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video screen inverter question

 
 
pmaczik@charter.net
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      12-04-2012


12-04-2012 05:51 AM

I have a hp pavillion dv9700t CTO entertainment laptop running Windows
7 with Nvidia GeForce 8600 video drivers.



I might have a bad video inverter. I sort of have a black screen
problem. I am connected to my LCD TV now so I assume my video adapter
and drivers are working. The odd thing I discovered is, I can see my
laptop video if I shine a flash light or a desk top condescend lamp on
it. I can read this text that I am writting now. I can see color but
it is like looking at my desk top through a dark filter lens. With
out the lamp my laptop screen just looks black.



Any clues as to what is going on.



Thanks for your time,

Pete
 
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Ken
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Posts: n/a
 
      12-04-2012
http://www.velocityreviews.com/forums/(E-Mail Removed) wrote:
>
>
> 12-04-2012 05:51 AM
>
> I have a hp pavillion dv9700t CTO entertainment laptop running Windows
> 7 with Nvidia GeForce 8600 video drivers.
>
>
>
> I might have a bad video inverter. I sort of have a black screen
> problem. I am connected to my LCD TV now so I assume my video adapter
> and drivers are working. The odd thing I discovered is, I can see my
> laptop video if I shine a flash light or a desk top condescend lamp on
> it. I can read this text that I am writting now. I can see color but
> it is like looking at my desk top through a dark filter lens. With
> out the lamp my laptop screen just looks black.
>
>
>
> Any clues as to what is going on.
>
>
>
> Thanks for your time,
>
> Pete
>

Does your computer display properly BEFORE it enters Windows? See if
you can boot from a CD and have the LCD display the characters properly.
That will tell you if you have a problem unique to Windows or not.
 
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Paul
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Posts: n/a
 
      12-04-2012
(E-Mail Removed) wrote:
>
> 12-04-2012 05:51 AM
>
> I have a hp pavillion dv9700t CTO entertainment laptop running Windows
> 7 with Nvidia GeForce 8600 video drivers.
>
>
>
> I might have a bad video inverter. I sort of have a black screen
> problem. I am connected to my LCD TV now so I assume my video adapter
> and drivers are working. The odd thing I discovered is, I can see my
> laptop video if I shine a flash light or a desk top condescend lamp on
> it. I can read this text that I am writting now. I can see color but
> it is like looking at my desk top through a dark filter lens. With
> out the lamp my laptop screen just looks black.
>
>
>
> Any clues as to what is going on.
>
>
>
> Thanks for your time,
>
> Pete


Since you've done the "shine a flashlight" test
and the LCD is actually loading the pixels
with the right information, your problem
is with the backlight (source of light
that normally drives the display).

As you surmise, on a CCFL lit monitor, the
backlight subsystem consists of one or more
cold cathode fluorescent tubes (thin, and as long
as the monitor is wide). A power inverter,
converts a low voltage DC, like 12VDC inside
the monitor, into a 700-1000VAC voltage at
a low current. Each CCFL tube draws around 3 watts
of power at that high voltage.

The inverter accepts low voltage DC as input, but
also accepts a control signal that sets the intensity.
The intensity control can be PWM or pulse width
modulation. The PWM might be at 100 Hz, while the
high voltage is at around 25KHz (above human hearing).
The picture here, is intended to show a roughly 50%
duty cycle driving signal for the CCFL (bursts of
25KHz voltage). I can't draw an accurate waveform
this way, without it being many pages wide.
_ _ _ _ _ _
____| |_| |_| |________| |_| |_| |________

A typical situation, is a user notices the
light comes on for two seconds, then goes off and
stays off. Power cycling the LCD monitor, might
give another two seconds of operation.

The startup of a CCFL has two phases. The higher voltage
"ignite" phase is first. There is no mercury vapor at
first when the tube is cold. The striking voltage is
higher as a result. After two seconds, the plasma inside
the tube, has more of the volatile mixture flying
around, so the tube conducts a bit more. The inverter
voltage drops. It is at this point, that a weak inverter
just shuts down instead of continuing the "burn" phase.

Reducing the brightness setting, sometimes gives relief
from shutdown. For a few weeks. But eventually, the
inverter will quit altogether.

If the load is temporarily removed from an inverter
(like the wires from the CCFL tube are loose), the
voltage coming out of the inverter can shoot up,
causing damage to the inverter. On the old
piezoelectric inverters, they'd go up to 4000V
and then the crystal would crack and it would be
ruined. On the transformer based ones, I suppose
they could arc between windings and be ruined.

They're sensitive to stray capacitance, because
the inverter connection to the CCFL is via
capacitive coupling. Only pure AC is allowed to
appear on the terminals of the CCFL. Any DC component
(or apparently, even some harmonics), can cause
degradation of the CCFL and a shortening of its
life. If you drove the CCFL with a DC supply and
no AC at all, it would probably be ruined by the
end of the day. In normal usage, they last for
something like 25000 hours. But various kinds of
defects in the inverter, can shorten that life.

Few people suffer from "worn out" CCFLs. If you
had one of those, the tube light turns "brownish"
as a hint. The vast majority of the time, the inverter
is cutting out.

Now, decent sized monitors, can use multiple CCFL
tubes, and multiple inverters. It would have to be
a relatively small monitor, for one inverter to die
and wipe out all the light. Some of the big monitors
I've seen at Best Buy, have 16 CCFLs and
4 (quad) inverters, and you can get a
"sun tan" in front of them

Companies like this sell replacements. But, these
are typically "generic" replacements, and we don't
know how well they match the load. I'm only showing
this page, to show a picture of the inverter. This
one is long and narrow. Some (multi-channel) are
perhaps a bit wider and shorter in length. The image
on this page, has been altered by the website, to
hide component details. The extensive usage of the
word "US" in this advert, means you're dealing
directly with China

http://www.ecrater.com/p/15188290/hp...t+CTO+inverter

To learn more about the art of inverter design and
the careful packaging of CCFLs, you can try this
book. This guy analyzes failed efforts by other
individuals, to build devices with CCFL backlights.
So sometimes, your equipment is doomed, by the
nature of the stupid design decisions made in it.
When you see bits of tin foil inside the LCD housing,
put them all back the way you found them. They're
there for a reason (sometimes a good reason, and
sometimes not).

http://books.google.nl/books?id=SPwq...page&q&f=false

I also have this in my archive. Possibly the same
content, and a bit easier to read. The author, Jim
Williams, I think he appears occasionally over on
sci.electronics.

http://cds.linear.com/docs/Application%20Note/an65f.pdf

*******

The other kind of monitor, uses LED (light emitting diode)
lighting. The LEDs could be tri-colored or the
light source could be white LEDs. A white LED,
if driven hard, has a life of around 50,000 hours
or so. The failure modes could be entirely
different, than the ones you see on a CCFL
monitor. There is no inverter. I don't think
I've run into anyone yet, who has needed to take
the back off one, so we don't know what's inside

Paul
 
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