Coincidentally, the F103 fuse finally bit the dust tonight and the SC was totally dead. The hint was no power at pins #1-3 on J106 of the accelerator PCB. Traced back to the power driver board and still no voltage. Ultimately, this guy's website saved me on that one:
Couple other things....
The ball lock switches had to be adjusted as they were not letting the balls roll out at the start pf multiball. Also, waiting on a new plunger as mine is nearly a spear from being played so much. It's firing the ball everywhere but "in the loop" at the beginning. I know the mounting of the shooter lane solenoid needs to be adjusted, but until the new plunger comes in this needs to wait as it would be pointless to do it with a "wonky" shaped plunger.
Back to the main problem....checked all SC optos in switch test mode. Took #2 and #3 transmitters and swapped them, meaning I desoldered #3 and moved it to #2's spot and vice versa - no effect - the problem did not "follow the move" so to speak. Getting voltage on all transmitters of about 1.58V. Took a photo of the transmitter in question - bright purple light showing clearly. I went to the accelerator board and disconnected, reconnected, and wiggled all the connectors trying to make the opto trigger out of "bad" - no effect.
HS2 is being a brat, but we'll get it worked out.