The scoring in Happy Clown is controlled by four Player Scoring Control relays: AA, BX, CC & DD. There is one for each player and only one should be active at a time. The 1 point scoring schematic is shown below as an example:
Happy Clown Balls Played (resized).jpg
So when the N/1 point relay fires and closes the N switch, only one of the four 1 point score reels should advance based on which of the four Player Scoring Control relays is active.

The first thing to check is that only one of the four Player Scoring Control relays is ever active at once. If that's true, examine the switches on each of those relays to make sure that they all open and close as expected when the relay fires and that there are no shorts between them.

If you find that more than one Player Scoring Control relay can fire at at time, look into the Balls Played Unit which is probably a stepper unit with at least 20 positions. It's possible that the Balls Played unit is not working well because it's dirty, worn, loose, etc. It may be activating more than one Player Scoring Control relay at a time. If it is, it's likely activating two consecutive players' relays (e.g. player 1 and player 2, or player 2 and player 3, etc.) rather multiple random players' relays.

The Balls Played stepper has two solenoids, one to reset and another to advance it by one position. You should be able to push the plunger of the advance solenoid all the way into the solenoid, release it, and have the stepper springs advance the stepper cleanly to the next step. It shouldn't be sluggish or intermittent and all of the wipers should land on or very close to the center of the contacts below.

/Mark

I think there's still a problem in your Balls Played unit. The XB/Last Ball relay should fire on the last ball of the game regardless of how many players there are:
Happy Clown XB relay (resized).jpg
On a 3 ball, 1 player game, the XB relay fires when the Balls Played unit reaches position 8. On a 3 ball, 2 player game, the Normally Closed switch on PB2 opens so the XB relay doesn't fire until the Balls Played unit reaches position 9 through the path shown in red. I'm pretty sure that the PB2 switch is working since a 1 player game works fine. So the problem may be with position 9 of your Balls Played unit.

If your XB relay does not fire on the last ball of a 2 player, 3 ball game, unplug the game before the ball drains and have a look at the Balls Played unit. (Or you can just shut off the power, reset the Balls Played unit and advance it manually 9 times.) Assuming it's in position 9 there should be a short circuit (or an ohm or less on your multimeter) between the maroon-white and maroon-orange wires through the Balls Played unit. If not you may find a wire has broken off or a contact is worn or something like that.

Yeah, me too. But I convinced myself that the schematic is wrong. I think the 3 and 5 labels on the 3-5 ball adjust are reversed. If the schematic were correct, the Last Ball relay would fire sooner for 5 ball games (Ball Count unit positions 8-11) than for 3 ball games (Ball Count unit positions 16-19).

You could compare the schematic and wire colors against the game to know for sure.

How does the resistance of the 9th position through the Balls Played unit compare to the resistance between the maroon and maroon-orange wires when unit is in the 8th position? Are they about the same?

Does the XB relay fire on player 2, ball 3? If not, the problem may be elsewhere in that path.

If you clip an alligator test lead between the maroon-orange and maroon-white wires does the came end at player 2 ball 3? If not, try bypassing other switches in that circuit.

I think that XB should fire sometime during the last ball, not after the last ball. Check to see when it fires on a 1 player game so you know when to expect it to fire on a 2 player game.

If there's a good electrical path through the Balls Played unit in position 9, work your way out with your meter to find where the resistance increases (with the power off, right?):
Happy Clown XB relay 2 (resized).jpg
Start with both meter probes where you had them before on the maroon-orange and maroon-wires, and the Balls Played unit in position 9. You should see no more than an ohm or so of resistance. If that's true, then move the probe from the maroon-white wire to the wire between the PB4 and QB switches to include a bit more of the circuit in your measurement. The resistance shouldn't change substantially. Keep moving that one probe further from the Balls Played unit to the other points shown on the QB and SB switches, each time including a little more of the circuit in your measurement. Since those switches should all be closed you should see about the same resistance at each point. Note that with the power off you may need to manually close some of those switches to measure the low resistance.

If you can measure the same low resistance between maroon-orange and the wire on the right side of the SB relay switch, leave one probe at the SB relay switch and start moving the other probe to the left in the schematic. Note that if you move a probe to the left of the B relay switch you'll need to close the B relay manually.

Somewhere in the red circuit is an open circuit that prevents the XB relay from firing. By moving the meter probes methodically through the circuit you should be able to narrow in on where the problem is.

Hello there, I am new on pinside and only own a Gottlieb Happy Clown ,and a Williams Space Mission Pinball machine, on my happy clown I have the following problem, on the fourth player score reel during the 4. game the 100s reel does not advance, and on the 5. game also on the 4. player score reel the 10s does not advance. Everything else works fine. Any ideas? Thank you in advance, greeting from cold Switzerland Cheers Ruedi