I think when the ball is kicked out it passes over the switch in the trough to increment the ball in play unit.

I'm guessing when the game resets, the ball in play unit isn't resetting correctly, out of adjustment.

This switch on the 1st Ball relay (ZB) has the job of preventing that from happening.

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HowardR beat me to it but here is what I had just written about this same relay:

I don't have a schematic for this game, so this is just an educated guess, but virtually all of the Gottlieb multi-player games of this era have a first ball relay. This relay is activated during game start and there is a switch on this relay that is supposed to disable the trough switch so that the first ball being served to the plunger does not advance the player unit. On your game, the first ball relay would most likely be located on the control bank mounted on the bottom board just behind the score motor. If it is, then the first ball relay would be in the active state when the relay is in the up (latched) position, which will happen when the control bank setup occurs during game reset.

The player unit (a stepper located in the backbox) keeps track of both the player up and the ball in play, so it will advance each time the next ball is served to the plunger from the outhole. The player unit advances when the ball rolls over the switch at the far right of the ball trough. The only time the player unit is not supposed to do this is at the start of a new game because the game reset puts the player unit in the so-called "zero" position - which means it is set for player 1, ball 1. That's why the first ball relay needs to disable the trough switch for the initial ball feed to the plunger after reset.

As soon as points are scored for the first player (usually either 10 or 100 points will do it) the first ball relay is released and from that point forward the player unit will advance each time the trough switch is actuated for the rest of the game (unless an extra ball is earned on games that can award an extra ball - but that is very unlikely to be relevant to your current issue).

If the trough switch isn't disabled on the first ball, the most likely reason is that either the first ball relay is not getting latched on game start, or the switch on the first ball relay that is supposed to shut off the trough switch is out of adjustment and is still making contact even though the first ball relay is latched. So you probably want to check for those situations.

- TimMe