On Fri, 29 Apr 2005 09:05:27 -0700, "John Smith"
wrote:

A crt fails/dims due
to air entering the tube, possibly?

Hi Brett,

I've seen air enter the tube - perhaps, but there are more
conventional explanations. Those episodes of "seeing" it enter were
slightly less than catastrophic (accidentally broke the final seal,
which, as it happens, is the right way to de-activate them).
Principally it is the oxidation of the cathode (one reason why tubes
have scavengers) from what air had remained from the production cycle.

Back when I did TV repair as a teen, and then later in the Navy as
part of a Christmas charity project, we used what was called a
"rejuvenator."

This piece of gear would apply several hundred to a thousand volts
between the cathode and the first grid (usually a cylindrical shield
capped with a plate with a pinhole in it) and hold the voltage there
for a couple of seconds (manufacturer's recommended time). This would
produce a current that would rip the surface of the cathode open
revealing a new emitting area and the tube would come to life for
another 6 months.

Well, during the extended project in the Navy, we found any number of
tubes that refused this Lazarus treatment. We simply held the button
down for 30-60 seconds as they sizzled. Who cared? It often
convinced the tube that there was more to life than consignment to the
dumpster (the guys loved to give them the heave-ho to thundering
explosion). The only downside to this aggressive treatment was that
in ripping open the surface, part of the current was a debris stream
towards the grid that would occlude the pin hole. Sort of self
defeating when you think about it (and good reason for the recommended
couple of seconds instead of our heavy thumb).

All rather remote possibilities for the White LED. LEDs suffer other
problems in commercial service. Usually with aging where the
half-life is quicker than expected. Another problem is with what is
called sun bleaching (usually the problem of LED traffic lights). I
would suspect either, or both, to be issues with vendors and poor QA
or selection. There are a world of "ultra-bright" specifications that
wander the map.

73's
Richard Clark, KB7QHC