Korg MS-20 CV, PWM & Filter Mods - cykong.com

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KORG MS-20 (Midi-fied)
MODS - CV & PWM
\| Synth Index \| Korg MS-20 \| MS-20 Mods1 \| MS-20 Mods2 \| MS-20 MIDI \| MS-20 Midi Clock \|
\| Proper V/Oct CV Input & Trimmer \| PWM Input\| Ext Filter Switch\| Filter 0V Unlock Switch \|

Proper V/Oct CV Input
Problem: The Korg MS-20 uses Hz/V as CV control instead of the more common V/Oct. Although, the TOTAL IN socket on the patch panel can be used as a V/Oct CV Input and by adjusting the MG/T.Ext knob, this is not as stable as a proper V/Oct CV in. Of course one could go use a Midi-CV Converter with Hz/V option, but, most of these e.g. Kenton ProSolo, would only come with 1 additional Midi CC -> Aux CV option, and as the MS-20 has HPF and LPF and more, I'd rather use a Midi->CV that has more controller options. A Kenton Pro2000 can do this but is way not economical for this task. Eventually, I would like to have midi inside my MS-20, with additional Aux CV's to control HPF and LPF.
On the The Old Korg Owner's Club mod page there are some mods for the MS-20 which include a CV IN tuning mod. I didn't try this, because I found a better mod that was suggested on Doepfer's german FAQ page and it says you can use their V/Oct Midi-to-CV with a CV Mod on the MS-20. It also provides a NEW V/OCT CV INPUT JACK.
Schematics: Click to Enlarge	As with the Old Korg owner's Club mod, the CV Tuner goes to Pin6 of IC4. It is actually not as bad, because this is connected to the one leg of the MASTER TUNE POT. So, the mod could be easily carried out without having to remove the Knob Panel (thank goodness)... Basically just solder a 100K resistor to the centre leg of the MASTER TUNE POT, then hook it up to a POT or TRIMMER, then to a CV INPUT JACK.

As usual, i wanted to keep things tidy and semi-detachable. There are extra non used PCB space at the side and i mounted a small connector there.

The new CV wire then goes to a 100K resistor then goes to the right side of the front panel, where i mounted a little 100K Trimmer. (It's also where I mount the MIDI LEARN button for the Midi Interface, more on that later).

What it looks like underneath:

It's easier to have a trimmer here, rather than a Pot. Initially I tried a Pot and mounted it on the back, and i found you could easily detune and move the pot by mistake. So a trimmer is a better solution, after you've tuned it, you can cover it up. Finer or further tuning could be done via the Master Tuning Pot.

So this is how it's like now, after all has been done and it's hidden under a protective cap.

PWM Input
The MS-20 has manual PW. Actually it has been discovered that there is actually a pad point for PWM, but probably not enuf space on the front patch panel for housing an extra socket. It's also documented on the The Old Korg Owner's Club mod page.
Some people mounted the PWM Input Jack Socket to the top of the MS-20. I didn't want to do anything to the top. I ran out of space on the front patch panel... but i do have space on the rear. So i thought, i could tap it to a socket at the back, and maybe tap a Midi Aux CV to it later on.
Schematics: Click to Enlarge	The PWM is Pad Point 56. On the schematics, it looks like you solder to the legs of R10 or R11. In fact it's actually easier, because pad point 56 is actually on one leg of the PW Pot. What I did was just tap a wire to the leg of the PW Pot. I dunno, this maybe semi PWM, since the original PW stuff is still in tact, but I really can't be bothered to do extra work on this if it involves removing the Pot panel!!
As usual, i wanted to keep things tidy and semi-detachable. There are extra non used PCB space at the side and i mounted a small connector there.

And the PWM Input Jack goes to the rear back panel:

(well, actually I can still use a long patchcord to route this back to the front panel)...

Filter External Control Semi or Full Mod
You can control the HPF and LPF with external CV. There are tricks from MS-20 users that if you plug the jack in half way, you have semi-external controls - i.e. the front knobs still works as well. Initially I tapped wires to the HPF/LPF sockets and route them to the back. This is just like inserting the jacks half-way. But, i like to have both options - so i can choose to have HALF External or FULL External controls of the filters. Well, I can have it by inserting an empty jack/patchcord to the HPF/LPF socket in the front to obtain full external control (since it's still tapped to external) but i rather have switches..... Sometimes, being too optimistic/idealistic could be a pain. there's a Chinese saying that goes "if I don't walk into hell, who would walk into hell?" meaning there's someone who would go do something people find hard or impossible. Well, my aim for modding is to add functions and make the synth work the way i wanted it to - so i feel i should go thru the pain in removing the 2x35 screws.... anyway, i wanna do midi and it'd require me to put extra stuff there, so i'd have to open it up.
So i went thru all the way to temper with the HPF/LPF sockets on the Patch Panel. For this mod, i just needed to replicate the switching action of the jack sockets. Since i have the External tapped to the TIP of the socket, basically it's just a matter of a SPST switch that acts like the inserting of the jack which breaks the signal on the switching action on the socket.
	So, de-solder the jack socket, bend up (or remove) the side-tip, so that it won't get re-inserted to the PCB. Then solder the socket back. 2 wires that tapped to the PCB, one to the tip (where i also have the external wire soldered to) and the other to the now blank side-tip solder point. These then go to an SPST switch.
i just realize there could be other ways of doing this, but what the heck......

There is just enough space on top of the patch panel for small switches, so it's be ideal to put these 2 switches above the HPF / LPF box drawing. BUT it might be troublesome if later i want to do additional work on the panel box, so these switches have to be easily removable if needed. So instead of mounting them directly to the front panel, i mounted them above the panel box as an extension:

(This is very DIY!) I needed some thin metal that I can use to support and mount the switches. It would be nice if i can mount the metal by screwing on the jack sockets rather than extra screws. So I found an old PC fan cooler that the fans are long not working, remove the fans and cut out parts of the metal grid, and luckily it fitted nicely!
So my final Filter Semi <-> Full switches look like this:

Semi External (Normal) to the left, and Full external to the right. One for HPF, one for LPF.

Filter External Control Sustain Mod
On The Old Korg Owner's Club mod page there's this simple VCF Sustain not lock to 0V mod. I didn't understand what it was, until i tried using external control of the HPF/LPF.
Problem: Whenever you release a key or trigger a new key, the HPF/LPF external control voltages would fall back down to 0V and restart from there....
Well this could be an interesting/kool filter falling effect. but what if i want proper external filter controls? This is what the VCF Sustain not lock to 0V mod is all about. By removing R171, the VCF sustain is unlocked from 0V.
Problems: 1. I don't want to remove the Knob panel to deal with the PCB. De-soldering R171 and let it fall out is too risky. 2. The filter fall effect is pretty nifty, i might use it sometimes, so i'd like to keep R171 there.... Solution: Add a switch that jumps/short R171.
Schematics: Click to Enlarge	R171 Location: Click to Enlarge	It's not difficult to find R171 on the main PCB. Since we're working from the underside of the PCB, it's basically 2 solder points next to the DECAY POT under an 8-pin IC (IC-20). Just solder 2 wires there to an SPST switch.
I could've mounted this near the VCF Semi/Full switches, but I actually did this switch after that and no way i'm re-opening up the patch panel! So I drilled a little hole on the front panel near the Release knob and mounted it there:

Next: Midi-fying the MS-20