Repair Korg MonoPoly Vintage Analog Synthesizer Reparatur

3. Juli 2019

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Repair Korg MonoPoly Vintage Analog Synthesizer

This Korg MonoPoly Vintage Analog Synthesizer came in for repair/restoration with several issues. The two main issues were PWM waveform was not working on each of the oscillators and neither the pitchbend nor the modwheel did work in pitch mode. The Monopoly itself was in great condition. So I started picking up the schematics and tried to find out, what could cause the PWM waveform not to work. By tracing back the signal I quickly found the culprit:

This JRC4558 Dual Operational Amplifier was faulty. I replaced that with NOS and the PWM waveform came back to life. Then I inspected what would cause the pitchbend and modwheel not to work in pitch mode. The components all worked, the sub circuits all worked, so the only thing left was a connection problem. Once again I followed the trace and the "good" signal was present at the connector coming from the wheel section to the voice board. But then got lost. What I found was a damaged pcb. I think at a time there was something dropped onto the MonoPoly or the MonoPoly itself was dropped. However there were a lot of cracked traces. Some I could see without magifying glasses, but some were just tiny and only the coninuity test could verify.

After I fixed all broken traces I protected the copper with photosensitive Green PCB UV Curable Solder Resist Ink.

Next problem: all the electrolytic capacitor (brands) looked suspicious, so I desoldered a few to finally jusitfy a re-cap. And yes, like very often, nearly all of the electrolytics spread out their nasty poison again.

They worked, today, more or less, but in the near future they will damage your synth. And if they do, its maybe too late. It is a good idea if you let your synths be checked by a tech like me, to verify the components inside. Your synth will be thankful. While the synth was already open I could easily access all the controls. I cleaned all potentiometers, switches, rotary. I also cleaned the keyboard. Customer said "it's ok", but I found some weak points while playing with it. I cleaned the metal side with a rubber and some IPA, while I renewed the rubber contacts with conductive paint. Before...

After...

Next was to re-calibrate the synth after changing components. While doing that I found the trimmer kind a weak. I could establish the correct tuning, but with some vibration it went slightly off by some cent. So I decided to repace all trimmer pots with NOS.

Ok. And now I got very angry. Hey you ! Yes I mean you ! It maybe worked for you, even it was the very wrong way to do this. I know you had a hum problem with your synth. In the studio, or on stage. But it is NO REASON, NOT THE CORRECT WAY and VERY DANGEROUS just to loosen the earth wire on the mains plug!

Caution

There are lethal voltage exposed in electronic devices. Unless you know what you are doing, don’t try this. If you following along, you doing so at your own risk.

I’m documenting repairs for my reference and yours. I do synth restorations and repair with a specialty in vintage synthesizers, drum computer and other vintage gear, based in Düsseldorf, GER. Get in touch about your synth situation!

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