Question about how high voltage works

[Electronic M]

If you haven't compared the negative voltage at G1 (control grid) of the H output tube to the value listed in the schematic then that should be your first mission. Screen voltage on that tube is also informative if G1 is right. Screen too high, something is shorted in the flyback, yoke, HV damper...Too low screen and something is open in those stages.

There's no good equipment to test a doorknob cap other than the TV. What I do is disconnect one side of the doorknob and stick it in a small glass bottle (a shot glass would also work) so it can't arc. And run the set and see if I get more HV. If the set has a glass CRT with dag you don't need the cap, and if it doesn't you can sub another HV cap or try it without it (HV may vary across the screen but it should somewhat function).