If the Player Unit Stepup solenoid doesn't activate in a 2+ player game when the ball drains, Inspect and diagnose this circuit with Alligator clip jumper wires
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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Because the Ball Count Unit is stepping up, the contacts in the Outhole Relay, Extra Ball Relay,
and Ball Index Relay have to be doing their job. The thing is, with the Coin Unit stepped up to
two or more players, the Ball Count Unit shouldn't step up after the Player 1 has drained the
ball. I'd maybe recheck the Player's Step Up Unit's wiper and wiper board, but even more likely,
the Coin Step Up Unit's wiper and wiper board..
When playing a 2 Player game, the circuitry should not be completed to the Ball Count Step Up
until the Player Step Up is on player 2 and then the ball is drained. The Player Step Up and Coin
Step Up needs to be aligned to complete the circuit to the Ball Count Step Up. When the Ball
Count Unit steps up, its EOS (end of stroke) switch makes, which energizes the Player Reset
Relay which cuts the circuit to the Player Step Up Unit..