It was the first module of this gear and I made it with no knowledge about microcontrollers, hence it contains way too much ICs. It scans 60 (not 61!) keys 100 times per second with large 4051-based multiplexed. Digital serial data containing key number and its state is sent to as-I-call-it Generator Controller. Likewise it is also discrete digital design allowing 8 voices control.

See the schematics, and timing of output signals - example while first C key pressed.