Mystery Synth - Korg 700s/770 Prototype?

Published on Apr 15, 2013

"I found this "mystery synth" at a garage sale this weekend. It appears very similar to a Korg 700s, but upon closer examination there are clear differences. The second oscillator has it's own waveform and pitch selector, for example. And the style of sliders and controls are certainly different than those found on a 700s.

If you know anything about this instrument, please let me know!

At any rate it's a great sounding synth!"

This one in via via Dan Goldstein who found the synth. Talk about an amazing find.
Click here for a pic of the inside of a Mini-KORG 700.  Update: also click here for one more.  Note two of the boards appear to match but this mystery synth has one more. I couldn't find any of the inside of a 770 to compare.  If you search for KORG 770 or KORG 700s you will see the controls are similar but different.  With that we are left with two possibilities, this is either a very nicely done DIY synth composed of vintage KORG parts or a KORG prototype that fell somewhere within the 700 series.



And details also posted on gearslutz: "Here's everything I know about it:

I found this instrument on CraigsList, posted at a garage sale here in Las Vegas. The seller said that he'd purchased it on eBay not knowing what it was, and that he believed it was "built by a guy that worked at Ibanez." That was all the provenance that he was able to provide. The instrument was clearly so bizarre that I had to take it home and find out more about it. It does work, mostly, and it certainly has a unique sound despite it's incredibly strange architecture, which I'll describe below.

I opened it up, eagerly hoping for clues about its origins, but what I found deepened the mystery further. As you can see in the photos below, the circuity of this synthesizer is totally discreet. There are some metal can-style Op Amps, namely 741HC and MN131A, which would seem to date this in the early 70s. There are no markings of any sort on the circuit boards, just hand-labeled numbers by the patch points on the circuit boards. I can't find any sort of name anywhere, though the words "Made In Japan" are on the 1/4" output panel.

The architecture of this instrument is incredibly bizarre. It features two standard oscillators. Oscillator one has triangle, square, and sawtooth waveforms, plus a "Chorus 1" and "Chorus 2" setting that seem to be chorused sawtooth waves. Oscillator two features triangle, square, sawtooth, a thinner square wave, and reverse sawtooth. Then there's a third sort of oscillator, which is switchable between noise and what I think is some sort of ring-modulator that operates on Oscillator One. There's a global tuning knob but no fine-tune knobs for the oscillators.

Things get weird after that. There's an attack-release envelope for the VCA, and that's the only envelope you get. There's a resonant low-pass filter and a resonant high-pass filter, but no resonance knob, so you're stuck with a single cutoff control for each filter. The low and high cutoff filters are sort of "ganged" together so that you can't move the Lowpass cutoff above the Highpass cutoff - they move together once they meet, if that makes sense. There's an LFO that seems to be fixed at a triangle wave, and it can be routed to frequency or filter or both, and there's some sort of vibrato delay switch that doesn't seem to do anything. There's also a Repeat switch and speed control that will cause the envelope to retrigger, which is pretty cool. There are some other bizarre controls too, including a "Bright" switch that makes the sound brighter, an "Expand" switch that doesn't really do much of anything, and a "Bender" switch that causes notes to quickly bend up to the pressed key (I remember a similar feature on the Roland VP330, for example).

There's a 3-position sustain switch that goes from "Short" to "Long" and determines if the envelope continues after you've released a key. It works fine, but as soon as you release a key the frequency of the note played drops to some lower, random value. Perhaps the sample-and-hold is not working right.

The cabinet and design is clearly Minimoog inspired, but it's not a Minimoog case. My first thought was that this was a home-made synth of some kind but I'm very doubtful after spending time with it. The silkscreening is very professionally done, the cabinet is all custom-made with interesting angles. There's even a sort of thumb screw on the bottom that lets you lock down the folding-up synth portion of the instrument for transport. Everything about the instrument seems to suggest it was professionally made. The only outputs are a Low and High audio output. There are no inputs at all, no bend wheel or mod wheel, though there's clearly space for such a thing. The lack of basic features (i.e. sustain & release envelope stages, fine tune, resonance amount, etc.) and the addition of unusual features (repeat, bender, etc.) make for one odd combination. And the fact that I can't find any label anywhere makes this a serious synthesizer mystery.

I've owned an awful lot of analog synthesizers and have read decades worth of magazines and web sites, and I've never seen anything like this. Does anyone here have any insight at all into what this might be? Perhaps it's a kit synth from the 70s, like a PAIA design of some kind? Perhaps this was a prototype for a product that never got built? The components date it to the early 70s, so perhaps some company was attempting to challenge the Minimoog and decided against it? If anyone has any information at all on what this could be, I'd sure appreciate it! I'm happy to answer any questions about this synth."