I was lucky enough to get a hold of a JP-4 a while back. She was in a terrible state - near dead.
I managed to restore the power supply (all electrolytics and regulators changed), I cleaned the key contacts, solved a bizarre grounding issue on the control boards, desoldered, opened up and cleaned EVERY fader, checked the track resistance (just to be sure), resoldered the faders. And finally did a full calibration... Magic!
Now when I turn off the VCO Sub, and turn off the VCO Waveform, and play any key, I get a low level 'vinyl' like random 'scratching' noise (and what appears to be bleed-thru from the voice cards at the pitch played). The Level slider is halfway up. When pushed all of the way up the noise/bleed increases in volume, but stays at the same ratio.
These noises are just about masked when the Sub AND Waveform are on, but audible when Sub OR Waveform are on...
I have removed, cleaned and reseated all of the chips/boards that can be done, without desoldering.
I'm kinda hoping it will just be some of the bypass electroytics that are on their last legs, or possibly some cold joints, but I thought I'd ask you guys for any advice you may have, or have you experienced this before?
Look forward to hearing from anyone!
All the best!