Grinder said:
A few years ago I purchased a gateway flat panel LCD monitor. The model
number suggests that its an 18" (as does a ruler,) but I recall it being
billed as a 19" At any rate, it has served well enough.
Early on the monitor developed some spots that weren't as bright as the
rest of the screen. You could see them if you had a constant color
background, but not if there was any sort of pattern. Now, though,
things have gotten worse.
While I was looking at the monitor, part of it became marked less
bright. It is at it's most pronounced at the top edge of the screen,
and fades as you move downward. So, if you were to bring up a solid
white screen, this monitor would make it appears as if you had a grey to
white gradient, top to bottom.
Is there any possible service that would cost less than the $200 to
replace this monitor?
Thanks for your thoughts and considerations.
The parts back there that affect output are the inverter
and the CCFL (cold cathode fluorescent) lamp. The inverter
converts +12VDC to around 1000VAC at 3ma or so. That connects
to the fluorescent lamp. The lamp draws 3 watts total power.
The inverter can run on a couple principles. It can be transformer
based, or piezoelectric based. The piezoelectric version, applies
a voltage to one end of the piezo material, and the mechanical
contractions at that end, cause high voltage electricity to be
generated at the other end. That is where the high voltage comes
out. (The piezo material is similar in concept, to what is used in
a gas barbecue starter.) The output of the piezo is supposed to be
a nice sine wave.
To get a range of contrasts, the inverter can be gated on and off,
at around 100Hz or so. Via pulse width modulation, the width of
the 100Hz enabling signal is varied, to give whatever lamp intensity
the user desires. So the lamp intensity adjustment does not have
to be achieved by varying the voltage across the lamp, but instead,
switching the lamp on an off rapidly.
The CCFL lamp is supposed to have a life of 15000 or 20000 hours.
I get the impression, that inverter failure is more likely than
CCFL failure, but your situation almost sounds like you have
two lamps, and one lamp is failing.
In the past, I have found a couple web sites, that carry replacement
parts. But the odds of getting exactly the same thing as was in
there, are pretty slim. Chances are you'll get a "nearest substitute",
if you were to order something. And the thing is, the lamp and
inverter are matched to one another. While you can attempt a
repair with new parts, there is no guarantee you'll get as
many years from the thing when you are finished.
Sometimes, a repair can be as simple as disassembling the connectors
on the inverter and lamp, and then reconnecting them. If the screen
goes dark, it could be a bad connection. But since your light
output seems to be steadily dropping, that almost suggests a lamp
failure. So maybe in your case, you'll actually have to order
replacement parts.
If you only have one monitor, I'd buy a replacement monitor first,
and that will leave you time for "surgery" and contemplation of the
symptoms.
Since there is high voltage in there, I don't recommend running the
thing with the parts sitting on the table. The inverter sine wave
might be at 70KHz or so (plus being gated at 100Hz), and at higher
frequencies, high voltage tends to travel on the surface of your skin.
That means you *might* get less sensation from touching it while it is
operating. But I'm not going to be the person to test that hypothesis.
(The only practical experience I've had with HF HV, is with a Tesla
coil, and that was painless. The resonant frequency of one of those, is
about 200KHz or so.)
Example of a spare parts site:
http://www.lcdpart.com/doc/ccfl.html
Here is an example of a DIY repair project. Some projects
are simpler than this one.
http://www.dnd.utwente.nl/~grit/backlight/ccft.html
Paul
This particular device doesn't seem that sophisticated,