[DonaldStott]

Depending on the age of the transformer I understand that thermal fuses can blow not just as a function of over-heating but simply through too many thermal cycles. I've no figures for 'how many' is 'too many' but it's just another age related symptom of vintage equipment i.e. thermal fuses will fail eventually

Thermal fuses are often inaccessible without major disassembly of the transformer and its windings, so by-passing the thermal fuse and using one of the other Primary inputs and a suitable in-line fuse may be an option. You won't, of course, have any over-heating protection and pay attention to Post #8 from broadgage above.

All the usual caveats apply and you do so entirely at your own risk!