switched out the caps and everything appears fine. The amp made it through one practice. Thanks for all your help!

The input jacks are grounded solidly to the chassis. The decoupling caps are the 100/50 rated caps in the schematic right? I was going to change those and the 25/25 caps. Hopefully that will work, because I don't really know how to "probe a signal path yet xP. 

Here is the right schematic.

Sorry about that schematic I don't know why it loaded up like that. I am not at home right now but if Enzo doesn't beat me to it I'll get up a better version.

I'll check the inputs, I hadn't thought of that.

Quote from: LateDev on June 30, 2015, 06:01:54 AM

You need to make sure that all the leads going from the front panel to the base board are properly connected, paying particular attention to any leads that look like they are local to the effects section.

What is the base board?

Hi LateDev,

I figured the problem is with the preamp side, specifically with the effects input (this amp has a normal input and an effects input) because when I have just the power section of the amp plugged into a speaker, disconnected from the preamp section there is no hum. Also, the hum goes up and down when if I turn the volume/gain knob on the effects side. If I turn it all the way down, and play through the normal side of the amp there is no hum. The other effects knobs effect the hum too, like if I turn up the depth and rate, the hum oscillates. The equalizer in the preamp also affects the hum.

Here is the schematic and some photos, I already changed out the 50/50 caps from the power section, they were visible damaged. One of the big 6000uf caps attached to the transformer looked like it had a little bulging, but they both tested good. 

That makes sense, just wanted to make sure.

Some more questions for you fine folks, if you're kind enough to indulge me. 
Since there is a hum coming from the preamp section I figured might as well change out those caps too while I have the head open. Took a look and they seem to be a different value from what the schematic shows. Schematic shows 100uf/25v caps, where the amp has 150uf caps. I'm thinking maybe these caps have been replaced before and someone used different caps. Which leads me to wonder, could that be the cause of the problem? Or is it a sign of a preexisting issue, something is causing the caps to go bad. Do you know if these ever came out of the factory with different caps, like would peavey just use what was lying around if the value was close?

Hey all,

Trying to repair my Musician 400 series head. Some background, I bought the amp used, gigged it for about 8 months until after a long practice, 2 or 3 hours the night before a gig, the amp started smelling really strongly of the burnt resistor smell. I thought/hoped that it was just too hot and turned it off. When I turned it back on the next day everything seemed okay, until the show when it totally crapped out started sounding all crunchy and weak.

Flash forward a few months later I actually get the time to open the sucker up I can see a few fried caps in the power amp section and some burn marks on the old cement resistors. I change out the some caps and things sound pretty good, there is still a bit of hum on the effects section of the pre amp, but the normal section sounds great, all the output levels look good, no DC going to the speaker or anything.

I turn it up and start pushing it hard and it still sounds good, but the light bulb limiter I have the amp plugged into starts to light up. The louder I play the brighter the light bulb gets, but when the amp is just on, or I am playing with the volume at 50% or less it's fine. What could be going on? is this normal? This is my first repair and I can't find anything about something like this on the web.