A channel is dead, the knobs crackle, or there's background hum.
Dirty pots or faders, aging capacitors or a broken jack.
Pot cleaning, replacing damaged parts, and a channel-by-channel test.
It depends on the model and on what we find inside, so we quote each repair individually. The diagnosis fee depends on the device, and you approve the exact repair price before any repair starts. If it can't be fixed, you only pay the diagnosis.