peavey bandit awful squeal

[cooldude666]

Buddy brought me his peavey bandit to have a look at. It's squealing. verrrry high pitched. The squeal is only on the Lead channel with the gain switch on. I can dial it out by bringing the high or the post-gain down to about 3. I feel like this next little fact is related- the amp has become microphonic. I can tap on the chassis and the tap is amplified. Where should I start on this hunt? I put the schematic up in my dropbox,

https://www.dropbox.com/s/quq9r9rg48...dit%20112s.pdf

[Enzo]

It makes noise. WHich controls affect the noise in ANY way?

Microphonics may indeed be the issue. Dismount the chassis but leave a speaker connected. Set the controls like normal. Now use something insulated to tap on each part on the board. A wooden chopstick from a Chinese restaurant is ideal for this. Looking for things that seem sensitive to vibration.


Other possibilities would include input jack grounding with broken solder. Does this occur when no guitar is plugged into the input?

[cooldude666]

almost all of the controls on the lead channel affect the noise. the clean channel is very quiet until everything is dimed and then there's a little hiss. on the clean channel the chassis is no longer microphonic.

tapping on just about everything on the board makes a sound and it all occurs with nothing in the input. When i tap the supply caps sometimes the sound jumps in frequency for a moment. C53 looks a little bulgy (although like i said- this isn't a problem on the clean channel just the lead).

[Chuck H]

I've seen ceramic caps go microphonic. When a component goes grossly microphonic it can seem like everything is. With very little distinction between the actual microphonic component and anything else. try subtle tapping and prodding on the ceramic caps. Bulging electrolytic caps should be replaced. In fact if one is bulging I'd replace them all. Interesting that the pitch of the squeal changes when that power supply cap is tapped. Though I wouldn't know the reason.

I'd start by replacing the power supply electrolytics, cleaning any jack switches and, siding with Enzo (there's a shocker), reflow the ground connections. It won't take long. If it's still acting weird try gentle tapping to isolate the microphonic component/s.

[cooldude666]

actually there really aren't any ceramics in here. there are 2 in the power supply. the rest are all film or electrolytic. I'll recap the supply and try reflowing grounds.

[Enzo]

You sure? A lot of ceramic caps look like resistors, except the body is usually a pale green. Look at the layout picture, and all those small rectangles that look like resistors, note how many of them have a C number inside instead of an R number. By tapping, we already know the whole thing is sensitive, we are now looking for some part that is EXTRA sensitive.

[soundguruman]

Re-solder the entire circuit board, especially jacks and controls.
It's really common for these amps to have bad solder joints. Most often on the input jacks.
That's what usually causes it.