mystic line extra ball design flaw
After ernie's rather brilliant diagnoses, we looked at how to fix the problem (well, resurrect the lie that single stepping to "extra" is gtting you closer to getting the extra ball).
There's a few options, but some require adding switches, modifying cams, or changing the odds. There is one fix that just requires a little rewiring, which is the one that is recommended.
Suggested fixThis fix takes advantage of the fact that switch 6A/6B is really not needed in the single step circuit. As mentioned earlier (see, you should have read the circuit theory section), it is there to keep the multiple step circuit from passing pulses when it shouldn't.
So the fix is to move 6A/6B to where it still keeps the multistep circuit happy, but doesn't inhibit the single step. That means moving the switch into the blue path, and there is only one place you can do that so it works all the time - wire #78 between the start relay and the 16 pulse cams.
You've now inserted 6A/6B in the circuit as shown in the highlighted circle. It does it's job blocking the 50V when it should, and doesn't affect 8A.
How could this happen?This bug was in multiple games spanning several years. Either nobody noticed, or nobody cared (especially since it doesn't really matter).
Ernie said that someone had tried to fix his machine by swapping the switches between cam 3 and cam 6. Unfortunately, the cam 3 lobe didn't rise early enough either, so the attempt failed.