A fairly simple synth with an LSI Key assigner/tone generator digital chip and a lot of analog filtering and a BBD ensemble effect it appears.
The SK series contains some fairly reliable stuff and from what I've seen keyer chips that might go bad as in the case of one of mine an SK20. Impossible to get the chip it appears. So do be careful when buying broken ones not to overpay.
While Yamaha normally does not keep parts around more than 5 years or so, I stock what I can. I have some sk knobs.
Contact Sound Doctorin'