[Hunts smoothing bomb]

I think we have still missed something here, I agree that the fuse most probably emmitted the flash that was seen through the EM87 when it blew but for a fuse to emit a visible flash it means that it blew with a considerable overload. (i.e a short somewhere)

There is also thefact that you mentioned a burning smell that was detected when it happened, this seems to have since been forgotten.

I think that the HT rectifier and main smoothing capacitor need to be identified and checked as if this failed in a manner where it was supplying AC current into the smoothing capacitor it could well be responsible for emmitting the smoke and smell and the blowing of the fuse.

Cheers