Peavey Mark III 400 b/g no output

[UnluckyFett]

Hi all, I have a Peavey Mark III 400 b/g that worked fine until put in storage. it looks like it got some moisture while there and now has no output. when hooked to a cab it make a pop noise when turned on and off but theres no hum when the volume is turned up or when a cable is plugged in and the end touched. i was thinking it could be something simple like corrosion bridging something but I'm not sure. I was wondering where should i be looking in the circuit for the problem and also how to test the transistors. I have an ok background in tube electronics but no idea about solid state and thought that this would be a good project to start with. Thanks

[Enzo]

Troubleshooting is troubleshooting, doesn;t matter the technology. The essence of it is...

Isolate the problem.

Is this a Bass Mk3? Or some other PV Mk3? For now I will assume Bass.

In PV solid state amps, the rear panel is the power amp, and front panel the preamp. Divide and conquer. Power up and speaker connected. Plug a signal - guitar or whatever - into the power amp in jack. SOund? If so, power amp works. If not, power amp OR power amp feed from preamp has problem.

Likewise, plug guitar into regular input. RUn a cord from preamp out jack to some other amp. ANy sound from the other amp? If so, the preamp is working, if not, it ain't.

And verify the power supply. The +/-15vDC supplies are important. If they are not both there and up to voltage, the whole thing won't work. The high voltage for the outputs is something like 45-50v both polarities. Since it thumps, I bet they are both there.

These are the same things I'd do for a tube amp.


Since the speaker thumps at turn on, we know at least part of the power amp works.


POWER AMP PROBLEMS:
If no power amp, drop the rear panel for access. In the corner of the power amp board, farthest from the big filter caps, is a cable up to the front panel. It carries power ground and signal for the preamp. Note whuch way it goes, and pull it off the power amp board.

Four male pins stick up from the board. The one closest to the corner is the input to the power amp. Power up and speaker connected, touch that corner pin with your finger. A working power amp would make hum out the speaker. Not real loud or anything, but easy to hear hum. If you get that, the power amp is working. I say working, because it means it is amplifying. Your problem is not bad sound, but no sound, and that is what we are troubleshooting.

If it no work, then the schamtic has lots of DC voltages on it. See if they are close. And there are two ICs on this board, a 14-pin and an 8-pin. The smaller 8-pin is the compressor IC, the amp wil operate without it. Note which way it goes and remove it from the socket. Now power up, is sound restored?

And if my DCs are OK, and pulling the little IC no help, I usually just sub the TL074 to find out if it is OK. The preamp has several TL074 on it, so you can borrow one from there if need be.


PREAMP PROBLEMS
Reconnect the cable from power amp to preamp.
First, check power supply. We already checked at the power amp where it is created, but if that +/-15v does not make it to the front panel, the preamp won;t work. And that includes one IC stage between power amp in jack and the actual feed to the power amp. Are the 15v rails getting to the front panel corner connector? And to the ICs?

If that is OK, I usually just apply a signal at the input and trace it through the amp.

[UnluckyFett]

First I would like to start with thank you so much for your help. Its the musician head. So I followed your steps and the power amp seems at fault. Theres signal out of pre out on the front, its noisy but good signal. when I touch the input pin I don't get anything. I pulled the 9 pin IC and still no sound. When I was looking at the board I saw some corrosion I didn't notice before. There was some that looked like it was bridging so I cleaned it and now I dont get thud turning on but a fading out pop when I turn it off. Also the collector of Q12 is very heavily corroded. Could Q12 be the problem?

[Enzo]

Did you substitute the 14-pin IC, the TL074? I don;t remember on the Musician, but there are usually some 074s in the preamp you could borrow one.

The schematic is chock full of little DC voltages, have you checked yours against those?

Use clip wires or just reconnect the preamp cable and use that - somehow apply a steady signal to the amp. Is it present at pin 14 of the IC? At the collector of Q9?

[UnluckyFett]

I haven't substituted the IC yet, maybe later today. I checked some voltages and this is what i got

Location Schem Value Actual Value
Neg side of C41 -15 -15.12
Pos side of C40 15 14.88
Base of Q2 1.17 0.98
Emitter of Q12 -0.02 -0.02
R19 side of R20 26 25.8
Base of Q8 -0.87 -7.02
Neg side of CR13 -0.15 -7.03

Sorry if thats difficult to read. The last 2 concern me. And how exactly would i measure the signal through the amp?

Thanks again for all your help

[Enzo]

They would concern me too. The base of Q8 and the anode end of the diode are connected directly to... ta da... pin 14, the output of the TL074. All the more reason we need to sub it to find out if it is the problem.

[UnluckyFett]

I swapped out the IC and checked those 2 voltages and got -1.14. I plugged a cable into the power amp in on the front and i get hum the it fades out. its like it charges and drains. I was thinking one of the transistors now but is it Q12 or one of the power transistors?

[UnluckyFett]

Or should i just buy all new transistors for it and see if it works?

[Enzo]

Why are we focused on Q12?

You swapped out the IC and now instead of -7v, you have -1v, which is not far from the expected -.87v

But the amp still passes no signal? And I don;t recall, did we establish there is no DC voltage sitting on the speaker wires?

SO connect a signal to the input conector. It is present at pin 14 of the IC? At the collector of Q9? At the bases of the output transistors? At the output pin - the connector for the speaker wires (or autoformer if there is one.)?

[UnluckyFett]

it passes signal just intermittently. It last 2 seconds then fades out and wont put out signal for another 15 seconds. and the picture is why i keep bringing up Q12.

[Enzo]

Ah. I bet it is OK, but you can get a new one from Peavey and feel confident it is OK. If that transistor was toast, you would probably have DC on your output.

How about that chasing the signal from point to point? If the test signal is coming out the IC but not the speaker, then the problem is between those two points. If we look for it stage by stage, we can hopefully narrow it down to just where it disappears.

[Enzo]

I don;t recall, did we remove the little 8 legged U2? If not, please do so.

[UnluckyFett]

I bought a lot of 5 TL074CN's and swaped the one from the preamp for a new one and i got the same voltages as the "bad one" i replaced and no signal. i put the one from the preamp back in and voltages check out and i get the same intermittent signal. also there isnt a U2 on the board. there is a spot for one but no socket.

[Enzo]

OK, so it passes signal briefly.

Once it is back to silence, with signal applied, how far through the system does the signal travel? Is it leaving the 074? COllector of Q9, etc?

[UnluckyFett]

so long story short i found a break in the foil where i believe -52v goes to the transistor board (its highlighted in the picture). i jumped the break and tried the amp. i got a loud buzz through the speaker. i shut off the amp quickly and i unplugged the speaker to check for dc. i turned it on and 3 seconds on blew the fuse. could the problem be somewhere around Q13-Q16 thats causing the fuse to blow?