transistor Q15 on the interface board (solenoid driver board) is the transistor that controls the knocker coil (the schematic is a bit hard to read, its blurry)

there is a 7407 I.C. that drives that, the schematic just lists it as 13.

Diode D2 is between the 7407 and to the base of Q15

If you cant get it working and you want to send it up I will go over that board just like i did with the mpu and power supply for you.