I would have started by simply trying to isolate the playfield from the boards.
See if the control lead to the coil is testing to ground. If it is... you know it’s a short or up on the board. Disconnect the connector on the board and test again... if still to ground then you know it’s a short on the pf. If not... move further up the chain to the connector itself. Unplug and test the pins while the game is live.
A coil firing is simply the controlled lead going to ground... use that knowledge to test that condition... remember the transistors are just doing that job by cpu control.
Newtown, CT
