That's very sad
Welcome to the world of trying to keep old TV's working!!
My guess, and it is only a guess at this stage based on your description, is that the insulation has failed on the tripler or line output transformer. The tripler, or more correctly EHT multiplier, is a highly stressed component- there's 25Kv in there trying to get out!
These items were very regular failure items on all TV's back in the '70's & '80's. Yours is probably the original one so has done very well in it's 40 odd year life.
The tripler is that grubby white thing standing up on top of the chassis in the right of your picture in post 44. It's also got the focus knob built into it as well.
The line output transformer (LOPT), is immediately below it and will have a thick wire coming out of it going to the tripler.
Carefully examine these items for any burnt areas or melted bits- it will be fairly obvious if this is the cause.
Also examine the print side of the circuit board these components are mounted to- often a print burn up can cause the problem you describe.
Make sure the set is unplugged before looking and you need to be VERY aware that the EHT connection (that round plastic disc with a wire coming out at the top of the tube) can hold a charge up to 25Kv (yes that's 25
thousand volts!) almost indefinitely under certain circumstances. Don't disconnect it or touch it. Just look visually at this stage.
I hope it's not the tripler or the LOPT that has failed as both of these are now very difficult problems to solve.
Fingers crossed
Nick