Thank you, Rolf. Sorry for the long delay, other things came up that kept me away from this problem.
Now I understand. It's actually quite simple, the end-of-stroke switches on all 4 100-pt score reels are simply all wired in series. So the only way for the 100-pt relay to stay active is for a playfield switch to be stuck, or for the EOS switch to not close. And, in fact, when I looked closer, the 100-pt drum solenoid wasn't activating, meaning, of course, that the EOS switch is never closed. Which has to be related to the player unit. And now, of course, after poking on that for a minute it's working fine.
My other big problem was the bonus not scoring correctly, and the 5,000 pt events not scoring correctly. I had thought they were related and associated with the score motor, since in both cases it involves advancing the 1,000 pt score reel multiple time. But it looks like I actually have separate issues:
If I manually step up the bonus unit it advances by one until it reaches the maximum of 10 steps. But then when I do the same with the step-down (reset) solenoid it sometimes skips multiple teeth on the gear, moving several positions at once. There's probably something sticking in the ratchet mechanism, hopefully I'll figure that out soon.
Meanwhile, if anyone can explain how the 5,000-pt relay and the score motor work together, it might help me fix that problem, too.
And there are two other similar units under the playfield. I assume one is the "extra ball unit", but I don't know which it is, nor what the other one is. Neither is labeled, and I don't have a copy of the manual (and haven't been able to find one online). Do youknow what they are? I can post a picture if it would help.