This just came in with “squirrelly” keys, and VCO 2 was drifting badly when warm. The keyboard contacts were cleaned, and VCO 2 was checked with an oscilloscope. The VCO was found to have drifting frequency output, but stable inputs. The 2600 had sealed VCO’s (these early modules were encapsulated in solid epoxy, later ARP ones had an epoxy sealing layer and can be opened and repaired) so an aftermarket replacement was ordered from CEM. The replacement part arrived after some months and was duly fitted. The keyboard contacts were cleaned, and the unit set-up per the service manual.
via This Old Synth – Vintage Analog Synthesizer Repair Portfolio Page.