The most obvious things to clean/replace are the volume and balance pots, but don't forget any speaker switches or relays.
Over time they get corroded and can act like little diodes. I had this happen with a Yammaha P2050. Replaced with new and much better relays, and added sockets for next time, problem solved.
Over time they get corroded and can act like little diodes. I had this happen with a Yammaha P2050. Replaced with new and much better relays, and added sockets for next time, problem solved.