A Customer brought me a Lab Series L5 with a high frequency oscillation around 5k Hz.
The noise is on the negative and positive rail voltages going to the Opamps, but I've isolated the power supply and power amp and they don't have the problem.

The noise is coming from the preamp and increases with both channel volumes and master volume. Weird side effect, the frequency slightly changes with the channel volumes. Ie, ..Normal channel frequency increases, Reverb Ch, frequency decreases when increasing volume. I've started pulling opamps and still haven't found the issue.

Any ideas?