Korg MS-20 issue

Sorry, if this isn't the place for this, or if keyboard repair isn't a thing that's talked about. But, I've been trying to find out if there is something simple that I can do to fix my Korg MS-20 synth. It's one of the originals, not a mini or one of the modular jobs. I've had it since 1980 and it's always worked just fine, with no issues at all. It still works fine, on short duration notes, but if I want a note to sustain, it acts like it's running out of steam and the note slowly goes down in pitch.

There were a couple of years when all of my equipment just sat there, because my computer needed replacing. Then, about six months ago, I got everything working again and that's when I noticed this. Like I said, for short notes, it works just fine. The only problem is the long sustain. It makes no difference if I'm using one or both oscillators or the wave settings or how anything else is set up, as far as I can tell.

I've joined a few forums that deal in keyboard repair, but none of them have responded at all. I don't know if I've stumped them with this, or if I just don't rate, because I'm a new member. So, if anybody knows anything about this sort of thing, or maybe knows of a forum I can go to that will actually respond to my posts, I'd appreciate it. If it just needs cleaning, I'm not sure what to clean, other than potentiometers.

It doesn't seem like it's an oscillator issue, because both do it and even when using both, the pitch go down in pitch in perfect unison. Maybe there are some capacitors that need replacing? If that's the case, I could replace them, but I wouldn't know which ones to replace. Would I look for those that might be leaking or bulging or just replace them all? I don't mind doing that, if it would fix my problem. I'm just looking for someone who knows about this and can say yes, that will fix my problem.

It's strange how it's actually appreciated in value over the years. An MS-20 in good working condition, without cosmetic damage can be worth anything from $1,600 to $2,000 or more. I paid $600 for mine, second hand. It was well worth the money.
 
Hey, I'll reply...

Congratulations on keeping an original MS20. My friend sold his, and has recently bought the reproduction full size version.
This may be a behaviour exhibited by all MS20s. You'd need to track down another owner to check this.
It sounds to me like the VCO control voltage is drifting away with duration.
There'll be some 'portamento' circuitry involved in there, which may be relevant.
A bigger capacitor at that point may help, or finding out what sucks the charge away.
If it were me, I'd open it up, and map out a circuit diagram, and get an ossilliscope on it.
But, that's me.
 
Wow, I sold my old one over 20 years ago for $300 (think I paid $250). One of the oscillators was shot.
 
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