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I doubt it's indefinite, the insulation of the fine wire inside eventually degrades, they usually short internally, not to the cage. But there are certain concrete things that can cause flybacks to be destroyed, not to just wear out.
- Tubes in the HV section shorting the filament to other parts in the tube... resistors going bad in the HV regulation circuit and letting the HV go out of control till that 40K volts finds a way to get out (like thru the flyback insulation!).
- Black dust accumulating in the cage can increase the likelyhood of corona and HV leakage, should be cleaned up.
- Shore up any missing insulation/wax on the flyback correctly.
- I think some of the 'GT' variants of some tubes may be sturdier and less likely to short, like 1b3GT and 3a3c. Just replacing the rectifier tubes in the HV section with new ones that are more up-to-date GT or A types etc. is a good idea.
Then there are always peculiarities to certain chassis's. For example on my CTC-16 I asked around and there are a few weak spots that need to be addressed, namely a couple of chroma board solder pads that should be shunted with a separate wire. Back then, the joints used to come loose from board heat warpage and this sent the HV sky high. I was also given some tips on getting the current in the flyback as low as possible (for reduced heat) by using some tricks with extra caps in the circuit. That's another biggie with flybacks, dialing in the horiz. efficiency adjustment coil, width adjustment, experimenting with different H.O. tubes etc. to get your current there as low as possible and still perform correctly picture-wise.
Adding some ventilation holes to HV cage lid can help too if it doesn't have any, but you have to make sure you don't leave new sharp points in the metal in the process which could induce corona (HV leakage).
I would suggest posting here and in the antique radio/phono newsgroup with your chassis number, lots of old tv repair guys there who remember what the weak spots were on what sets.....Frenchy
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