[Tig] OT-CRT Monitor longevity

[Dan Desmet]

One of the main reasons for short lived CRTs was people leaving black on the
monitors when not in use.  Quick (and simplified explanation): when the
filament inside the cathodes (guns) are on, these cathodes produce electrons
that are ending up hitting the phosphors at huge velocity (thanks to the
24,000 Volt anode or screen) thus producing light.  In addition to these
electrons the cathodes also emit heavier ions that fly with these electrons
to the screen and disappear without doing any damage (that is with a certain
level of beam current i.e. with active video levels).  However, when there
is black on the monitor the beam current falls to almost zero (instead of
the 450 micro Amps at full white) and these heavier ions kind of hang around
the cathodes (like a small cloud) and eventually fall back onto the cathodes
where over time they build up as a poisonous layer smothering the cathodes
and preventing the normal amount of electrons to leave the cathodes and thus
reducing the light output.

I always suggested to my customers that when the monitor was not in use to
leave it on but with a grey field (20-30 IRE) on it: this kept the electrons
sailing smoothly and taking all the impurities with them, and at the same
time the grey field would take care of any white uniformity issues that had
built up during the day by fixed patterns displayed on the screen.

In fact this is the reason why clever TV repair men 30 or so years ago would
fix consumer sets by "reviving" the guns.  Basically discharging a big
electrolytic capacitor on the filament contacts, thus overheating the
filament and cathode which had the result that the layer on the cathode
would break off giving a couple of months of new life to the CRT.  In most
cases however the only real solution was to "regun" the CRT (taking out the
old guns and replacing them with new ones making sure they were re-installed
exactly in the same position to preserve the convergence and white
uniformity of the CRT.  Tricky procedure as the CRT had to be cut after
releasing the vacuum inside.

Regards,

Dan Desmet
www.dandesmet.com