[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 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 the 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.