A Confession

[ohohyodafarted]

Also, you could get lucky and find the problem is very close to the outside layers of the HV winding. If it is near the outer diameter, you could unwind a few outer layers till you get to the defect, and just re-coat with wax and use the flyback with a few less turns on the HV winding. You would have a bit less HV at the crt, but it's worth a try if the defect isn't too far in from the outside. You could probably remove 10% of the turns and still have enough HV to operate. I think you would still be able to operate at 20KV. Cross your fingers and hope you find the defect very near the outer edge.

[Penthode]

I have been able over the years to resurrect bad flybacks. You will generally find the insulation has cracked or else the is a carbon track across the insulation.

I would ascertain where the arc is by examining the flyback when briefly plugging in the set. That is if the carbon track is not immediately obvious. You could then remove the flyback and give it a thorough investigation. You most likely can remove the carbon track and then seal it with glyptol or silicon and it will as good as new.

Nothing is lost trying and there is a lot to gain.