I have a unit I'm trying to repair that has some strange issues
1. The right channel has a permanent slight hum, which only appears when the compress section is switched in
2. Occasionally a loud hum with a modulation appears which gets louder & then sometimes just reduces back to nothing & leaves the slight residual hum. Other times linking for stereo & then unlinking clears the hum. When the unit is linked for stereo the loud hum appears in the left channel, but only when it is also in the right channel. When this loud hum is present the make up gain switch becomes very clicky. This would make me think perhaps there is a flaky cap, but since when I swap the channel audio boards the hum stays on the same side it appears to eliminate problem being a cap.
I have swapped the audio boards & the fault is still with the right channel. This leads me to believe that the problem lies somewhere either in the wiring harnesses or the compress controls, but I haven't found a fault there.
If anyone has any ideas I'd love to hear them because I've spent a lot of time on this already.