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Vintage Korg MS20 keyboard tracking issue

PostPosted: Fri Dec 28, 2018 4:12 pm
by superfuzz
Hi. Picked up a minty but faulty vintage Korg MS20 a while ago. Keyboard supposedly not working. After some testing I noticed the keyboard does trig the EG and osc, making a clicking sound like an osc at its lowest frequency. Tried the patchbay but could not trig anything with the keyboard there either.
Hooked up to a sequencer everything works gloriously although out of tune after sitting for 30 years.
I have pulled the keyboard and cleaned it. Some corrotion but not too bad. Measured everything for continuity and there is nothing that is stuck on or broken off. Connection to the mainboard is good also.
It sounds good with a sequencer as it sits now, but a dead keyboard is very annoying. Does anyone have a clue as to where I should start to debug? I have fairly good solder skills and can recap if needed.

Re: Vintage Korg MS20 keyboard tracking issue

PostPosted: Sat Jan 05, 2019 3:11 pm
by JamieOxford
Do you have an oscilloscope? Or know someone that will lend you one?

Looking at the schematic, the problem should be in the highlighted area. The top half being the pitch voltage and the bottom half being the trigger (which you assume is working).
I'd check the Keyboard CV Out and Trig Out to see if you have any voltage there.
There are also 2 test points highlighted on the cropped schematic - 1 & 2. There should be some voltage there when a key is played. There are a few other test points in the KBD area - 48 & 49, and some in the Trig area - 20, 21, 31, 32.
There are also 2 pots which control KBD CV on the front panel, which should be working.
If you have something visible at TP1, but not TP2 - it could be IC4 or 5 that has failed.
Test Point 1 should measure between 0 & +8.4V according to the Block Diagram (6). If you have no voltage, or no variation in voltage between keys, there could be an issue with the resistors in the Keybed. You can usually test these in place with a multimeter - it may not give the correct values as it's in circuit, but it should give you an idea if they're passing signal or not! Also worth checking they tie to ground correctly.
Check out the video here at 12:18, you can see them poking out the bottom.
Test Point 21 should measure up +5v down to 0 when keys are pressed.
You could also try looping the Keyboard CV Out to the CV In to see if that does anything. The CV Inputs break the KBD connection to the oscillators, and if there's a mechanical issue with the jack, it might not remake the internal connection.
Otherwise it's a problem further down the chain, but you said EG 1 & 2 trigger fine externally, and all other functions work?

Best of luck!


MS-20 Service Manual

Re: Vintage Korg MS20 keyboard tracking issue

PostPosted: Sat Jan 05, 2019 4:42 pm
by superfuzz
Thank you very much, this is most helpful.
I do not have a scope, but as I build effect pedals and fix my valve amps I have a justified need for one.

I have measured the resistors in the keybed and they seemed ok, will do it again and verify they are about the same value.
I tested the KBD cv adjust pots and they work fine, when I maxed them the keyboard makes a nice solid tone. Interesting thing is that the top right key makes a much higher pitch than the rest of the keys. Voltage on KBD CV OUT is 0.08 on all keys but the highest one that is 0.11. There is some logic there that eludes me.
TRIG OUT is 5.10. Routing KBD CV OUT to CV in just lowers the pitch slightly on the top key, not on the others.

I have tested all functions when sequenced and besides the keyboard tracking there is absolutely nothing wrong. Even the external signal processor tracks accurately on guitar.

I will check the test points when I have disassembled it again, needed it assembled for a project. I suppose I should pull everything and reflow ground points and verify all jacks on the patch bay. Only wish there was less nuts and bolts on this thing ;-)