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KORG MS-20 (Midi-fied)
MODS - Adding Internal Midi-to-CV Converter

| Synth Index | Korg MS-20 | MS-20 Mods1 | MS-20 Mods2 | MS-20 MIDI | MS-20 Midi Clock |

| Choice of Midi-CV | 9V Supply | MCV-4 Tampering | Midi Trigger Switch & LED | Notes |

Internal Midi-to-CV Converter INSIDE the Korg MS-20

Choice of Midi-to-CV Converter

Task: i wanted to install a small midi-to-CV converter inside my MS-20 so that i can have full Midi control of notes Pitch, On/Off, Pitchbend and additional midi CC's to Aux CV's to control at least the HPF and LPF.

Some people will choose Synhouse Midi-jack or Midi-jack II.  Problem is they don't handle pitchbend and also there's no Aux CV's, despite it's small and compact. (also, Midi-jack II Hz/V version has a very limited key range...
A Kenton Pro Solo is too expensive for this task and it only has 1 Aux CV.

Now that the Hz/V is sorted out (see "Proper Hz/V input"), so there are more choices in Midi-CV Converters.
Basically I needed a small compact model (that fits inside the MS-20) plus it has to have at least 2 Aux CV's.

Some Midi-CV Converters for modular synths would fit into this nicely. I haven't explored into these, and at the time of planning the Midi-CV, i wanted something relatively cheap and I rather not need to build (i.e. not kit form).

My choice was a Doepfer MCV-4.

The MCV-4 is pretty compact and it does CV/gate + 3 other Midi CC/Aftertouch to Aux CV's.

I got mine from a shop in Germany, slightly cheaper than getting it from the distributor in UK.

It has a Midi Learn button to set various parameters including Midi Channel and Aux CV Midi CC#, which is good.

It requires 9V DC power.


9v Supply
So first thing, I need to build a small step down to supply voltage to the MCV-4. The MS-20 has regulated +15V/GND/-15V thru out, so it's not difficult to tap a +15V from the PCBs and step it down to 9V.

Schematics & Stripboard:
+15V to 9V Using LM317
Click to Enlarge

I used a simple circuit using an LM317 regulator. It gives a solid 9V DC to power the MCV-4 and can also power other stuff that needs 9V.

The +15V/GND (and -15V which is not needed here) comes from the ESP Pots board (smallest) PCB, there's a connector (it's marked 2006) next to connector 2010.  On the PCB it says H6. This is the +15V/0V/-15V connector.


Simply solder wires to the underside of the PCB here to get +15V/0V/-15V!

I used a connector to connect it to my 9V board. I've also tapped the -15V for possible other future uses (see later).

Mounted the 9V board next to the transformer:
This will supply 9V DC to the MCV-4 (and maybe other stuff that would need 9V).

MCV-4 Tampering
Mounted the MCV-4 inside:
I did a little bit of tampering - removed the Gate out jack socket and soldered some wire to the PCB (so that i don't need to use jacks). These comes out to the jack sockets i mounted to the rear panel of the MS-20 (which are also tapped to the corresponding contacts on the PCBs):
MCV-4 MS-20
GATE (Midi On/Off) TRIG IN
CV1 (Midi Note Pitch) New modded V/Oct CV IN
CV2 (Midi Aftertouch) HPF CV IN
CV3 (Midi Volume) PWM IN
CV4 (Assignable CC, default=CC#01) LPF CV IN

There are also an Midi Activity LED indicator and the Midi Learn Switch. I removed the LED, tapped 2 wires there to a connector. Tapped another connector to the midi learn push button.

The LEARN SWITCH would need to got the front panel, preferably next to the CV Trimmer. So I drilled a hole there and mounted a new push switch:

Final looks:

Midi Trigger Switch & LED
I need to do something to the TRIG IN socket in the front, if the MCV-4 is active and since it's giving S-Trig (i.e. constant voltage when there's no activity) then, by just tapping to the TRIG IN socket would deactivate the internal keyboard triggering - since it won't go to 0V when a key is pressed, only midi in would trigger note on/off.  So another jack socket + switch tampering time.
This time an SPDT is needed.

Switching from external trigger from Midi and from the front panel jack socket. there's actually no need to tamper with the socket this time. there's a jump wire (J-25) next to the TRIG IN socket on the PCB.

This is what comes after the TRIG IN socket.

So one side of the switch goes to right side of J-25 (near the TRIG IN socket) as faced from component side above, the other side of the switch goes to the midi trig, and the centre goes to the left side of J-25.


There is enough space to mount an LED just between the J25 wire and the TRIG IN socket, originally used a chunky LED, and the holder was getting into the way a bit... i also mis-measured the damn thing, causing me having to enlarge the LED hole on the front panel, making it not look as nice.... i just kept reminding myself: it's functionalities that count, and not cosmetic....

In the end i ordered some LEDs from Farnell they're BLUE and mountable, just small enough and now my internal metal casing closes up properly.

The ultra miniature switch i used would fit in between the TRIG IN and OUT sockets (well, actually i have to trim the sides of the sockets a little). the it's a matter of measuring and making holes on the inside panel box and then on the front panel. It's difficult tho, but i managed all right.


So, this is how it looks like:
The rear TRIG IN socket i mounted at the Rear Panel needs to be re-configured. Since we cannot have 2 S-Trig going to one. So I re-wired the TRIG GATE IN socket s that when nothing is plugged in, the Midi TRIG will pass to the trigger switch on the front panel. If a jack is inserted, it'll break the Midi Trig and the external S-Trig from this rear socket will go to the switch instead....
The MCV-4 has a jumper switch to select V-trigger or S-Trigger. On mine, selecting S-Trigger does bugger all, it won't even trigger. So mine is configured as V-Trig BUT using program change in MIDI LEARN to reverse the polarity, so it's REVERSED V-Trigger.

The LED is therefore REVERSED - it actually goes OFF when there's midi note activity!

Since there's no switch to the CV, when using MIDI, the internal keyboard will act as a key transposer - i.e. it would transpose the incoming midi if you press any keys other than C1.
Also, vice versa, the Midi would transpose the internal keyboard.... maybe i need to add a switch to the CV In...



Next: Midi-Syncing the MS-20


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