**UPDATE: What I initially thought was simple laziness and/or lack of soldering ability by a previous owner actually turned out to have been a temporary mitigation of a legitimate issue (fried transistor), which I have now repaired.
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I considered putting this under "worst hacks", but it's not even a hack. Just a big ol' facepalm.

I picked up a Black Knight 2000 last weekend. After resurrecting it by replacing the corroded battery holder, I went through the diagnostics. On the solenoid test, I found one of the upper playfield pop bumpers wasn't firing.

While I had the playfield up today, I decided to peek at the hardware for that pop bumper for anything obvious.

Well, sure, here's why it's not working ... the wires appears to have broken off:
UPF-LPB-1_(resized).jpg

So ... where are the loose ends of the wire? Looked around up in there, found a pair of red/white wires tucked up out of sight, pulled them down ... oh, hello!
UPF-LPB-2_(resized).jpg

...I guess two-outta-three functional pop bumpers was "good enough" for them.

Good thought, especially looking more closely at the "break" -- definitely a cut, seeing how well the inner strands line up.

Should I solder it back on to see how it behaves, to better troubleshoot?

Well, well, well ... looky here:

UPF-LPB-3_(resized).jpg

This is Q75, which I found in the schematics goes to the "snipped" pop bumper. Fantastic, thanks for the tip!

So now, a technical question: I have some spare TIP122 transistors here among my repair parts ... but the schematic has a note related to Q75, which reads, "SPECIAL SELECTED TIP122 FOR WILLIAMS, P/N 5162-09410-00" ... what's special about this? Are my "standard" TIP122 transistors no good to replace Q75?

Oh sweet, the ones I have are 102, not 122. Will do some surgery later when I get to it.

Re: Q74 -- measuring in diode mode from middle to either outside leg yields approx same value, and both outside legs together gives a 1.5xxsomething value (no beep), so I think this one's okay.

Ok I'll let you know how it goes after transistor R&R surgery -- not sure when I'll get to it. Stay tuned!

Good tips, thanks. Presumably, the resistance should be equivalent to the other two bumpers' coils, so I'll have a look at that when I go diving back in there. Had to close up the machine for the day due to other priorities.

Also, shameless plug for more help, please see also my switch matrix issue thread: https://pinside.com/pinball/forum/topic/1989-black-knight-2000-switch-matrix-entire-column-out

Ok ... checked out the coil resistance and manual functionality, compared it to the other two, everything seemed to be fine.

Had a few minutes to do the surgery -- wife went out for a bit and the kids are watching a movie. Removed the old transistor, installed new one, reinstalled board, re-connected the bumper wires, and fired it up.

Bumper did NOT lock on immediately. Good sign.
Went through coil test, bumper operates properly.
Went to switch edge test. Switch operates properly.

SUCCESS!!! Thank you all!

Boy am I glad I posted what I thought would just be funny. Thanks guys! Now onto my Switch Matrix Column issue.

Yup, you'll see when you read that one later, just finished doing that a few minutes ago. Good continuity from all switches back to the connector in the backbox. Let's have all future discussion on that one over in that thread. Thanks!