Vintage Korg MS-20 VCO-2 Pitch Control Not Working

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Vintage Korg MS-20 VCO-2 Pitch Control Not Working

Post by BeeMee84 » Mon May 24, 2021 12:59 am

I have a vintage Korg MS-20 that was languishing in storage for many years. I recently fired it up, and everything seemed to working except *every* single knob was dirty and made a ton of noise. I meticulously opened it up and spent a day inside cleaning all the pots, keyboard contacts, etc.

When I put it back together and tested it, it initially sounded great. (I started testing it with sounds only using VCO-1.) When I turned VCO-2 up in the Mixer section, it sounded good, but slightly out tune, so I tried to adjust it with the Pitch knob. It didn't change the tuning at all, but when the knob got to around +4, there was a burst of crackling, and then all sound went dead (including VCO-1). By turning off the MS-20, and back on, sound returned as normal. But the Pitch knob on VCO-2 still doesn't work, and causes the whole synth to 'die' at +4, until I power off/on again.

My guess is that there must be a short in the Pitch pot, and I should attempt to replace it.

But in case there are any other analog repair wizards out there that can offer any different troubleshooting advice, I thought I'd give it a try. Would a multimeter help test a particular pot? And, if a pot replacement is in order, any recommendations where to get replacement parts?

Also of note: there some seems to be some 'intermittency' between the A2 and A#2 keys. Sometimes one plays, and the other doesn't. Sometime they both play. My guess is a problem with the contacts, which I can take another look at, but there is also a board of resistors that runs the length of the back of the keyboard assembly. Any advice for things I could look for beyond the obvious (ie, bent contacts)?


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Re: Vintage Korg MS-20 VCO-2 Pitch Control Not Working

Post by Yekuku » Mon May 24, 2021 5:54 pm

Hi Brett and welcome to the forum.
It sounds like a faulty potentiometer, it could be dirty so it is advised to clean it properly and re-check.
The pot is 100KOhm (VR2) , you can use a multimeter to measure its resistance or you can read the voltage on the wiper (middle pin) it should vary from 15V to -15V depending on the pots position.
VCO 2 Pitch trimmer (VR6) can also trigger such a behaviour.
About the keys problem it sounds like dirty / oxidised contacts, a good clean with WD40 on the contacts, hopefully it will bring the operation back to normal.
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Gear: Korg MS-20, MS-50, SQ-1, ZX-81

Re: Vintage Korg MS-20 VCO-2 Pitch Control Not Working

Post by liveinabin » Mon May 24, 2021 9:35 pm

Please let us know how this goes. I'm taking delivery of a very dirty MS-20 soon so I'll be following this closely :)

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