Isolated, this transformer denies absolutely any existence of a fault as far as I can determine. Correct continuity and sensible resistance measurements on a really extensive set of tests.

In circuit and power on, I had the right AC voltage selection range on my multimeter telling me over and over again there was 0V on the busted mono versus 129-130V on the good one at the diode junction.

I did that so many times because I absolutely did not want to remove the transformer. And I just didn't think it was a likely fault.

It's definitely like living in a world where you just cannot rely on your senses.

I am starting to realise why no one is keen on working on these things.