So, the saga of the Rollergames magnet. My lovely tech tale.
Symptom: Magnet is locked on.
Parts replaced: Magnet (it was crispy) and driver board under playfield.
Maintenance: CPU was thoroughly inspected for dirt or other shorts. Q78 and Q79 tested as good according to my meter.
-Disconnected 1J19 on the CPU board, which disables special solenoids. Ran a wire directly from the high power driver board to Q79 tab. Magnet still locked on, confirming the problem is not in the wiring between CPU and magnet.
-Touched the wire to Q77 (lower jet bumper) instead. Magnet did not lock on.
-Hooked up a light bulb through the disconnected CPU driver. Ran the coil test and left it on Deep Freeze Magnet. Bulb pulsed on and off exactly as the magnet should. Bulb did not lock on.
So my only logical explanation is that Q79 and/or Q77 -tests- as good, and is capable of driving a low current LED, but as soon as you hook it up to the 50V magnet, it loses all confidence and locks on. Is this possible, or are there any better ideas than replacing a seemingly good driver+predriver transistor?