Oscillating Boogie

[drewl]

Hmm, I have a Mesa Boogie Rectoverb which had a board short that I fixed a few months ago that has a strange oscillation at different volume levels.

It happens on both channels, tone controls have no effect, only at certain combinations of gain, volume and master volume settings.

Any ideas before I rip into it?

[drewl]

Alright let me clarify:
If I whack on the guitar neck while fiddling the master output volume the amp will start motorboating-
A low frequency oscillation that is all over the place, on the power supply being fed to all the tubes.

I probed the phase inverter inputs and it went away, but not all the time.
I'll start by check for a loose joint and possibly intermittant pot.

[drewl]

Seems the 2nd half of V4 which is the effects return stage is where the problem is.
I lowered the gain of that stage after troubleshooting the effects return circuit and the problem went away.
Weird stuff.