Got my GI working again. Summary follows in case future peeps run into GI issues.
Problem was that Centaur GI was out.
I drew up a circuit diagram for one of the circuits, and everything was good, green and white wires (grounds) were on one side of the bulbs in the circuit, and red and orange wires were on the line side.
I was reading 7V AC at any point on the GI circuit, which means stuff wasn't getting grounded properly.
Centaur has finicky GI compared to most other pins, because they are controlled by an add-on auxiliary board and a Triac for some special effects.
Failure points for Centaur GI are:
- Triac (can be bench tested easily with a multimeter). It's a little round metal connector mounted to the same metal bracket as the power rectifier board.
- Auxiliary lamp board (there is a bulb beside it with no lighting insert that must work in order to load the circuit. Failed bulb equals failed circuit. It was not my issue, but I'm told cold solder breaks are common on the auxiliary lamp board connector and reflowing these will solve GI issues sometimes. You find this under the PF near the O-R-B-S lights.
- Main lamp board - Q14 is in line with the GI.
In my case, the triac tested ok. Jiggling connectors did nothing, so didn't seem to be a connector problem. Swapped lamp board with a known working one and the issue resolved. Replaced Q14 with SCR 5064 on the original board and put it back in, problem remained resolved.