Traces around the edge of the driver board can short against the rail. I have had the same thing happen when the driver board is flopping around because all the plastic mounting things where snapped off. Rubbed against the bracket until the solder mask came off and then locked a coil on.

There is high voltage at that connector and the pitch is only a tenth of an inch. Sticky flux and then enough conductive coil dust land on there I can see it throwing sparks in case like that.

Q16 out to coil and +230v are right next to each other at that connector. If Q16 is locked on or the bracket is shorting against trace that is a path to ground for high voltage.

Nice!

Make sure your rectifier board has the right fuse for the 43v. That would short 43v to ground through Q3 when it is turned on. I would expect the fuse to blow.

I would leave the feature lamp fuse at 10 amp unless it blows or seems to be heating up. With the aux lamp driver board there is like 12 extra feature lamps lamps vs older games. I don't think you need to 2x the fuse amp rating, but 20a is what bally specified. Maybe 15a better choice.

The solenoid fuse in later games was also specified to be higher amp depending on the number of flippers used. The way the flippers are staged and EOS switches I don't think you need up the amp of the fuse unless you have a problem, which I don't think you will. I think it was 5a for two flippers, 6a for three and 7a for four flipper games. The future spa manual rectifier schematic still shows a 5a fuse. They may not have started changing the fuse with flipper count until the -54 rectifier came out, im not sure without looking.