Shorted electrolytic cap across the switch. Shorted TIP-102 on the driver board. Bad TTL 7402/7408 on driver board.

Start at the solenoid and check/replace the electrolytic cap across the solenoid switch.

Next test the TIP driver transistor. Power off. DMM set to diode test. Black lead on middle leg/metal tab of associated TIP transistor. Probe the outer legs with the red lead. You should see .4 to .6v. If not replace the tip driver transistor.

Next test the associated 7402 and 7408 TTL ics. Power off. DMM set to diode test. Red lead on ground. Black lead touch the associated pins on the 7402 and 7408 ICs. Again you should see .4v to .6v. A lower or short is indication of bad TTL chip. Use the schematic on IPDB to find the associated 7402/7408 pins.

Remember, pop bumpers in williams games are "special solenoids". The scoring switch circuit is completely isolated from the driver circuits. Whether or not it scores will have nothing to do with why it locks on.

If it was a WMS factory driver board i could help you better. I assume rottendog drives things in a similar fashion, but i really have no clue.

That is the scoring switch with the broken blade. Your pop bumper in theory should still pop with that blade missing, just receive no points. That electrolytic cap, barrel shaped thing, in top left of picture is the cap to check/replace.