CRT monitor getting brighter over time

[Mr. Land]

On Dec 26, 8:51 am, Wiebe Cazemier <halfg...@gmx.net> wrote:

PMFJI....

BTW2, I don't think some component is going bad. It's not as if it started
doing this suddenly. In the course of the 4 years that I've had it, it slowly
turned brighter.

Which would suggest that the value of one or more components is slowly
drifting due to heat/age/leaks/etc.

BTW, how exactly is the beam current driven up when the voltage decreases? I
would have expected it to be the other way around.

Lower cathode voltage == larger potential between cathode and plate
==> more current.

 

[William Sommerwerck]

"James Sweet" <jamessweet@hotmail.com> wrote in message
news:Hdxcj.17001$HQ6.9429@trndny01...

Viewsonic is a mid range monitor, I've had a few of them and they provide
good performance for the price, but there's better monitors out there.
Some
models were very prone to failure, I replaced at least half a dozen
optocouplers in the power supplies, they would fail slowly and cause the
voltage to creep up until it blew the vertical output IC.

The ViewSonic I have is unbelievably good. It uses a Mitsubishi Diamondtron
tube, and the convergence and purity are "perfect" right to the corners. I
used to own Sonys, and they were never that good.

 

[JANA]

The HV multiplier is more complicated than just wires. It consists of a
number of high voltage capacitors, diodes and resistor combinations to
multiply the voltage from the flyback. There are tap-offs for the focus
supply, screen supply, and the high voltage for the CRT anode. Because
feedback regulation is required, the HV multiplier in your TV set can be
fairly sophisticated. There can be a fair number of components involved.

The flyback is really a high voltage; high speed switching transformer with
a number of secondaries and usually a single primary. The HV multiplier is
usually combined together as part of the flyback assembly.


A very basic idea of how a high voltage multiplier is constructed:

http://www.rmcybernetics.com/images/main/pyhsics/voltage_multiplier_schematic_diagram.jpg

http://www.spellmanhv.com/tech/images/an_08a.jpg




--

JANA
_____


"Wiebe Cazemier" <halfgaar@gmx.net> wrote in message
news:acb33$47724392$d4cc82be$20027@cache5.tilbu1.nb.home.nl...
On Wednesday 26 December 2007 04:11, Jerry G. wrote:

When I used to service monitors, I changed many of these HV multipliers
to correct the fault you are describing. In most monitors it is part of
the flyback assembly. On the odd occasion, I have had CRT's starting to
go defective to also cause this problem.

In these monitors, what exactly is going bad? The HV multiplier is nothing
but
a bunch of copperwire wrapped around some core, right? How can that wear
down...?

Personally, I would consider changing the monitor for a good LCD type.
Then you will be good for a number of years without any problems.

No flame intended, but I really don't understand why people are advising me
to
get a new TFT screen. The issue I described is a very minor. The drift has
been so slow that every few months the colors needed a bit of calibration.
The
colors are still extremely pure, black is pitch black, white is brightly
white, the image is sharp, high voltage regulation is perfect (so the image
is
stable under all brightness conditions, RGB convergence is good, etc. In
other
words, it displays a perfect picture.

Even if you didn't dispise TFT's like I do (see my other posts), you would
be
mad to replace this screen only because the HV bias needed a minor
adjustment... If it were 10 or 15 years old, perhaps there would be some
logic
to it, but after 4 years of use and the MTBF not reached by a long shot...?

 

[Wiebe Cazemier]

On Thursday 27 December 2007 05:21, JANA wrote:

The HV multiplier is more complicated than just wires. It consists of a
number of high voltage capacitors, diodes and resistor combinations to
multiply the voltage from the flyback. There are tap-offs for the focus
supply, screen supply, and the high voltage for the CRT anode. Because
feedback regulation is required, the HV multiplier in your TV set can be
fairly sophisticated. There can be a fair number of components involved.

