Build a simple 8-step sequencer based off the CD4022 IC. The sequencer is clocked by the MS-20's LFO square output
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!
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.
Im not sure, but I'm pretty sure all this would void your warranty.
Opening the MS-20 mini is fairly simple:
Remove the 6 screws on the back
Take caution and remove the black coated metal clips holding in the beads
Gently remove the 2 cables from the jacks
Now you can remove the back completely and work inside
To install the PWM CV In jack:
Locate the test point labeled TP-13
The three pins above the test point are the potentiometer pins for the PWM knobs.
The pin directly above the test point is where you want to solder the wire for the PWM CV In.
Locate a ground via like the one pictured below. Solder a ground point to the via and solder a wire to it.
Solder the two wires to a jack as shown below
Now you have a Jack to send in CV signals to control your pulse width's modulation.
The Passive 4X Multiplier is simple:
Solder 4 jacks together. Same pins on each of the four. You can do as many or as little as you would want.
Many more to come!!