Got the schematic ?

I'd be checking the circuit and finding out if the Bonus count unit has a set of switches that close when it reaches "zero".  There's also the switch located in the ball trough (under the apron). From memory, the ball count unit is advanced when the ball rolls over this switch.

I have the schematics. re the trough switch when this is operated it resets targets and the switch bank but not ball count unit, this unit does turn when a game is started. Im pretty sure its a switch somewhere and ill check the bonus switches