The flyback is really a high voltage; high speed switching transformer with
a number of secondaries and usually a single primary. The HV multiplier is
usually combined together as part of the flyback assembly.


A very basic idea of how a high voltage multiplier is constructed:


http://www.rmcybernetics.com/images/main/pyhsics/voltage_multiplier_schematic_diagram.jpg

http://www.spellmanhv.com/tech/images/an_08a.jpg

I detect some cognitive disconance on my part. I work a lot with audio and
other low voltage electronics, and the full wave and half wave rectifiers are
quite common, and I can see it's the same thing. That is, as far as the basics
go.

 

[jango2]

BTW, how exactly is the beam current driven up when the voltage decreases? I
would have expected it to be the other way around.

Potential difference between a cathode and final anode increases
when ;
1) Final anode voltage goes up OR
2) Cathode voltage (normally held at 180 or 200) is decreased
Cheers!

 

[Dave W]

I have a Panasonic CRT TV with a similar problem. It comes on dark and
defocussed, but comes right after about half an hour. The effect can
be corrected by adjusting the screen and focus pots, but after warmup
it's too bright and defocussed. The screen and focus controls are
tapped off a potential divider from the EHT, inside the output
transformer assembly. The resistance to earth at the screen output
(probably about 500V) is 17M ohms, so the resistance at the top (25kV
EHT) must be in the region of 800M. My theory is that this resistance
is unstable. I would need a new output assembly costing about Ł25 to
repair it.


On 25 Dec, 13:59, Wiebe Cazemier <halfg...@gmx.net> wrote:

Hi,

My Eizo T766 CRT monitor has been turning brighter for years, ever since I got
it. I used to be able to correct it using the cut-off and gain controls in the
on screen menu, but the red had reached 0%, so I was out of lee-way.
Therefore, I resorted to adjusting the flyback transformer.

Anyway, my question is, what could be causing a monitor to turn brighter? Every
other monitor I know turns darker over time.

And on a sidenote, when calibrating the flyback transformer, what voltage is it
you're adjusting? Acceleration voltage?

Regards,

Wiebe Cazemier

 

[James Sweet]

"Dave W" <davewi11@yahoo.co.uk> wrote in message
news:61b48aec-3016-4941-b1bc-4fa2832d4c7c@w38g2000hsf.googlegroups.com...
I have a Panasonic CRT TV with a similar problem. It comes on dark and
defocussed, but comes right after about half an hour. The effect can
be corrected by adjusting the screen and focus pots, but after warmup
it's too bright and defocussed. The screen and focus controls are
tapped off a potential divider from the EHT, inside the output
transformer assembly. The resistance to earth at the screen output
(probably about 500V) is 17M ohms, so the resistance at the top (25kV
EHT) must be in the region of 800M. My theory is that this resistance
is unstable. I would need a new output assembly costing about Ł25 to
repair it.


On 25 Dec, 13:59, Wiebe Cazemier <halfg...@gmx.net> wrote:

I suspect bad capacitors, that's the usual cause of things changing as they
warm up. It could be affecting power supply voltage.

 

[b]

On Dec 28 2007, 6:58 pm, "James Sweet" <jamessw...@hotmail.com>

I suspect bad capacitors, that's the usual cause of things changing as they
warm up. It could be affecting power supply voltage.

in both of these cases i would start by changing the cap in the RGB
output and b+ stages. i have cured many faults this way. the caps are
usually rated 160 v or more and has a low value - between 4.7 and 47
uF in my experience.

 

[Wiebe Cazemier]

On Saturday 19 January 2008 14:22, b wrote:

in both of these cases i would start by changing the cap in the RGB
output and b+ stages. i have cured many faults this way. the caps are
usually rated 160 v or more and has a low value - between 4.7 and 47
uF in my experience.

For now, the monitor is working perfectly, but I'll keep it in mind.

Are there others ways I can recognize those caps, by where they are situated
perhaps?