It gets power from Lots of paths for G to be energized. But, before bothering to go through them all, check to see if it is getting stuck electrically, or mechanically. Once energized, watch it as you turn off power to the game. If it drops out immediately, it is electrical. If it holds in with power off, it is getting stuck mechanically.

G: Bonus Score Control RelayTargetAlpha-173.jpg

The switch pointed to with the Red arrow is a normally open switch on the G relay itself. This is the "hold" switch for the G relay so that if the relay has been energized, it is supposed to hold in until something else drops it out. In the original poster's case, it was this switch that apparently caused the problem. If you have cleaned the hold switch on the G relay, then I would instead suspect the 15th position switch on the bonus unit itself. Due to the vibration, it is fairly common for it to drop out as the bonus unit steps around. Make sure it is clean and has good "over-travel" to insure it is closed and remains closed all the way around until it hits the valley in the cam.

Well, I don't have one to look at. Based on the schematic, it shows them as normally closed (one is a make break switch and therefore has a normally open contact too). So, normally, the schematic is drawn in reset condition (player 1 Ball 1) and some brands with power turned off. The schematic says 15th position switch so I would expect that home position is position 0. So, yes, the contacts should remain closed in every step position until the switch stack reaches the valley. From what I can remember, at that point, it would step past the valley and land back on the high portion of the cam.

So, basically, positions 0 through 14 should maintain a closed condition on that switch stack.

You may want to grab the bakelite wiper disc and try to wiggle it. It should be pretty solid and only rotate. If it is really sloppy ( worn shaft or bearing or loose set screws) it is very difficult to keep the switch in adjustment. I had a Hot Shot which also used the stepper bonus scan system and due to the number of plays, the bonus unit was very sloppy due to wear.

Correct, the schematic shows the start or sitting position as "0" with the Yellow wire going to it.