Peavey M-3000

[Blastart]

Folks, I am working on my power amp. It started making a dirty hissing sound with occasional sputters and then ceased any sound. There are no apparent hot spots (visible) so the easy trouble spot is not an option for me. None of the electrolytics are showing any leakage. If I were to guess (which I am) I would say it is probably a transistor or a capacitor or a combination of the two. Since there is a thermal switch or breaker on one of the transistors that is probably causing the shut down. Kind of like the fuse in a tube amp power supply. The transistors (power) are motorola sj6357 - 8226 the second set of numbers is below the first and there are 8 of them. the third in the chain has the sensor on it.
What is my logical course of action. Do I pull the transistors first and test each of them one at a time? switch out the quad op amp tlo74 (it has a socket) start checking the electrolytics? I don't want to just spin my wheels here. Any help would be appreciated. This has been a dynomite amp for years and I would love to get her well again.
Thanks for your time; DB

[Rick Erickson]

If the fuses aren't blowing it's not likely to be a power transistor problem. You can blindly throw parts at it and you might get lucky, or you can run a signal generator into it and check with an oscilloscope to see where the signal stops. You should have a schematic and a basic understanding of safety precautions and service procedures before attempting a repair on your amp. Solid state amps are very unforgiving when mistakes are made.

RE

[Enzo]

Email parts@peavey.com and ask for the schematics. Then we all can refer to things.

The SJ6357s are MJ15003 transistors, you can get them from Allied or in fact right from Peavey. That assumes you might need any. 8226 is a date code - 26th week of 1982.

It makes no sound, but do any lights come on? Any sign of life at all? First thing to look for in any dead thing is power. Is there DC on the main filter caps? is ther AC on the power transformer secondary wires? Is the main fuse or breaker OK? DOn't just look, measure.

Why do we assume the thermal switch is the problem? That is a rare failure. But check it. It is just a switch that opens when it gets hot. You could have a failure of the power switch, the line cord, the power triac if that model has one.

But you need to isolate the problem: there is power in the unit or not.