Synergy DK

via this auction.

Update via the comments regarding the details that follow: "It was a Crumar GDS that was used on Carlos' TRON soundtrack, not a Synergy. Also, I'm not aware of that Larry Fast ever used a Synergy. That was just his artist name."

Details:
"Here is your chance to own one of the rarest keyboards in existence (there are reportedly only about 100 working models left in the world!) as used by Wendy Carlos (Tron and "Beauty in the Beast") and Larry Fast. This particular model has the MIDI in/out/thru on it, and looking inside, seems to have been done by a skilled technician (or perhaps the factory). This means it is the II+ model. The sounds are very digital in nature, but have a nice warm tone to them. There are plenty of electric pianos and strings, as well as some neat digital drum sounds. The keyboards velocity sensitivity drastically change the sounds, and this is part of the nature of the Synergy's programming.

I also have 14 original sound cartridges with this, 4 of which seem to be blank. The cartridges include: VCART 1-3 "Voices", CARLOS WC-1 "ORCH-PLUS", CARLOS WC-2 "Bells and Whistles", CARLOS WC-3 "Missing Links", DEMO "Demonstration of internal voices" and TCART1 "Processor and Panel Tests" and TCART2 "Oscillator and D/A tests"... These are included in the auction.

From the web: The Synergy is a Phase Modulation/Additive synthesizer and has lots of DX type FM sounds. The Phase Modulation algorithms are user defined with up to 16 oscillators available.
The envelopes have up to 16 stages/per osc, and are loopable. Program/sounds consists of 2 'boundaries' which are 2 separate timbres. The keyboardist can morph between these timbres via velocity and key number. The DK SYNERGY II synthesizer originally retailed for around Six Thousand 6$k (Thanks matrixsynth.com!)

Also from the web:

http://users.ece.gatech.edu/lanterma/synergy/
http://www.synthmuseum.com/synergy/index.html
http://www.synthony.com/vintage/dkigds.html

The rear of the Synergy has 1/4" unbalanced outputs 1 and 2, and balanced XLR outputs 1 and 2. There is also RS-232 output for interfacing to a Kaypro computer for programming. The knobs on the front offer user control of the sounds, but in order to really dive into this synthesizer, you need the Kaypro."