Korg micropreset M-500

[tonewill]

Hello,
I'm repairing a Korg Micropreset M-500 synthesizer that has one problem left that I'm not sure what the cause or solution is.

When portamento is on, the pitch of the instrument will drop up to a semi-tone. The higher the portamento time, the lower the pitch drops. I've attached what I think is the relevant part of the schematic, if anyone can suggest a likely candidate please let me know. There are trimpots, but I'm reluctant to adjust them without knowing what they are trimming.

Just to clarify, I do know what portamento does.

Thanks for any help.

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Description:	Korg micropreset M-500 schematic, portamento section.
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[nisios]

Hi tonewill.

I repaired a micro preset 500 with the exact same problem (and a bunch more), but this one was puzzling alright.

The portamento wiring runs along the pcb in two parallel tracks if my memory is correct.

Its a high impedance circuit and you need to keep it that way or else, you loose some precious electrons through the pcb and your pitch drops accordingly.

The solution is to clean thoroughly the pcb near that high impedance circuit. Along those tracks that go to the pot.
Use isopropyl alcohol, or regular alcohol if you don´t have isopropyl (its high impedance, but not condenser microphone kind). I like also like to use a pcb cleaner from kontakt chemie.

Also dry it well under an incandescent lamp. Humidity is low impedance.

In my case it was completely solved this way.

Hope its the same thing with you.

[tonewill]

Thanks a lot for the information, it's worth giving it a try as soon as I can. I'll report back if it works, may not be for a while yet though.

[humancalledjohn]

Hello,
I know this is a bit late, but I have had a Korg M-500SP for a while, and it recently broke. I am trying to fix it, but all the schematics I can find are blurry to the point of all writing and detail being unreadable, except for this post.

I was wondering if you still have the full thing, as this one is cropped, and the full thing would be of great help for the preservation and repair of this wonderful little synth.

Many thanks for any response

[humancalledjohn]

Stupid me ,

After searching for multiple weeks, I found a mostly workable version in this thread minutes after asking! https://modwiggler.com/forum/topic-94307.html Some of the labels are a bit fuzzy, but it's miles better than any other I could find.

I think I must have seen the thumbnail, and assumed it was the same low resolution scan as it looked very similar and I had had no success with searching flickr so far.

Thanks anyway, I hope this is helpful to someone else having difficulty searching in the future.