Working on a 1972 Fireball for a friend (who lives an hour away), and during play, the 100 pt relay in the head is intermittently locking on. Maybe one out of 3 hits makes it lock on. I've attached the relevant part of the schematic and want to make sure I'm understanding this correctly so I can fix it for him on the next visit. I think I've circled (in red) all the pf switches that turn on the 100 pt relay on the left part of the schematic. The hold switch is circled in green on the right side, and the EOS switches on the drum units that should break the circuit and relax the relay are also in green on the right. I've examined the EOS switch on the player 1 100's reel very carefully. It's missing a contact (which I'll replace), but I don't think that is affecting its operation - with a meter, it measures shorted or open when it's closed or open. There's nothing wrong visually with the EOS switch. So I'm pretty sure it's opening - which to me means that one of the playfield switches on the left is still closed and keeping the relay energized. Is that correct? I suppose a short somewhere else could do it as well?

We also noticed that when the 100 pt relay is locked on, energizing the 1000 pt relay (with a pf switch) seems to consistently relax it. I can't figure out what that means, but perhaps it's a clue. I've attached the 1000 pt relay circuit as well, in case that helps.

Thanks in advance for any ideas.

Fireball problem marked (resized).jpgg 1000 pt relay circuit (resized).JPG

Thanks phil-lee - I'll check all those out when I'm there next week.

Good ideas, Mark - thank you. I only know two answers to those questions right now:

It does happen for all 4 players.
It does happen when I manually activate the relay armature.

I had some trouble identifying switches on the 100 pt relay because the wire colors are so washed out, but while it was locked on, I did open each switch on the relay manually with a tiny plastic screwdriver, and it didn't seem to matter. But if I can identify the hold switch, I'll block that one with some paper and see what happens.

The answers to the other questions, I don't know yet. I won't see the game until next Wednesday, so when I'm with the machine, I'll look at the yellow pop bumpers and block the hold switch. I'll look carefully at those playfield switch relays as well.

Thank you!

Thanks Skidave - I wondered about residual magnetism. Turning the power off and back on did reset everything - but I didn't actually look at the relay once the power was off to make sure that it had released. I was assuming that it did release since the problem was gone when the game restarted, but I'll look before I turn the power back on to be sure next time I'm at the machine. I'm not above a piece of electrical tape, if that fixes the problem. Thanks for mentioning that.

Next Wednesday...