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Keio Electronic Laboratories "Minikorg" 700 synthesizer mods

(December 2009)

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.


interface install

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.


interface close up




switchThe switch above the din jack (obscured) the previous owner had installed, to switch between internal and external key control.


Recent Investigations

When these mods were done, I wasn't on the internet and had no access to any third party info regarding this synth. Since then, I've come across C Y Kong's site, which details his mods to a couple of 700 synths and also the mighty 800DV. He mentions that some external key CV retrofits don't gain access to the "effects" available from the synths own keyboard, i.e. REPEAT, BENDER, and PORTAMENTO. The mod I've documented above does give access to bender and portamento, but not the "repeat" function, which is a bit like a variable-rate tremolo except that it actually re-triggers the note each cycle. It allows the simulation of, say, the picking of a mandolin, with just holding a single note.

Recently I opened her up again to investigate what was happening here. It turns out that in disconnecting the 20 volt rail that the keyboard uses to derive a note CV from, one disconnects that CV from the repeat function. This function (from point 11 on the filter PCB) is provided by a twin transistor multivibrator, which swings the voltage between 20v and zero and back in a pulse wave whose duty cycle varies with rate - at the fastest rate it's pretty much a square wave, at the slowest it's a brief pulse to zero. When it swings back to 20 volts, it triggers the envelope again. So when you switch repeat "on" at the front panel, it is this pulsed 20 volts that the keyboard sees, to divide the note from, thus external CVs can't access it. I'm not too fazed by this, as I find, of all the key functions available, REPEAT is rarely needed in use, and can easily be simulated by programming rapid staccato notes in a sequencer.

One other thing of interest I found at C Y Kong's site was this diagram for external audio input to the filter. It's the sort of thing I wouldn't use that much, but it was so easy I couldn't resist. I converted the low voltage audio output to a switched mono input jack for audio. It needs quite a healthy signal to work well.


external audio in


Above: with just the top rear panel removed, access to the trimmers on the voice board is possible to allow scaling and tuning adjustment to match the external CV.

The 700/800DV synth circuits: a thought.

Gordon Reid calls the 800DV one of the greatest monosynths ever built. It is basically two 700 synths in one case, with ring modulation and some sophisticated keying options thrown in. Having used one for a couple of years I can attest to this: it can be a MONSTER!

Check out C Y Kong's page regarding this synth. Now have a look at this circuit diagram. The colour coding and labelling allows one to easily follow what is happening. Have a look at the subcircuits - so simple! Just a few trannies. I think these babies are just begging to be cloned, maybe in a modular approach. It would need a custom PSU to provide +20 to -10 volts. Colin Fraser has generously provided us with the secrets of the filter IC. Now all we need is the VCO!