On Sunday, 19 January 2020 22:40:18 UTC, Cursitor Doom wrote:
On Sun, 19 Jan 2020 13:39:12 -0800 (PST), John-Del
wrote:

Lots of circuit maladies will simulate a weak CRT. Start with voltage readings, move to waveforms. Low high voltage will cause the picture to dim as the drive to the tube is increased for instance.

I was given to understand elsewhere (which is why I'm double-checking
here) that a CRT which has gone dim and reaches its peak brightness
before dimming again towards the end of the intensity control sweep is
irretrievably ****ed and not worth further investment in time. It
would be nice if that advice is indeed incorrect!

That is very incorrect.
1. Check all your electrode voltages, using a true rms meter on the heater if it's LOPTF driven and you have one. Also check video drive amplitude on all relevant electrodes (many TVs put different signals on different electrodes, using the tube to effecively calculate the resulting output).
2. If still not upto snuff, there are 4 reasonable ways to improve it
a) Zap type rejuvenation. Used to be popular, but don't do it. It doesn't last long & causes severe smearing as emission drops again, making the tube unusable. Not all tubes can be helped with zapping.
b) cook type rejuvenation. Just wind the heater voltage up eg from 6v to 9v and let it sit a while, maybe 15 minutes. Often the tube then works nicely..
c) voltage boost: increase heater voltage. 10% gives slight improvement, I've used upto 66% as an experiment in a severe case (zero emission was detected, worked fine for the next 10 years I kept it)
d) A sort-of obvious one that sometimes is overlooked: if it's an old tube with plate glass guard, often there's a great deal of filth in that gap, clean it. Sometimes nicotine on more modern screens can be less than obvious too.
f) Finally, and this is usually to be avoided: increasing EHT. May be safe to do on historic low EHT tubes, but 1950s/60s onward tubes (typically) have high enough EHT already that x-ray output would be increased problematically. X-ray output level must be checked if this approach is used.
f) Really finally, re-gun it if worthwhile & everything else failed. A major rebuild job, and very few are done now. But Hawkeye's equipment was bought and is still getting used.

Oh, and there's one superbasic approach that can get old tvs just good enough when they're not quite the turn down the black level to ensure all black areas are fully black.

And note that when colour tubes are improved by most of the above methods, you can expect poor colour tracking. The worse it was, the poorer the tracking will be. Usually not a big deal, but can be.

There's more. Obwarning: don't do any of this unless you know how to do it all safely.


NT