Vintage Synth Repair

Hi everyone.

My ARP OMNI Mk.1 has a drone whenever the release slider is set more than zero. When set to zero it's fine. No drone. When I press down on more than two keys however, it comes in whether the release is zero or not.

When I bought it was totally shot even though cosmetically you could eat off it. I had the repairs done and it lasted a year or so before this showed up. 

I'm gonna take it in soon, but would like to know what the matter may be beforehand. I use it quite often. Last time it set me back around $400 that including my ARP SEIV which also had an issue.

Thank you.