I developed an unusual problem with my Abra Ca Dabra: There are many items on the playfield that score 500 points - rollovers (which don't complete the sequence) and drop targets that aren't lit. 90 or 95% of the time, those items score correctly. But occasionally, instead of scoring 500 points, it may only score 300 or 400, and followed by the sound of a buzzing relay. At that point, if I hit any 500 point target or any rollover, it scores nothing. If I hit a lit target, it will score 1000 (or whatever the bonus value is) and that usually stops the buzzing relay.
Investigation has determined that buzzing is the M (100 point) relay in the backbox. When the problem occurs, that relay remains energized and won't release until I push the armature away with my finger.
I know that 500 points are scored in 100 point increments - so when the 500 point switch is closed, the 100 point relay will receive five pulses from the score motor. But when the problem occurs, only three or four of those pulses seem to go through (although the score motor turns the correct amount) before the relay stays energized and locks up.
I've cleaned and adjusted the switches in the M relay to no effect. I even took the relay apart and cleaned the armature (I had a stuck relay issue with Sure Shot and that was the solution as seen here: https://pinside.com/pinball/forum/topic/gottlieb-sure-shot-bonus-count-issue-#post-6084166). But that hasn't fixed the problem either.
Any suggestions?