Gallien Krueger 200MB bass amp with large DC offset

[jpcar]

Hi,

Can anyone suggest what I should do next?

There is -42 vdc on the output of this amp (GK 200MB bass amp). See schematic below

I have tested the transistors with a diode test (Q21, 20, 19, 18, 13, 12, D20) and I can't find a short anywhere.
The +/-15v and +/-12v rails are all working.

[Tassieviking]

The first thing you should do is make sure the speaker is not connected to that output as it will most likely destroy it.
The next thing after that is a bit beyond me at the moment.
Best of luck with it.

Here is a later service manual :

[jpcar]

Yes good point!
This DC voltage will fry a speaker coil in a short space of time. I have a 8 ohm dummy load connected at the moment.

Thanks for the service manuals. I'm using an older one because the component numbers seem to be different on the newer schematics.

[g1]

Does the schematic in the first post exactly match your unit?
From what I can see, there are 3 basic versions.
The last IC connects to a transistor which will be either Q4,Q5, or Q12.  Which is it in your unit?

[jpcar]

Yes, I'm pretty sure it's the version of the schematic first posted. That last IC (U20) connects to Q4.

[g1]

What are e,b,c DC voltages for Q12 and Q13?

[jpcar]

Q12,13 are mostly -42vdc, except Q12(B), Q13(C) are -26vdc.
I've added more voltages to the schematic.
There's a resistor on the board that's not in the schematic, R214: it's 1ohm bringing Q4's Collector to Q9's Base.
When I probe Q7(C) the voltage starts low and rises to around -20V after 30/40 seconds.

[g1]

You have pretty much -42V at both emitter and collector of Q20 and Q21 (and Q19)?
That would seem like a short.

[jpcar]

Yeah, I just don't see a short anywhere. None of the transistors are showing a short with the diode tester. I just tried removing Q19, and now there's -22v on Q20,21 (E & B). Hmm. Removed Q21 and it's still -22v. Trying to remove Q20 but it's trickier to get out!
Is there a better way to go than removing parts?

[g1]

A few things you haven't mentioned checking are Q9, D17, D20

[jpcar]

Q9 has +42v at Emitter & Base, and -42 at the collector.
D17 and D20 are both testing good with the diode mode on the multimeter.

[g1]

If nothing is shorted on the (-) side, then something must be open on the (+) side.
You will need more precise measurements across components (one probe at each end) rather than to ground.
What is voltage across R198? R209,R210?
B to E of Q9,Q12,Q13?

[jpcar]

If nothing is shorted on the (-) side, then something must be open on the (+) side.

That makes a lot of sense!

Voltage drop across R198 is 70mV.
R209 and R210 is 30mV

Across Q12 (E-B) -70mV
Across Q13 (E-B)  0V

[jpcar]

And Q9 E to B is -700mV, with Base being negative probe. The schematic shows R199 should drop +650mV, but this is -700mV, so that seems to be a problem.

And R210 and R209 I guess should be dropping that -42v to around -1.2v according to the schematic.
D24 and Q18 seem ok. Maybe a resistor is open somewhere.

[g1]

Is B-C junction of Q9 giving proper reading on diode check?  It seems like Q9 might be open.