Peavey Amp Screeching Issue, Voltage Leak?

[christopherrgerhardt]

On a friend's Peavey-Musician, Series-400 bass head; at any volume other than the lowest low I get a high-pitched ring that increases with either gain or volume. I want to make it clear right now that this is not feedback. Sounds similar to some voltage leaks I've heard in the past where a bad electrolytic was leaking DC into the ground. Problem is, I can't find it. I don't have a capacitance meter and am without a scheme to see what values are supposed to be present in some of the worn down components.

The amp is solid state and high wattage with HUGE filter-caps, so I want to see if any-one has had this issue before before I blow up another cap-drain, lol

The two pics are of the amp itself to maybe jog a memory and of a component in the power-amp, RIGHT before the output, that i"m having trouble identifying. it's in pretty bad shape and if it's a choke I bet it's the culprit.

[teemuk]

Yep, if it's indeed in the output of the power amp it's likely an inductor in parallel with that power resistor next to it. A typical part of a so-called "Zobel network", which in this case is counteracting the capacitance of the following load.

I don't know about "bad shape", though. Yes, it looks crude and dirty but in many amps a similar inductor is nothing but couple of turns of magnet wire wrapped around some former (which is usually the power resistor the inductor is in parallel with because that's the most convenient solution). That's enough to get the few millihenries of inductance the generic Zobel design requires.

So, in this case the visual appearance can be a bit misleading and I think the chances of such component failing are somewhat slim. In fact, that inductor doesn't look burned or anything, it just looks like what it is: a paper former for magnet wire, soaked with lacquer, which consequently looks kinda shoddy.

You can resistance test that thing and it should read a short circuit. I just doubt that had the inductor, or its solder joints failed, the resistor next to it would likely been toast by now because it's merely serving as a "damper", and if it winds up in series with the load alone it very likely goes down in flames. Anyway, the condition of the inductor should be fairly quick thing to check - and if neccessary, to replace.

[Enzo]

The only thing wrong with that inductor is that you think it is ugly.  It IS ugly, but they all look like that.  And as teemu suggests, if it were open - broke off at one end - then that 22 ohm resistor next to it woud likely be burnt.

If your big filter caps were all tired and dried out, the amp would hum, not squeal at high pitch.

If you turn the amp controls all to zero, the noise goes away?  Then the power amp is fine.

Does it make the noise even with nothing plugged into the input?  If so, the amp is oscillating somewhere.  But if you have to have something plugged in, we suspect the input jacks.   What if you plug a guitar into the input, but turn the guitar volume control to zero.  Still noise?  or does that also kill the noise?

So you have a PV Musician 400 series.  Peavey will send you any schematics you ask for - customerservice@peavey.com