I am repairing a T.A. for a friend. So far everything works, but if a one player game is over, after the 5th ball, the 4th player in the backbox is lit, and another ball is ejected for the 4th player. This ball will score, and when it drains, the game is finally over (instead of after the 1st and only player). This also happens with two or three player games. I have the schematics and can read it, but i would be grateful for a hint where to look here (unsure if i should look at a relay in the bottom first or rather the player unit in the backbox, or even one of the score motor switches?). I do not want to fiddle with too many switches at once.

Wow such good know-how here, thank you all. As for the 3/5 ball game, i have tried that, it behaves the same with 5 or 3 balls. The other hints are great, i will try that tomorrow (the game is in a private event hall, i do not have it in my workroom) and report back with the findings. Greetings from Switzerland.

It almost works. Meaning that sometimes its normal game over and sometimes it behaves as with the fault above. After the last ball it always checks the bonus unit first(with double bonus since it is the last ball) and when that has finished, the BX relay is activated. I have made a video of the bx relay in this moment. It seems to me that it flutters a bit (pulls in, then very quickly resets and pulls in again). Could it be that a score motor switchs timing is a bit off? I have just now realized that i cannot upload a short video here.

Thank you HowardR your hints are very helpful. I put up my (very first) youtube video of the BX relay here:

I also have checked the bxr reset coil with a test lamp and it does not reset the BX at this time. Therefore, seeing that O and the player unit and the coin unit and DX and E are in the path of current to BX, should i look at the coin unit first? It seems to me that the O relay is adjusted and clean. But the mentioned units have not been touched in a long time, i think. I also noticed that the correct behaviour is usually when i have raised the playfield to watch, but when i have lowered it the fault appears more often. Maybe there is a broken wire involved in the moved cabling.

Addendum: After some time, the problem has disappeared. I had cleaned several relay contacts, among them the AX and BX relays. However i cannot tell what was the root cause of this erratic problem.