CRT Heater to cathode leakage
Where the CRT heater is fed from a transformer or a series heater chain, H to K leakage can seriously impair the display. Low capacitance transformers were sold to remedy such problems.
Another way is to create a winding using well insulated cable on the line output transformer, as later TVs tended to do.
Deciding the number of turns is a bit hit and miss but start expecting about 1 turn per volt. Keep away from the EHT overwind.
The original heater connections should have been referenced to a potential with 1 or 2 resistors and possibly a capacitor. This was to prevent static build-up of the heaters and discharge through the H-K insulation. This will need to be removed and referenced instead to the cathode (if colour the cathode of the gun with a problem)
The horizontal deflection circuit is unlikely to complain about the small extra load, but you might have to re-adjust slightly for optimum.
I used this method in the 70s when a large batch of TVs had the heater reference circuit missed during manufacture. Most of the CRTs failed H-K. I can't remember which colour gun it was.
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John
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