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Gallien Krueger 200MB bass amp with large DC offset

Started by jpcar, October 20, 2023, 12:54:05 PM

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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 :
There are no stupid questions.
There are only stupid mistakes.

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

Quote from: g1 on October 24, 2023, 01:40:43 PMIf 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.