Korg DW8000, Mono/Poly, MS-20...

I checked it in unison mode: it really seems that it's (mostly) always 
the same keys that show the problem (they don't make any sound).
However, the same keys, sometimes also work.
Maybe it's just they need some cleaning...
I'll try to open it up and see what I can do.

Thanks for your suggestions!
Best,
Marco