Does the Add Player Unit relay (P) activate and hold for 1/3 motor revolution when the ball goes across the Trough switch?

On a 1 player game, when the ball drains (after some points have been scored) the Add Player Unit relay (P) should fire the Add Player Unit solenoid 4 times through these 4 circuits.
If that doesn't happen, starting with these switches on BX, P, U and I, Inspect and diagnose this circuit with Alligator clip jumper wires

https://pinside.com/pinball/forum/topic/gottlieb-big-shot-repair#post-6305153
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features
Example of a pinsider actually doing this https://pinside.com/pinball/forum/topic/red-baron-tech-question#post-5858156

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If the U relay drops mid game then this switch on AX or the hold switch on U most likely isn't making good contact.

pasted_image (resized).png

Let's make a guess based on little to no information that the problem is in the wiring or switches between the AX relay's switch that has a Red-White wire on one terminal and a Slate+Black wire on the other terminal, and the U relay coil. So thinking about the operation of the machine as a whole, we don't want the kick over of the first ball across the trough switch to advance from 1st to 2nd ball. So after the machine has reset and the 1st ball has kicked over the trough switch, connect a jumper wire, as you suggested, from the Red-White terminal on the AX relay switch to the Slate+White wire on the U relay coil.

If the U relay now stays activated, the Add Player Unit relay (P) now activates as it should, and probably the game plays correctly, then the problem is to the left of the AX relay's switch's Red-White wire on the schematic fragment in post #12 above. Most likely the AX relay's switch or the U relay's switch.

Assuming all that works out, the next step would be to make another guess about whether the problem is in the AX relay's switch or the U relay's switch, and place the jumper wire accordingly for the next test.

Good thinking!! Remember though that the schematic is drawn with the game reset and unplugged. So since the H relay is the Tilt Hold relay, with the game plugged in and reset, the H relay make/break switch you refer to is in the opposite position from how it's drawn on the schematic.

I don't know what you mean.

Check the U relay's hold switch then.

For a switch to work 3 things are necessary:
1) When open, there should be a small space between the contact points (duh)
2) When closing, the long blade's contact point should push the short blade's contact point enough to move the short blade
3) The contact points should be clean, which they usually will be if #2 is happening

I'm glad you got it fixed. And looking on the bright side, now you know how meticulous you have to be when diagnosing this kind of problem.