Does My Korg MS-20 Need A Cap Job?

Hi. I have a great old Korg MS-20 mono synth (the real deal, not a mini or kit model) and it’s continued to work just fine for years. I had to take a break from playing for a while and when I got back into playing, it has developed a problem.
Short notes, it plays just fine. I did have to clean the pots of dust, but other than that, everything works. What the issue is, when I play a long sustaining note, the note slowly goes down in pitch. This is the only problem I’ve found.

I’m an old electronics guy, but my skills are rusty. I’m thinking it might need new capacitors somewhere, but that’s where my skills have failed me. I don’t know which caps would need to be replaced, and where they would be found. I do have a repair manual in PDF, so I think I could find them, but I didn’t want to take the thing apart until I need to. The last thing I need is to have MS-20 pieces sitting around and possibly getting lost. But, I know I could probably clean some more on the inside, too.

Another thing. The small screw holes that are plugged with what seemed to be rubber plugs? I’ve lost the plugs, because they just deteriorate, over time. If I need them, I’d like to find a place where I can get new ones. If I can’t find them, do I need t put something in the holes?

I’d appreciate anybody’s help, if they know anything about what I’m talking about with the caps or plugs for the holes. And if you could tell me where to find these caps, that would help, too. I can replace them, if I know where they are. Thanks for any help.

Caps are easy to get… easy to replace… if it’s old and has issues unless you want to test each cap with an esr meter the just replace them… And if you go through and find some faulty ones you might as well replace them all anyways…

I agree, replace them all. Specifically the electrolytic capacitors. They have a lifespan of about 10-15 years. I don’t think that will solve the pitch issue, but it will remove a large variable when you are ready to troubleshoot. It will also improve the sound quality and reliability. The pitch could be a dirty contact somewhere. Check the key-bed and switches/pots. Sometimes, the calibration trimmers go bad and cause issues like this. It is a good idea to replace the caps in an instrument of this age.

Electrolytics may degenerate, but this becomes an issue for the powersupply (you might hear a slight hum). But it is for sure not a problem for the pitch. The holding cap C6 for the keyboardvoltage is a normal mylene 220nF cap - not an electrolytic capacitor.

The issue of (not) holding long notes is a known problem of the MS-20 in general. I never had a MS-20, but I remember that even new ones showed this behaviour. If I remember right then the cause is either the FET Q23 before or the OpAmp IC5 after C6, through wich a leakage from the capacitor happens.

And a short PS: The recapping myth comes from old large studio consoles, which ran 24/7 over many years. Inside these consoles we find temperatures which are close to the upper region of the supported temperatures (in our SSL4000G we measured an average(!) of 45 degrees celsius - the caps are specified to work find up to 55 degrees…). Under conditions like these the electrolytic capacitors will “dry out” for sure after some ten or twenty years. But a synth never was switched on continuously and also never became that hot continously.

I once recapped a Roland System 100 for a customer (though I told him, that I do not expect any change, he ordered it - so I made it…). I made recordings before and after for analysis - I did not find the slightest difference comparing the spectra and transients. He had spent 350 Euros for a zero improvement…