I'm bench testing a Bally MPU and can't seem to get the 7th flash without the cheat (jumper from U12-3 to U14-14). I am providing the board 5, 12, and 21.5 VDC at the correct test points, but no flash without the cheat, which doesn't let me test U14. PIA input from ZCD circuit is constantly low, making me thing U14 may be bad (Clay's guide).
The whole "zero crossing detector" nomenclature makes no sense to me. The incoming voltage when installed in a game is 43VDC, voltage divided down to 21.5VDC at the test point. There is no zero crossing, it's a positive waveform after rectification on the rectifier board.
In short, is it possible to boot the board up with 21.5VDC on the bench? Or is a bench supply too clean of a signal to allow the ZCD circuit to function properly?