Mk1mod0, I'm not an expert (at anything), but this would be my thought: something is shorted to coil voltage and causing the drop. No fuses are blowing? The next step is probably to unhook any coil on the 43v line, leaving one connected and work through. Time consuming, but you'll find the problem!
You'll have to disconnect one side of the coils anyway (at least) to test the coils (via resistance).
Also - a backwards or shorted diode may cause this behavior (possibly). Test them all while messing with the coils.
If you're not blowing, say, your GI fuse, then one of the above should do it.
If you have the schematic, check your wire colors on each side of each diode.
I hate when something shorts in the out hole area. Always takes forever to find.
Good luck! It's not gonna be easy, but you'll find it. If someone has a better idea, I'm all ears!