[Jeffhs]

The CRT in that set must have been a very good one, if it worked 38 years. I'm tempted to think the set was used quite a bit, which will weaken any CRT over time. I've seen and owned a few b&w sets with 20-year-old CRTs that still produced a bright raster and a decent picture, but color tubes seem to weaken more quickly due to increased beam current and much higher HV (25kv+ in a color set as opposed to 18-20kV or less in a b&w one). I don't think the instant-on system had that much to do with it; in fact, with the filaments always on, I would think there would actually be less wear on the CRT guns and much less thermal shock to the heaters than there would be in a standard TV.

I agree with Charles's advice to replace the CRT in this set with an identical one, if at all possible. Tube substitution is an emergency procedure, to be used when the correct tube isn't available. Zenith's 1950s-'60s tube-type TVs had a warning printed on the tube layout chart, stating that only tubes identical to the type originally in the set should be used as replacements, but this applies to any tube-type TV, radio, stereo, etc.

BTW, I think the use of a 30KD6 in place of the original HO tube in this set might have something (or everything) to do with the problems you are experiencing with the remote. I'd put the correct tube in there; that may solve the problem, and give you a brighter picture to boot. Some of these sets were very particular as to the horizontal oscillator and/or output tube -- it absolutely had to be the same type as the one being replaced. In fact, in some ultrasonic remote TVs, the horizontal oscillator running far off frequency, or the output tube radiating much more signal than it should (well outside established limits which could and often did, long before HV shutdown circuits became a required part of all color TV high-voltage systems, cause an X-ray hazard as well), can trigger the remote receiver -- causing all hell to break loose, as in the TV changing channels at random, the sound muting and unmuting itself, the set turning itself on and off ... name it.