How noisy is a healthy Peavey TNT115BW?

[Brian H.]

My son has acquired a Peavey TNT115BW bass combo FROM THE TRASH!
It stunk of mildew, and barely passed a fuzzy, distorted signal. I cleaned the power amp in/fx return jack, and now we're cookin' with fire. What luck! The Black Widow shows no water damage. As a matter of fact, the whole cabinet looks fine, but still smells some after bleaching.
Anyhow, after getting the amp working, it plays great at all volumes, but has an audible buzz. It's constant, regardless of gain or level settings. I'd assume that if the PS filter caps were bad, it would fall down and maybe offset at high power. It's not bad, only noticeable in the house. Is this normal for these?
Incidentally, this amp sounds nice with guitar, also.
Enzo, I know you know this one!

[Enzo]

Plug the guitar into the power amp in jack, but zero the volume on it. ANy effect on the hum? That will tell us if the hum is preamp or power amp related.

Not sure what you mean about the power supply caps. Are you assuming they all fail together? If one gets leaky the other won;t get leaky to balance it. Measure the ripple on the two high voltage supplies - is it the same on both? Are the rail voltages about the same as well?

Half of U1 drives the power amp, it has its own +/-15v rails derived from zeners. One of those could be sagging. Are both 15v rails about the same and clean on that IC? The other ICs run of the regulated 15v rails, so check those rails as well.

And just on the odd chance, pull little JFET Q1 to see if it is leaky.

[Brian H.]

I'm exposing my ignorance

Thanks, Enzo! I knew you'd be all over this one. You should get paid, like the I.T. help-line folks.

I had the notion that if one of the filter caps was bad, it could show up as a dc offset component out of a push-pull pair due to a lopsided potential referred to ground between high and low sides during peak demands. I'm sorry for my lack of knowledge here.

I plugged into the power amp in, and the noise is gone. I guess my next step is a change of venue. I have cfl's everywhere, but switching the light off that was overhead did nothing. We'll try moving the amp tomorrow.

[Enzo]

OK, it's in the preamp then. Check the two regulated 15v rails for ripple and level. Look at the op amps in the preamp, is there one or more with DC offset on its output pins? Any get hot?