If you have the schematics, check to see if you can find the relay that drives the ball count unit. I believe on multiplayer games, the ball count unit is sequential: Player 1: ball 1 -> Player 2: ball 1 -> Player 3: Ball 1 -> Player 4: Ball 1 -> Player 1: Ball 2 -> Player 2: Ball 2..... and so forth all the way to Player 4: Ball 3. In order to advance the ball count unit to ball 2 on a single player game, the score relay will run to pulse a relay that advances the ball count unit four times.

Also make sure the ball count/player unit is properly cleaned and advances properly when triggered.

You've got an issue with the player unit in the backbox. That's the starting point. After each ball, on a single player game, it should step four times. If it's going from player one to player two and ball one to ball two, and is set for a single player game, it's only stepping once while the ball count unit is stepping correctly. I had a similar issue with a Surf Champ, and it was one switch on the player unit.

yes, tighten stacks on player unit. there is a contact that is closed when on player2 that should advance it, likewise on 3 and 4. these circuits get opened up by the coin unit (adding players etc), something else you can check

I just finished bringing a Duotron back to life thanks to help from several pinsiders. Before I sorted out my power issues I went through all steppers and sanded with 600 grit, cleaned with isopropyl alcohol and then Super Lube. Maybe that solved some issues I would otherwise have had. Either way the games are similar so let me know if you need pics. Pinball Ninja and pinrepair.com were also helpful. Good Luck!

hope this inspires you..this was my minty magnatron from a few years back..loved the backglass art and had 2 of the best shots for special on any pinball machine.