Weird oscillation on output at high volume

[Axtman]

I have a weird intermittent problem with my Peavey Standard Mark III guitar amp. Everything is fine at low to medium volume. When I crank it up to high volume the amp starts pulsing DC on the speaker output. It is kind of hard to measure it but I think it is about -5 volts. The pulses are about 1/2 second apart. If I turn the amp off then start it again everything is fine....until I crank it. My guess is that something is possibly overheating.

I have no idea where to even begin to look. Any advice would be appreciated.

Schematic: STANDARD_MK_III.pdf

[TASBrendan]

Does it do it if you use the line out or preamp out to another power amp? You can start isolating the pre and power amps this way, at least give you an idea which half its happening in.

[The Dude]

I'd remove, clean, and reinstall any and all multipin connectors. It could be a poor/oxidized connection issue.

[Axtman]

Dude,
I have cleaned and retensioned all the multi-pin Molex connectors. I am going to do what TA SBrendan recommends and see if I can isolate it to the preamp module or power amp module.

[Axtman]

After way too much fooling around I found and repaired the problem.

The amp would act up after being on for a little bit. After the amp was off for a short while, I would turn the amp on and it would work normally but then act up after on for a bit. Therefore, I determined the problem was a thermal issue. I let the amp go into the weird oscillation then used freeze spray around the board. I found that the 7815 positive 15 volt regulator was the offending component.

I replaced the regulator and the amp no longer acts up.I did replace the 25uf/25 volt caps because they had slight corriosion on them. I also replaced some 1N4003 diodes. But the regulator was definitely a problem.

Tip: If you don't have freeze spray just turn your compressed air can upside down.

[xtian]

Now if only FLIR cameras would drop in price...