Korg MS20 calibration

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Korg MS20 calibration

Postby Yekuku » Mon Apr 14, 2014 9:20 am

hi guys, I am done recapping my MS20 and I am looking for info about how to calibrate trimmers VR1,VR2,VR3 that are found on the "jacks board" klm-128C. they seem to affect S&H, MVCA and noise but I cant find any calibration procedure in the service manual.
any ideas?

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Re: Korg MS20 calibration

Postby dstrength » Mon Apr 14, 2014 7:32 pm

Though I haven't adjusted them on my Ms-20, from looking at the schematics, I'm guessing they are for centering up the outputs of each section.

VR1 feeds the offset terminals on that weird CA3140 op-amp voltage follower in the S&H. I think you can adjust it to center up the 5V pk-pk output.

VR2 adjusts the gain of the inverting output buffer in the Modulation VCA. Try feeding it a 5V audio signal (vco) and a 5V CV, then adjust the output for 5V pk-pk.

VR3 adjusts the pk-pk output of the noise source. Again, adjust for 5v pk-pk.
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Re: Korg MS20 calibration

Postby Yekuku » Mon Apr 14, 2014 8:35 pm

Many thanks for the help Dstrength, I appreciate it.
I have noticed that there are 4 more undocumented trimmers , VR1 and VR2 on the signal processor board ( KLM-129B) and VR1 and VR2 on the filter daughterboard (KLM-307A).
Any ideas about how to calibrate these too ?
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Re: Korg MS20 calibration

Postby dstrength » Mon Apr 14, 2014 11:23 pm

Oh man, the pitch to voltage section is over my head. And the series of six invertors? WTF! On the ESP board, VR1 is another of those offsets on a voltage follower op amp, TL081. My guess is VR1 is for trimming the zero output state. VR2 I have no idea.

On the filter, my MS-20 has the Korg35s, so no daughter board. The schematic is here:

http://machines.hyperreal.org/manufacturers/Korg/MS-synths/schematics/KLM-307.GIF


Those two trimmers near the diode chains are for setting the Max resonance points: the resonance at which the filter oscillates. This should be similar to the FC adjustment procedure. This gives you the ability to change where on the panel resonance (Peak in MS-20-glish) pot will lead to self oscillation. Set the filter Frequency around 5, VCO levels at 0, the dial up the resonance to see where it starts to oscillate. The manual suggests going for 500hz at a level of 10. But if your into the acid sound thing, you might want to give yourself some more room. Say, start oscillation around 7 or so.
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Re: Korg MS20 calibration

Postby Yekuku » Wed Apr 16, 2014 9:39 am

Many thanks for the info and the support.
I will try the calibration later this weekend and I will keep you updated with the results.
thanks again.
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