Apologies for not providing any further updates/ or pending mod descriptions for this project any longer. I describe many mods here (more than the classic mods for the original), including mistakes and spelling glitches, and feel I rather want to spend time on other synth modding/development projects.
Any appreciation as regards the info here should be directed to Pedro Eustache, who practically “made” me do this… Did you know that this guy has quite a long MS-20 history? So, off I went for a K2 in the hope to transfer some of the old mods and leave some mark of my own (how immodest of me).
Apart from the links above, classic resources are…
As regards innards, there are the usual suspects, as it were – stock 4580 and 074C op-amps, a bit of an uncommon wave selection system for VCO 2 (the rotary switch carries CV for an IC switch, while the VCO1 wave select switch carries actual audio), oodles of 411 switches for toggling between the two filter types, and the rest pretty much, I guess, following Korg’s lead.
The latter seems to be clearly the case for the VCA as this is not as per old Korg schematics but looks familiar to what you see in pictures of the insides of the new Korg MS20 Kit. Not sure if both have common ground somewhere else.
I haven’t A/Bd this with any of the Korg ones, so it’s hard for me to call out differences. There are vids, partly comparing apples and bicycles, but I guess if there are any substantial differences between the newer Korgs and this, then maybe on account of cumulative effects. For fun I changed amps in the old filter to 4558s and in the new filter to ne5332 and would not really do it again. Much more sonic differences lie in how hot you drive the filter inputs. Before I get ahead of myself…
The unit does not spit out or understand 1V/oct without alterations, and “gate,” just like the MS20, is actually an s-trigger signal. This can be easily changed though, as even an Doepfer page of old shows. Best use extra input sockets for CV/GATE. As fort the CV input, go for the pitch CV point pictured below. For the GATE to STRIG converter, wire your NPN transistor collector to the tip pin of the TRIG IN socket of the K2.
NB PW2 point pictured above is incorrect! (Please read PWM section below).
VCO Hard Sync: this is probably the easiest mod you’ll ever encounter. Wire a diode between the points marked sync out (anode) and sync in (cathode) via an on/off switch.
First pitch to VCO2 then hard sync is engaged:
PWM para tutti: for PWM on VCO1, wire some cv input socket via a 1k resistor to PWM1 point marked below. I use PWM on VCO one by means of an on/off switch that connects the LFO out socket tip lug via a 1k resistor to point PWM1. Sounds like this:
For PW/M on VCO2 is first tested going on the +IN of the comparator amp responsible for doing POWM, but found that I can have a much more usable range if I do this – wire a 100k pot with pin 3 to +15V and pin1 to GND and wiper to a 100k resistor. That resistor then goes to pin& of IC48 (-IN of op-amp comparator).
Individual wave outputs: feed any of the points marked below to the outside world via 1k resistors and output sockets. On mine, I use extra mixer inputs for noise and ring mod.
Selected Wave Outputs, i.e. VCO1/2 outputs can easily be tapped from the VCO Mixer pot pins (pin3 respectively) and just wired to output sockets via a 1k resistor each. Good for FM stuff!
Here’s some filter FM from VCO2:
Mixer section: the mixer bis a simple op-amp setup with 100k input resistors. Increase/decrease the volume of VCO1/2 by changing the respective resistors. You can also add more inputs by replicating the pot-100k resistor setup of VCO1/2 (see service manual).
On Pedro’s unit I went full tilt and have extra volume pots for noise, RM, and sub1/sub2 (extra sub oscillator running under VCO1).
What I also did is to wire EG2 per intensity knob to VCO2 – description will follow soon!
Listen to what’s possible in terms of easy filter variations! Two passes each – first OLD VCF, then NEW VCF, then NEW VCF in parallel mode (HP and LP in parallel instead of serial config), then added direct VCO to VCA signal…
Resonance boost: well, if you must! For the OLD VCF (FILTER1), decrease T163 and R177 to 4,7k each (4k3 if you wanna go bananas). Comparison:
For the NEW VCF (FILTER2): pull the resistor top terminals marked by yellow dots in the picture above via 10k resistors to ground. Comparison:
FILTER NEW parallel/Serial switch: description will follow real soon – easy mod for setting HP/KLP in parallel mode. Sounds like this:
FILTER NEW route VCO to VCA IN directly parallel to VCF sound: this is cool for vowely, phasery stuff.
Switch EG2 to VCA on/off: one annoying design aspect of the MS/K2 is that EG2 is always routed to the VCA. Yet, since this EG is more complex than EG1, you sometimes would like to have EG2-filter, EG1-vca. You can patch the latter but need to mod for having EG2 not affect the VCA. Do as per image: cut trace and rewire eg2 point via a 10k resistor and switch in series to VCA CV in point. You can, of course also use a toggle switch or rotary switch and route LFO or fixed voltage to that VCA point (drone action etc…).
Hotter VCF OLD into VCA: I like driving the VCA harder as this makes things a bit crunchier in a good way. With all the complex filter switches, you need to do this separately for filter old and filter new. For old, simply replace R14 with a 1k resistor.
Hotter VCF NEW into VCA: same can be done with R249 (68k, see mixer section pic above), which I reduced to 33k.
Demo of increased VCA input levels:
Sustain “fix” for EG2: the original Korg and any of the clones sport a peculiar design decision where EG2 resets to zero when sustain level is set to 50% or lower. This makes a couple of things weird but can be changed super easily. Thanks to paperCUT at MW, who pointed out that this EG can work like any other synth ADSR by removing R171 (100k) in the original. On the K” you can do this too!
Simply remove R355 on the front of the PCB…. or…. cut the trace between the SUSTAIN pot wiper and the upper VIA in the red circle as you see it in the picture below. NB the lower VIA in that red circle is the other terminal of R355, so in case you want to make this mod reversible/use a switch/jumper etc. simply reconnect things with a 100k resistor.
Faster LFO range: I swapped C220 (100nf) for a 47nf film box cap in my unit as I find the modulation generator to slow.