"David Nebenzahl"

Just noticed something a bit strange about my CRT monitor (Dell-branded
Sony Trinitron), wondering if it means anything to anybody here.

This only happens when it's first turned on, before it warms up much. I
use Firefox as a browser and have it set to a little more than half the
screen height, so the browser window takes up about half the screen. I use
tabs, and I noticed when switching between a tab that's mostly black and
one that's mostly white that when I do so, the entire screen "jumps out" a
little. I hesitate to use the term "bloom", since I believe that has a
specific meaning for certain CRT image artifacts, but that's what it seems
to do. It's quite noticeable, and actually a little bit alarming.

So I'm thinking: something having to do with AGC? Some kind of
overcompensation for increased voltage levels for the brighter screen?


** There is an EHT " acceleration" voltage applied to the inside (
graphite coated) surface of the CRT of about +24kV.

The higher this voltage is - the smaller the image gets.

To keep the image size reasonably fixed the EHT voltage is normally
regulated by a feedback circuit.

When the screen brightness goes up, more current ( ie electrons) flows from
the cathode of the CRT to the phosphors and hence more load is placed on the
EHT supply. With more load the voltage drops and the image gets larger.

All CRTs do it.

When the CRT is old or not fully warmed up, the sensitivity of cathode
current to control grid voltage is lower than usual - hence the EHT
feedback loop is less effective.



..... Phil