You may need the schematic to find this fault. SYS80 has a sound board and a power supply that power the sound board. You need to check voltages at the sound board connector.
Pin 1 - +12VDC
Pin 3 - -12VDC
Pin 5 - +5VDC
Pin 10 - +30VDC
Pin 6 - GND
Pin 4 is one of the signal inputs. But you need to check voltages to the Sound Board as the first step.
