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Interesting behavior of CRT (image "blooming")

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.

--
I am a Canadian who was born and raised in The Netherlands. I live on
Planet Earth on a spot of land called Canada. We have noisy neighbours.

- harvested from Usenet
Reply to
David Nebenzahl
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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.

Reply to
William Sommerwerck

.

What you described is exactly "blooming". The reason it is more noticeable in the early warm-up period is that the CRT circuits are not developing quite as much oomph, so to speak, as when they are fully warmed up to operating temperature. Is this something new suddenly or has it developed gradually. ALso, as the CRT ages, it takes a higher current thru the CRT to produce as much brightness as when the CRT was new, due to phosphor aging with use.

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

Reply to
hrhofmann

"David Nebenzahl"

** 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

Reply to
Phil Allison

On 12/29/2009 5:45 PM hr(bob) snipped-for-privacy@att.net spake thus:

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.

--
I am a Canadian who was born and raised in The Netherlands. I live on
Planet Earth on a spot of land called Canada. We have noisy neighbours.

- harvested from Usenet
Reply to
David Nebenzahl

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