
There is no sign of heat stress on the replaced R112 it reads open circuit.
only a slight dis-coloration on PCB under the two 15V Zeners which is to be expected.
Everything else looks as per normal.

As this power stage requires both 45V rails and 15V rails to function then would it be fair to assume that failure of either 15v Rails could cause the power stage to blow something as well as blow the main fuse?
Any help would be most helpful, Phil.


OK,,I'm going too fast and the ol brain has lost the plot. :lmao:
I forgot I better check the power transistors before I go any further.
Diode test with DMM reads as per normal so maybe they are ok.

Now having had some food and time to ponder my over thinking lol,
maybe I'll first replace that open resistor *R112 (330R 7W)*
Logic says the failure of the -15V rail caused the problem.
So get that back and power up with lamp limiter see if if the amp shows signs of life.

If no one can find a better schematic I'll try and draw up a schematic of this power stage as it's not the same as the schematic I posted.
Oh yeah another mistake on that schematic is the power transistor TR10 is PNP not NPN.

Too late now but tomorrow will be sunny and I'll try and get some pictures of the PCB


T64 and T65 are BDV64 and BDV65.  Common replacements would be TIP142 and TIP147.

Those are all darlington so I'd agree that there must be another schematic for this version.
edit: the schematic you posted is for MG100, not G100. 


Thanks G1,, Yep I started to think they must be darlingtons but DMM diode test from B to C reads .650 ,same from B to E
I figured Darlington should be 2 diode drops? :-\

I believe one has to remove transistors for an accurate test but
I'm trying to avoid messing too much or replacing parts without really knowing what has broken. 
I've forgotten how to do a lotta stuff as I don't do much repairs so I have to relearn diagnosis, I'll go search how to test darlingtons.

Anyway meantime I replaced the big resistor for the -15 rail and powered up with 60 Watt limiter this morning and no luck. (the resistor on the back of the PCB)
Lamp full bright and only 4Volts on both rails so obviously a big short.

I guess my problem now is how to find what has blown.
Looks like there is only 7 Transistors and 1 Opamp in the power stage so I may have to draw this up which is a pain but might help work it out.

Meantime here are some pics.


I Removed T65 (TIP142) NPN.
Used this battery LED test and found
V CE = .005V
V BE = .68V

Simulation says V CE should be .9V
While V BE  should read 1.6V
Which lines up with the great Google

Can one assume it's dead?
You can tell I've not messed with Darlingtons very much lol.
I noted when I removed the device I noted a very faint 142 under the Marshall logo.


Collector to emitter short is a sure indication of a bad transistor.


 Thanks for the clarification Enzo :tu:
Was fairly sure they were stuffed,, time to order some parts.


Q, What would cause that 10W 330R resistor to go open? the 10Watt  on the track side.
Reason I ask is that the original was a vertical mount 7Watt unit and obviously replaced by mounting a 10Watt on the back.

So this will be the 3rd replacement ??? and I'm concerned there might be a deeper issue. Looks like both the original and now this one has gone open with no sign of overheating.

The original vertical mount was hard against the pcb (same as the others) and normally when they run hot over long period they darken the pcb and copper trace, Much like the darkened area under the Zener diodes which are bottom right just near that 10Watt 270R.
(See pics)

Zeners always run hot in this type of regulation so that tarnish is normal and often the big drop resistors run hot as well but there is no sign of heat stress on both of these big resistors that supply the voltage drop.
As they are 7 Watt units maybe they don't get hot enough to cause tarnishing BUT Yet this one has blown the original 7W and now Also the 10W.

So I'm thinking, will it blow again??

In case you're wondering about the 270R, it was all I had on hand to see if the two 15V rails were working. It will be replaced when parts arrive.
The -15V rail uses 330R while the +15V rail only uses 270R.

As a schematic for this amp seems hard to find and time to spare I drew up a map of the output section so others may reference. :tu:
I've tried to keep the part numbering the same as what is on the PCB.
I've been over this 5 times and fairly sure it's right but always the chance of a mistake.
It simulates ok but seems to have trouble swinging positive.
The output at opamp is offset about 3V neg but seems to come up at output side of R86.


Ding ding ding!  We may have a winner, at least for the power amp and supply.
How does this match up?


Clearly about the only reason for that resistor to burn is a load on the -15v rail.  COuld be a bad zener or filter cap there, but also could be out on the rail, like a shorted IC.   If the G8080 drawing works, the power amp uses the 15v rails


Thanks heaps *G1* yes a big help. :tu: :tu:
AND Certainly a lot easier to read than what I found. 8|

I have been over this again today and found my mistakes.
Helped verify the Collector of TR6 was wrong. R88 & R89 are in series and connect to base. whoops
R91 and R82 are indeed 1k8 Not 8k2,,  :duh
Only difference I can spot is R98 is 220R on this PCB.

I never totally trust sims but It can give you a glimpse of what might be going wrong.
Even with correct values this circuit still struggles to output a positive swing on a simulation. Headscratch??

@ *Enzo* yes as you might guess from screen shots there seems to be a problem. Right at the input, pin 5 is already badly offset.
I'm assuming it's partly due to that fancy feedback arrangement.

Going on the rule of thumb that the positive input of an opamp has to have a dc path to Common then that path is through a combination of R103, R105&106, R107&108 as well as Speaker & R109.

Yes the 15V rails come from the same supply as preamp and after removing 42 volt rail connections I powered up via limiter to establish it had a working supply.
I must have missed a link as after 2 minutes it smoked R102 which is the Neg side of power amp.

After talking to the owner again The Amp has obviously worked fine for a few years and used in a studio with no issue.
So I'll just put it all back together next week and see if we can get a stable output. Cheers, Phil.


Clearly there is a lack of gain on the high side of your sim.  The key to debugging this is to examine the currents in the transistors.  May be a bad model.

If you post the file and models I could try it out. Which simulator are you using?

PS:  Here are some voltages and currents off my sim attached.  I hope it will give you some ideas.


The whole power amp is one big feedback loop.   If there is DC on the output, it will come back to the op amp, which then tries to correct it.  The fancy arrangement is simply doing its job.

Does teh op amp have good 15v on both power pins right at the IC?  Did you sub the IC?


Hi Nick, Thanks for your input it all helps.
I use Circuit Maker and a good chance I'm using models that are not quite right, I have no doubt there are better programs around and many models exist but I'm not up to speed on all that. I don't do enough repairs to make it worth learning new programs.

Meantime after a good rest, (someones got to mow the lawn)  ;)
a day later yep the stock output darlington models are not happy in this circuit.  :-[
I've had trouble with that before but that's what happens when amateurs start messing with electronics. I Created a problem that does not exist, my own stuff up.  :lmao:
So it looks much better now so I'm relieved to know that it works.

I've re-read the values of resistors on the pcb but also measured them to be sure.
Small Rant;
I find those small blue resistors very hard to read the codes and
If I ever find the clown who decided to paint the base colour of resistors BLUE I will throw a bucket of blue paint over his head. :trouble :grr :loco
Even without glasses I can still read the colour codes on those cream base resistors but not the blue ones :duh
Rant over.
@ *Enzo*
I've disconnected the 40V rails from circuit and lifted R91 & R92 and powered up,, which shows 15 rails on all opamps including the power amp chip so I think it should come back to life. :tu:
It just bugs me as to why that 330R resistor failed twice.
Thanks all for the input, Phil.

Jazz P Bass