Interesting behavior of CRT (image "blooming")

[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?

The effect seems to completely disappear when the monitor is fully
warmed up.

Not a critical problem; I'm just curious.

 

[William Sommerwerck]

Blooming has a number of causes, the most-common (I believe) a drop in HV.
"Something" is keeping the voltage down a bit before the monitor fully warms
up.

 

[Phil Allison]

"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

 

[David Nebenzahl]

BTW, where in Holland did you grow up? I traveled all over Holland
after AT&T bought part of Philips in Huizen and Hilversum.

What makes you think I grew up in Holland? Oh, my sig: that's not me
(even says so). I grabbed that from another newsgroup. Probably should
change it.