Quote from: g1 on October 24, 2023, 06:29:25 PMIs B-C junction of Q9 giving proper reading on diode check?  It seems like Q9 might be open.

You nailed it!
I hadn't checked Q9 out of circuit. In circuit I was getting around 300mV in both directions which I suppose is not ideal. I guess an open circuit is less obvious than a short. I desoldered Q9 and the Collector leg broke in half. It wasn't making contact - hence the open circuit! But luckily there's enough of a stump on the transistor case to solder a wire to, so I tested the transistor and it's fine. Just soldered it back into the board and all the negative voltages in all the wrong places have vanished. Gotta put the amp back together and see how it sounds now!
Thanks for all the help, g1!
I learned a lot here. I didn't expect the open circuit situation. I haven't seen that much of it before. I guess a clue was that there was negative voltage all over the place but it wasn't drawing much current. Nothing was getting really hot. A short circuit would probably be clearly different in that way. 
 

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.

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

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.

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?

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.

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

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.

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.

I found the problem!

You're right though. I don't think this was the best way to approach it but I couldn't think of another way to isolate the problem.

The fault was C78, 79. One of them was almost shorting.
I don't know why I didn't suspect these sooner! They looked ok. Not bulging or leaky. Never trust a capacitor I suppose! I thought I checked them but I guess I didn't check them properly.
I learned a lot.
The problem was on both sides of the channel so I suppose I should have focused on what's common to both. That area from the bases of Q79 and Q81.

Thanks for all your help!

In an attempt to get further with this...

I removed a few transistors so I can try to isolate the problem. Q73, 75, 76. So now the signal is not getting any further.
The results are good channel in green and bad channel in red. Blue is the same for both. These are all 500Hz sine signal with peak to peak measures.
I tested DC voltages and all the transistors are getting same voltages on the good and bad channels. I tested all the resistors and they all seem good. 

In the schematic I'm just showing the results on one side but the other side (bottom half) has exactly the same results.

There's a strange signal (200mv) at Q79 base that continues all the way through the circuit out to the speaker terminals.

I have switched out Q80, 83, 79 (and Q82, 84, 81) with the good channel.

Any advice on how to hunt down the problem? I'm not sure I'm approaching this the right way.

DC idle voltages for Q73 and Q74 are the same for both channels.

Q73:
C= 66v, B= 1.7v, E= 1.1v

And for Q74:
C= -66v, B= -1.8v, E= -1.2v

I tried removing Q82 so that the signal just flows through one side of the amp.
On the good channel, I see a signal that's amplifying only the positive half of the sine wave and not the negative, as expected. But on the bad channel, I'm seeing the full wave, just not amplified much. Looks exactly the same as if Q82 was there.
Next I'll try removing Q80 and see if I get the same results.

All DC voltages are the same for both channels. I have now swapped Q79,80, 81,82 and 83,84 from the good channel to the bad channel and the problem remains the same. Very low signal output from the collectors of Q80 & 82. I tested all the DC voltages around Q83, 84 and the resistors and diodes around there. Both channels are the same.
I tried removing Q75, 76 to see if they were somehow were pulling down the signal at C of Q80. It remained the same. The only difference was slightly higher DC voltage because two less transistors were being powered.
Is there any way to isolate part of the circuit to see if it's working? Like just apply signal to the base of Q80 or Q82 to see if one side works? I'm not sure if that's a bad idea. Everything is so interconnected. Not sure where to go from here.

I'm stumped with this.

The signal comes in. Is present at the base of Q80 & Q82, and then drops to a very low signal at the collectors of Q80 & Q82. The very low signal is at the base of Q75,76 and Q77, 78 and gets amplified back up at the collectors of Q75,76,77,78.
On the other (working) channel, the signal at Q80,82's collectors is about the same as their bases, so by the time it gets to the Q75,76,77,78's collectors it's many times larger.

I just can't figure out why this is happening on both Q80 and Q82. I swapped those transistors with the good channel just in case, but it's exactly the same.

All transistors are getting their power from the +60 & -60 rails.