MS-20 Mini PCB Photos and OSC Sync Mod

Decided to take the plunge today and completely remove the MS-20 Mini PCB to see the component side. I couldn't just open up the MS-20 without a reason, so I decided to add in an OSC Sync switch. 

If you don't care for the details and just want to hear it, below is the sound clip of a PWM and Sync demo with some explanation.

I opened up the MS-20 as I've done in the past (remove 6 screws on the back and remove back plate)
I noticed the front panel was attached to the plastic sides so I just removed the screws holding in the front panel. I also removed all the knobs (gently pull them out) and the nut on the headphone jack.



To remove the metal bracket, I had to undo the nuts for a few of the knobs and undo the connector for the volume knob.



I was hoping Korg would do something like label the i/o for extra features like they did on the Monotribe and other products. There were no such markings on the PCB.

I did notice a lot of empty spaces on the board where parts were intended to be but left out. This is fairly common on PCBs, but usually there aren't THIS many!

SYNC Mod:
This was a fairly simple mod to figure out. Took maybe 10 minutes to find the solder points and it worked first time. 
I used this blog as a starting point for the mod: Remork Inc.'s Mod
I knew I had to locate the transistor cluster for the oscillators. I decided to solder to the diode pads since finding the proper pinouts for the transistors would be difficult due to their scant markings.

Oscillator transistors for VCO1 and VCO2

I located the diodes for the mod and soldered to proper pads



I soldered a 1n4148 diode to a switch and soldered it to the wires

Make sure the wire that goes to the diode on the switch goes to the diode for VCO1. The other wire goes to the diode of VCO2


I also patched out the raw signals for VCO1, VCO2, and VCO1's: square wave, triangle wave, and ramp waves. Maybe I'll make another post about that mod.

15 comments:

  1. Wow..i would like to have this mods on mine...Bravo!
    A question:the sync may be modulate by MG and EG2 or manually only?
    It would be possible also a simple mod like this:a switch to disengage the modulation for one oscillator?

    Regards
    Gianni

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    Replies
    1. For now, the sync is only modulated manually, but it will be easy to add a jack for CV modulation via the fine pitch knob (without removing the entire PCB)

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    2. Looking a little further into the extra parts, they kind of look like possible extra parts for an MS-50. The MS-50 had the same shape as the MS-20 so it's possible the PCB could be the same. But the locations of the jacks are a bit different. But hopeful thinking leans toward an MS-50. If that is the case, We can hope for the vocoder and SQ-10 too. Here's to hopeful thinking *beeeeeerrrr*

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  2. Hello there! I have a small problem with my MS-20 mini's modulation wheel. The values for "up" and "down" position differ and I would like to know which trimpots are responsible for it. I work in a music shop, but that doesn't help me much, because the technicians at KORG/Germany don't have the schematics yet. I hope you can help me, because it's the only thing that I don't like about my mini. Please send me an e-mail to: mail [at] pepemusic [dot] de

    Thanks in advance for your help!
    Pepe

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  3. I have to say a big THANK YOU. I modded mine yesterday. I would never had done this without your tutorials. OSC Sync switch is sick! its a must!!!
    Thanks again. you are the man :-)

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  5. i'm guessing the unpopulated components are for the MK2 MS-20 OTA filter.. check out the 6 diodes. (clipping diodes in the feedback path)

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  6. Thanks for the guide and pictures! For anyone thinking about doing this mod I highly recommend using shielded wire from the board to the switch/diode to prevent osc 2 from capacitively locking on to osc 1 when they are tuned close to each other and the switch is off. I used normal wire at first and had the aforementioned problem, but after re-doing the mod with thin RCA cable, soldering the copper shield to the sides of the scale select switches, it worked perfect.

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  7. I have to say a big THANK YOU. I modded mine yesterday. I would never had done this without your tutorials. OSC Sync switch is sick! its a must!!!
    Thanks again. you are the man :-)

    ReplyDelete
  8. It'be great to be able to get an AD for EG1, instead of the AR behavior.
    It would be handy for fast arpeggios.
    In fact, it's easy to get that response routing the Lfo pulse response to the EG1 retrig.

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  9. It'be great to be able to get an AD for EG1, instead of the AR behavior.
    It would be handy for fast arpeggios.
    In fact, it's easy to get that response routing the Lfo pulse response to the EG1 retrig.

    ReplyDelete
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  11. Great tutorial! Thanks a lot! if finished the mod; what would happen if you would route the lfo > vco 2 cv in. Doesn't that affect the syn? (to create sweeps and stuff?)

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  12. oh, and if it that would work, could you show/tell how you would do that? "via the fine pitch knob (without removing the entire PCB)" ?

    Thnx in advance

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  13. This blog helped me immensely to mod my MS-20 mini! Thank you so much!
    https://www.youtube.com/watch?v=Rr_9ogJY5mA

    ReplyDelete