[Chas]

There should be a capacitor across the primary of the OPT. This cap as OEM would be a metal foil/paper, often it fails. Then, the transformers windings are at risk from HV flash-over cause by a static burst received by the radio. During static, the plate current ceases for a tiny fraction of a second, the magnetic field in the transformer core caused by normal plate current collapses and a voltage pulse rises so high without the cap the winding flashes over through the glassine layers. Often the radio keeps working but the arc has set in motion by the creation of acids, that, slowly eats at the wire at the point of the arc until the wire fails and cannot pass current, the transformer has failed.

Often this cap is part of a high frequency tone control, combined.

The cap should be replaced with one with known dv/dt specification to absorb spikes without damage. Often that is not a concern just as the correct use of the hum bucking coil is often ignored.. The radio will play in either circumstance but with a low level hum and possible loss of a new transformer. Common metalized caps do not have a published dv/dt rating. I have seen these fail open when misapplied in a pulse situation, just one "hit" and mf value cut by 1/3 and the generation of faint ticking sound in an audio system as the broken metal film creates spikes of its own.

Chas