Peavey XR8000 channel oscillating

[diydidi]

I Have one of these units for repair. The last stereo channel has a strange fault. If I turn the EQ controls up, it starts oscillating. Sounds like my signal generator. The frequency can be varied by adjusting the "high" pot. No input is connected. How would on remedy this? Can I stick a small capacitor somewhere?
Strange thing also is, that the channel actually passes signal.

[km6xz]

I do not have a schematic of a XR8000 but if there are a number of stereo channels life becomes easier. Do a stage gain measurement of a good stereo channel and the bad one. The gain might be too high. Measure the gain with treble pot at center and then at 3/4 on both good and bad channels. Does the oscillation vary in frequency with tone control changes? Does it happen to both channels? Oscillations can occur for many reason, all it needs is in phase positive feedback with a path gain more than 1. Can a strong or very weak signal start if oscillating. If only on very strong, suspect a rail bypass capacitor has opened. If it has normal gain like a good channel I would suggest the tone circuit are for positive feedback. The odds are that the gain is high however.
Do you have a schematic for those channels?

[Enzo]

Try this