Vintage Synth Repair

I recently acquired an ARP Omni I in pristine physical condition, not a scratch on it! When I turned it on, there was a drone, which I assumed was a stuck note. All the switch LEDs light up, and all the buttons appear to be working. I replaced all of the tantalum capacitors in the unit with electrolytic ones just last week, and now I get no sound from the synth. I also replaced the decoupling caps on each board, to no avail. 

I have a plentiful supply of some of the CMOS chips, but I figured I'd pick your collective brain to see if there were any places I should be looking to get this beauty back up and running?