Had this problem on a Gottlieb Joker Poker EM years ago. My repair log mentions the score motor kept running upon power-up. The solution was that the AX relay was in the latched postion. I manually tripped AXR (AX reset relay) with the game off to release AX, and the score motor stopped turning. Not sure if the relay layout is the same in a 1976, but it's worth a try for you to check this.

You need to look at the credit unit in the games head (where the number of credits show). This unit has two coils with plungers, gears, and a spring wound around the center spindle. Check the following:

1). Manually press the plunger on the credit advance coil to add games. Then press the decrement plunger (the other coil on the this unit) See if it takes only one credit off as it should, or jumps to zero. If it jumps to zero, the pivot arm that rachets back onto a gear to only allow one game to come off is likely sticking and not moving to stop the gear. Check that this pivot arm has a spring on it that is not broken or missing. Everything should move freely on this unit when you "step" the coils either way. Many times someone lubed this unit and it gets all gummed up.

2) If only only game comes off when you manually step it, then check for broken wires, dirty or broken contacts on the unit which may not be letting the game know a credit was subtracted. I would really check this credit unit out first before looking elsewhere.