Peavey 400GH Mark 3 Bass

[tdlunsfo]

I have this 400gh board confusing me. For starters, the amp would turn on with a loud buzz. I found a physically leaking C36 and C37, replaced them, and started it slow on the variac in which it blew the 1A fuses. I removed the regulators and no difference, the fuses start glowing hot. All the diodes check out, and when I remove them, the fuses stop glowing but now R32 and R30 start to smoke. Output board and Preamp boards are disconnected. As far as I can tell the 16v rails only go to Q16 and the rest go to the preamp and it appears the 15v and 16v rails are separated (with the exception of Q16). I obviously have something else going on with the rest of the circuit, but why is this "something" burning the 16v fuses when all the 16v loads are removed?

Note: I get good diode junctions on both Q6 and Q7.

Schematic link: https://music-electronics-forum.com/...etch?id=868215

[TomCarlos]

If I am not mistaken, it has been suggested that you should not run the amp with a disconnected power board. That could damage Q6 and Q7,

Do not connect a speaker until you resolve this problem.

I would start by disconnecting and checking Q6 and R30 since you reported the resistor as smoking. I would then check Q7 and R32. Next, I would pull Q8 and R34 and check those. Do the same for Q12 and R42. Then check each of the power transistors, Q9 to Q15. Check each of the emitter resistors. Check the diodes CR24 and CR25.

The 16v rail goes to CR23.

400GH_MODULE.pdf

[nosaj]

If I am not mistaken, it has been suggested that you should not run the amp with a disconnected power board. That could damage Q6 and Q7,

Do not connect a speaker until you resolve this problem.

I would start by disconnecting and checking Q6 and R30 since you reported the resistor as smoking. I would then check Q7 and R32. Next, I would pull Q8 and R34 and check those. Do the same for Q12 and R42. Then check each of the power transistors, Q9 to Q15. Check each of the emitter resistors. Check the diodes CR24 and CR25.

The 16v rail goes to CR23.

400GH_MODULE.pdf

Hey I've done that
nosaj

[Helmholtz]

If you still have overcurrent with both regulators removed there must be something wrong with either the rectifier diodes or C36/37.
Check these parts again especially for correct polarity.

And don't forget that the autoformer shorts all DC at the output to ground. So you could have high currents but zero DCV at the output.
In effect the autoformer protects the speakers from DCV.
I would disconnect the autotransformer during repair.
Don't reconnect it or the speaker before you have 0VDC at point x.

[tdlunsfo]

Updates: Systematically I tried removing the regulators and then the caps with no change in current draw across the fuses. Lifting the legs of the rectifier diodes stopped the fuses burning but the diodes check out with the DMM. The caps were installed correctly, I marked the grounds before I installed them. I went ahead and reinstalled all the parts in the 16v circuit and strangely the fuses stopped burning, so it must of been a bad solder joint somewhere on either the caps, regulators or diodes. The 47 ohm resistors are still burning, so I took Tom's advice and hooked the output board back up and boom, It powers up with no DC on the speaker and passes signal. I've repaired similar boards (400BG) in the past without the output boards installed but apparently the 400GH does not like those boards removed.