I purchased an ARP-3604 keyboard (s/n 1620) on eBay that looked to be in fair condition. It had some mold but looked structurally sound. It was packed reasonably well but was significantly damaged during shipment as the following photos show!
I originally bought my Avatar for spare parts for my Odyssey but ended up restoring it. I picked up this keyboard to use as a controller for the Avatar. I restored it both electrically and mechanically and simply painted what was left of the vinyl covering.
I made a keyboard cable out of 3 coax cables and 3 individual wires for +/-15 volts and ground. I replaced the balanced output connector on the Avatar with an Amp/Tyco circular connector (#20606-1) for power and terminated the three coaxes in 1/8" plugs to match the Avatar inputs. I covered these wires with a braided wrap that really look nice.
The keyboard generated a trigger and gate but no CV. I traced this to a dead potted module which was simply a high impedance op amp used to 'hold' the CV. I replaced this with a LF398 true sample and hold circuit with a very long hold time.
This keyboard worked fine with the Avatar. When I used it with my MOTM, I discovered that my envelope generators would not function correctly. The keyboard trigger is generated from the gate so there is a delay between gate and trigger. This would cause the MOTM-800 envelope generators to start with the decay cycle and then begin the attack cycle when the trigger occurred. Although this was fairly quick it was quite noticeable. I added a CMOS flip-flop in the keyboard to synchronize the gate and trigger. Later when I designed my trigger delay module I modified my MOTM-800 modules to operate with a delayed trigger. Detailed information on this modification is on the DJB-008 page.
ARP-3604 keyboard modifications
I build a new wood cabinet to match my modular using the ARP enclosure as a template. I replaced all of the rubber bushings in the keyboard so the action is good and the keys are aligned. I interface this keyboard to my modular through my CV keyboard interface. My Kurzweil Expressionmate ribbon is attached to the top of the keyboard.