The relay in a Roland MKS-20 commonly fails, resulting in low (or no) output in one or both channels. Lots of people have searched for that part (AG2029) unsuccessfully. I’m happy to report that we now have a good replacement:
We’ve tested this relay in a couple of cranky MKS-20’s, and it brought them right back to life.
We don’t have any repair instructions. But it should be easy to locate the relay inside the MKS-20, then you’ll need to remove that circuit board and desolder the old relay, then solder the new one in its place.
Sounds like it’s time to do some signal tracing then. There could be any number of components that could be at fault, so you’ll need to use an oscilloscope to see where the signal is dying.