Regarding my similar coil issue, I did some exploring under my cab last night and noticed the coil diode on the pop bumper coil vibrated loose from the solder on one of the lugs. I read in another thread somewhere that on this era Williams machines, without the coil diode intact (to prevent voltage spike) on a special solenoid that one single pulse of the pop bumper coil could essentially fry the driver transistor. I did indeed notice that when I replaced the TIP 102 and turned the game on for the FIRST TIME, the coil was not locked, but the moment the pop bumper skirt triggered the coil it immediately locked on and was permanently locked on thereafter, even after shutting the machine off and turning back on. After this happened I tried yet another brand new TIP 102 and the exact same thing happened, no lock until the coil was triggered once, then it was stuck locked on. I figure I just fried 2 brand new TIP102's.
I haven't had a chance to replace the diode, but does this sound like a plausible explanation for my problem?