RCA Flybacks

[Alastair E]

I wonder if--

Your flybacks, (Line Output Transformers) are suffering a damp-coil condition like some British Bush/RBM B/W sets.

The HV Overwind was a pitch covered/wax impregnated coil in a doughnut shape, and some years of dis-use,--especially with a set stored in a damp barn or basement would allow moisture to creep into the coils via tiny cracks that appeared in the pitch/wax over the years since it was made, causing low EHT (HV) and worse still, falling EHT as the transformer warmed up, to the point the picture would expand and eventually completely disappear and the horiz-out bottle plate glow under the stress.

A period of 'cooking' the HV overwind coil by passing current through it carefully monitoring the heat in the coil to around 100-120 deg C for some hours, (I usually leave 'em overnight at this temp on the 2 or 3 Ive done) cures the issue.

Some guys have had success by storing the complete transformer in/on central-heating system hot-water storage tanks for some months...
--I prefer my method--Much quicker!

--A good coating of the insulating-varnish of your choice then seals it all up keeping the coils dry....