Quoted from justininia:Is there any way to test back to the board to make sure everything is working correctly?
If you take a piece of wire with an alligator clip at each end....clip one end to the "tab" at the top of the transistor Q53 (I'm trusting you that's the correct one).
With power on to the machine, briefly touch the other end of the wire to ground. This will complete the ground circuit and the coil in question should fire (5 ohms should be fine as tested).
If the knocker coil fires, this tells you that all of the wiring, connectors, mechanical things, etc. from the point of that transistor to the coil are fine. If that's the case, I would replace the transistor and the pre-driver transistor (I assume there is one).
If the coil does not fire when you ground the tab, you've got a problem somewhere else - between that transistor and the coil itself. Wiring, connectors, bad coil, etc. Fix that first before you deal with the transistor.
This test doesn't tell you if the transistor is bad or not - it simply confirms the "flow" through the machine. But it will isolate the existence of a board problem versus a playfield or connector problem.