Find a service manual?
If you feel confident enough to test and solder components, figure out which transistors/caps/ics control those stages and replace them.
Old stuff that typically goes bad:
-4000 series CMOS
-pretty much everything
m-audio Radium 61, Korg Kontrol49,
Nord Modular rack, ER-1
Custom built modular w/ ~30 modules
TX-216, VSS-100, PSR-470
Ableton Live, Reason, Cubase, plugs