Vintage Synth Repair

-If you can take the "ground loop" out of the equation, the most likely cause is power supply filtering capacitors that have gone dry  and arent doing what theyre supposed to.

I see this as a regular thing here in my shop.

Caps are easy to get and fairly cheap and replacing them drops troubleshooting time way down

Just doing that may make your troubles flee

Scott in Burrrrrrrr-mont