Quoted from Durzel:
I'm aware that GI dimming has to be enabled in settings for it to work in test, etc - I've ruled out settings.
That was going to be my suggestion, but you've already ruled that out. It very well could be the zero crossing signal. Because of the way GI OCD reads this signal directly off the ribbon between the CPU and power driver, it *might* work, even if the power driver has an issue. It's safer to make sure the game is working before installing, though.
The board you have is an older design, and the current limit on it is lower due to the small transistors. I shipped them with 1.5A fuses, anything over 32V is fine. Fuse information is here:
1.5A can be borderline depending on the bulbs (2SMD will pull a bit more current). Also on the page listed above, "Note: Early boards were shipped with 1A. If this does not blow unexpectedly, it will better protect your LED OCD from damage in the case of a short. It is, however, safe to increase to 2.5A if you are having trouble with it blowing in normal operation." So, don't just go increasing the fuse for no reason, but you can increase (I suggest taking baby steps) if it is blowing and you're certain there is not a legitimate problem with the game causing it.