This was the first ever Korg synthesizer, the 700, released in 1973 (Gordon Reid puts this in perspective). It (and its relatives the 700S and 800DV) sounds like nothing else before or since, due to quirks in its electronic architecture. When I found mine, over ten years ago, I had a very rudimentary understanding of electronics, and so I enlisted a mate to design for me an interface that would allow external control of the VCO pitch and gate, and the two filters. I didn't have a scope back then, but I had a barely-legible circuit diagram, and I made a lot of measurements with my multimeter while fiddling with the synth to try to understand what was going on.
The previous owner had installed a modification: a five pin din jack on the rear panel, with a switch, marked "Accordion". In the documents that came with the synth was a copy of a letter this guy had received from Keio Electronic Laboratories dated August 6 1975 in response to his enquiry on how to interface his accordion with their synth: they diagrammed a resistor ladder to be accessed by the keys, and also helpfully offered to sell him the resistors at 50 yen per piece!! (Presumably this worked quite well, as I found the "accordion" din jack on another of his synths, but that's another story..). I ended up keeping the switch and the din jack for use in my external CV mod.
External CV and Gate mod
Basically what happens is that the keyboard divides a voltage down from the 20 volt supply, and this CV value both determines the note from the VCO, and triggers the envelope. The three octave keyboard goes from 2.5 volts at the lowest 'C" to 20 volts at the highest "C". So any external control has to be able to provide a "gated pitch CV" as a single value essentially, whereas most systems use separate pitch and gate CVs, whether they be volt-per-octave (V/Oct) or hertz-per-volt (Hz/V) systems. My friend Malcolm came up with a system using two op-amps of an LM324 quad package (the other two were used for filter control). My diagram from the time is shown below - cute, isn't it? As I say, ten years ago I probably didn't understand fully what it was doing - but it worked!
The external 5 volt gate signal appearing at pin 2 of the IC "switches on", via the comparator and the diode, the pitch CV present at pin 12, coming from my Kenton Pro-2000 (Hz/V scaling). This CV is then presented (via the switch) to point 38 on the voice PCB. The IC seems to work fine off the 20 volt rail from the PSU. The external CVs for the filters are scaled and passed to "No. 3" and "No. 6" which refer to the points as clearly marked on the filter PCB.
Notice I refer to "cutoff" and "res" in the diagram. Clearly I was labouring under the delusion that the synth used a conventional single resonant low-pass filter like the Rolands I was more familiar with at the time. It doesn't - and this is they key to its distinctive sound character. It uses two filters in series with fixed (well, almost..) resonance and variable cutoff - represented by the high and low "Traveler" sliders on the front panel. The connection to pin 3 on the internal PCB should be labelled "Low-pass filter" and pin 6 should be "High-pass filter".
In practice, probably the main reason this mod was so usable was because Kenton make such bloody good midi-cv converters! To get the range of the VCO to match a midi controller keyboard, I had to transpose the converter up 19 semitones - I don't know if many MCVCs would let you do that. This matches the bottom C of the internal keys to midi note #60, middle C, and gives a usable range of three octaves and five semitones.
Above: the interface pcb mounted on the rear panel of the synth, next to switches and minijacks for the filter CVs. Just in front of this is the minikorg's voice PCB, the big black chunky "Keio IC" is the VCO. Along the top edge is a row of trimmers for tuning and scaling the key CV etc. The board to the right of this is the filter board, with more trimmers.
The switch above the din jack (obscured) the previous owner had installed, to switch between internal and external key control.