Quoted from Spyderturbo007:What controls the light for the Ball number? I'm noticing that when it goes to Ball 4, the light doesn't come on right away. But, if I hit different switches, it will turn on and then off again.
Coincidentally my Bally Rocket III of same era started doing similar yesterday. Per the post above, investigate the bulb holder itself and spider/circuit board on the "Ball Count" unit. This is my first EM and I like the way everything is conveniently labelled. Here's the Ball Count unit on mine. This game can be configured for up to 7 Balls so steps 7 units on one lever and total resets back to 0 on the other and the fingers on the spider complete the circuit to the relevant ball-in-play lamp.