Reposting as the original was over a year ago and became too disjointed over time. So, problem is:

When all drop targets are down, and special lanes light. The add a game coil sticks, buzzes and the coil plunger jumps up and down. Also, the switch just below and in front of the coil arcs blue green sparks. But, hit any 100 point target on play field and it stops. Then hit 100 point again and it starts again. Or, manually engage the alternating relay switch set and it toggles the buzzing / stuck coil on and off. Also found that if the "bottom roll over adjustment" jones plug is reset from conservative to liberal, the problem still happens and hitting any 100 points doesn't toggle it off and on.

Actions taken to date to try to fix it, on the assumption that a switch / contact is open or closed when it shouldn't be: Checked every 100 point scoring play field switch multiple times. Followed schematic wiring to check every switch / contact leading to or from the add a game coil and the deduct a game coil. Checked every switch / contact on every score reel, the 100's and 10's at least twice. Checked, cleaned, gapped every playfield switch / contact of any kind. Double checked bottom roll over switches due to jones plug change mentioned above. Checked every switch contact on the score motor and on the alternating relay.

What am I missing? What pics would you like to see? (I mean of the game, get your minds out of the gutter)

Thanks, I thought so too. Now that I think about it will try to post a picture or video later tonight after work.

Pictures of the credit unit when the buzzing is happening. And the playfield when it is happening. You can see the arch of the switch and in one pic the plunger that is in motion. The playfield shows the special lights lit and all the drop targets down. Any 100 point score toggles the buzzing and arc'ing off and on. Even in the two upper in lanes, when a lane is lit 100 points it toggles, when it is unlit and 10 points it doesn't. I also have video but not sure how to post it on pinside.

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Finally, 11:30 last night. Found the problem and solved. Upper left corner playfield roll over has two switches stacked. Somehow had twice missed that the top switch was just barely making contact when it was supposed to be open. Deaf dumb and blind!!!!

Thanks, other than that some idiot painted the cab white, in really clean shape and back glass almost perfect.