Jedinite,
Yes you are correct, they are pretty much bullet proof.
The blown fuse would indicate there was a shorted piece of equipment, or at least a short of some type on the secondary side of the isolation (Tice PB) transformer.
The interior of the case has some switches, and caps, and everything is well secured.
However, if someone did the original "lift the ground" mod from an early service bulletin and did not cover the end of the ground wire, and secure it, it could have come in contact with a metal part within the case and caused a short.
You can PM me if you have additional questions.
Bruce
That would take out a fuse too.