I was wondering if somebody might be able to help suggest what to try here. One of the four ADSRs is broken in mine, the last one. The first thing I checked was the voltages present at the IC supply pins, and immediately noticed a problem. The +12V supply is present at the header and R15, but everywhere else appears as about -0.6V. So the TL084 and 555 and powered between -0.6V and -12V, or -0.6V and ground.