Mark Rehorst wrote:

It looks as though either the transformer or mechanical filter have
croaked, but I can't be sure. Any suggestions for how to test a
mechanical filter? Do those things die?

I don't know if it applies to this radio but in that era there were some
mechanical filters that had something like foam rubber inside that turns
to a mushy goo over time.
I have heard of people repairing them simply by cracking it open and
cleaning with a solvent.
I seem to recall Kokusai as being one of the affected brands but my
memory is foggy on this.

Good luck,
Bill