ballys solenoid expander board might be a early version of williams A/C relay. what this relay does is expand the ability of the solenoid driver board to fire more solenoids or flashers(with resistor boards). a solenoid needs two things to work , a power line that fires the coil from the driver board , and a common return line to complete the circuit. most machines solenoids work on a common return line that are daisy chained from one coil to another. in this case some groups of coils have a dedicated common return line that are wired to one side of the solenoid expander relay , and another group of coils to the other. by doing this some driver transistors can fire 2 different coils , but the switched common return lines of the solenoid expander board will only allow 1 coil to fire. and the noise you hear coming from the solenoid expander board is the relay clicking off and on. if you see a light near the solenoid expander board that looks like it does nothing. this light thats driven by the light driver board , is used to switch the board on with the use of a photo receiver. so in the self test mode coil test the solenoid expander board will only pull in when it needs to fire a certain coil and may not pull in to fire other coils.
