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From: Andy Cuffe on 28 Jan 2007 14:07
I have one of these TVs that's starting to show signs of failure of
the blue and green polarizers. There is a slight lack of blue from
most of the screen and there's a noticeable green cast to what should
be black. I took apart the light engine to clean the dust out and I
can see a slight burn on the blue polarizer. The green filter looks
ok, but I find it hard to believe this set originally had such poor
blacks (it's only 2.5 years old).

Has anyone had any luck replacing the polarizers in LCD projectors
like this? From what I've read, they aren't available from Zenith/LG.
Has anyone found a source for new polarizing material that could be
made to work? I assume the polarizers used in other types of LCDs
such as overhead projector panels, or laptop displays wouldn't be able
to take the heat. Has anyone tried it?
Andy Cuffe

acuffe(a)gmail.com
From: ZZactly on 29 Jan 2007 21:56
I got the same problem in one a bit older and bigger.

I've studied the theory of operation and there is a BIG BIG problem
changing polarizing filters. Orientation.

If you get it 90 degrees off the color affected will appear in
negative. Anywhere in between no amount of gamma correction will fix
it.

If we assume, and are correct, that the polarization is along the
plane of either the vertical or horizontal boundries, we achieve a
50/50 chance of gertting it right.

My unit appears to possibly use more then one for the blue channel.
The filter is gone, and everything that is supposed to black is blue.
But then there is some modulation. See nowhere in the actrive video
can it produce white. It gets the dingy brown, except for in the
menus.

PTVs have long had problems with blue. It was dealt with many ways in
CRT days. A blue push circuit, Sonys used a controlled defocus.

Of course these things are a different animal. But blue is still blue,
and I believe it is still a problem for engineers. It is probably
because of it's short wavelength. In an LCD, the blue is obviously
hotter, as anything that passes blue has to absorb or reflect
everything else, including heat and IR, which are at the opposite of
the visible spectrum.

I'm sure that the polarizing filter from a camera shop would probably
work, but which way does it go in ? The material itself is not hard to
get.

It appears that they glued the material to glass to provide some
thermal damping. But on glass, it obviously didn't work very long.

For reasons like this I highly advise customers against buying any LCD
based PTV. Even DLPs melt the color wheel. It is just too damn hot.

Actually your symptom doesn't sound like the primary polarizing filter
for the blue channel, but these wild designs do make troubleshooting
by symptom difficult at best. Why can my Zenith display white only on
the menus ?

LCD technology is fascinating, but IMO is not ready for the mass
market. It just ain't ready,. polarizing filters should be available
like light bulbs and air filters. They probably never will be.

The main question, if we intend to replace polarizing filters is, once
the light comes out of the lens is it still polarized ? If so we can
determine the axis at the light output. Follow the light path backward
and figure out which way to put it in. I don't see any other way.

JURB

From: Andy Cuffe on 29 Jan 2007 23:03
On 29 Jan 2007 18:56:16 -0800, ZZactly(a)aol.com wrote:

>I got the same problem in one a bit older and bigger.
>
>I've studied the theory of operation and there is a BIG BIG problem
>changing polarizing filters. Orientation.
>
>If you get it 90 degrees off the color affected will appear in
>negative. Anywhere in between no amount of gamma correction will fix
>it.
>
>If we assume, and are correct, that the polarization is along the
>plane of either the vertical or horizontal boundries, we achieve a
>50/50 chance of gertting it right.
>
>My unit appears to possibly use more then one for the blue channel.
>The filter is gone, and everything that is supposed to black is blue.
>But then there is some modulation. See nowhere in the actrive video
>can it produce white. It gets the dingy brown, except for in the
>menus.
>

I took the filters out today. The green and blue filters are
definitely damaged. There is a very noticeable rectangle in the
middle of both bad filters.

Using a sheet of polarizer from a bad LCD display and rotating it
until the TVs filter goes dark I could see that the faded part is not
completely polarizing the light any more. The blue is also a little
dark in the middle. This explains both symptoms with the picture.

Figuring out which way to cut the new filter isn't a problem. You
just put the old filter on top of the new one (so the glass is NOT
between the old filter and the sheet of new filter). Then rotate it
until it's as clear as possible. Then it's just a matter of cutting
the new filter.

I tried it with this LCD screen filter and it worked surprisingly
well. Blacks are much better than with the original filters. The
only problem is that the filter plastic I used doesn't pass as much
light as the original stuff. The whole picture is about half as
bright as it used to be. It's still an improvement since black is
better and there's no tinting to the picture.

I ran it for about 3 hours and it seemed to hold up. If I can find a
more efficient polarizing filter I think this could be a practical
repair.
Andy Cuffe

acuffe(a)gmail.com
From: ZZactly on 3 Feb 2007 11:00
Bravo, you've made more progress than I have.

I understand that orientation can be determined the way you did,
however in my case the filter had already been removed by the time I
got to it. Thus my problem.

Is what you used a polarizer from a direct view LCD ? If so I think
that explains the lower transmission, it probably has some anti-glare
or anti-reflective properties that are not needed in a PTV.

Problem is I think the new filter(s) are going to run hotter due to
those properties.

I think we will be able to repair these things if 1, we get them while
there is still some of the original filter to determine orientation
and 2, obtain the high transmission polarizers.

Oh, I forgot number three, have the hands and skills of a microsurgeon
and quit drinking coffee three days before working on them. For a
fraction of the pay I might add.

Mine is coming back when the work slows down. We can barely keep up
right now. Actually I am surprised I wasn't handsomely bribed to work
today, with super bowl Sunday coming tomorrow. But I have this policy
of not working Saturday anymore so, this would have to be one sweet
bribe, and I think they know it.

Anyway, if you have a lower light output than before, I think it will
burn up faster than the original. I don't think you can really call it
repaired. If it burns up in a few months, the customer will blame you,
not the manufacturer. I could say it right now, the filters should all
be glass, or embedded in glass, not glued on like a decal. In
addition, I believe the manufacturers know of this limitation.

Congrats on making progress on this, but that doesn't solve my
problem. I still have the burning question, is the light still
polarized at the lens output ? If so, with it running, when the blue
blacks disappear that is the orientation. With a bit of mental
gymnastics, looking at which way the mirrors go in the light engine, I
think I could make a heck of a good guess.

JURB

From: Andy Cuffe on 3 Feb 2007 18:37
On 3 Feb 2007 08:00:44 -0800, ZZactly(a)aol.com wrote:

>
>Congrats on making progress on this, but that doesn't solve my
>problem. I still have the burning question, is the light still
>polarized at the lens output ? If so, with it running, when the blue
>blacks disappear that is the orientation. With a bit of mental
>gymnastics, looking at which way the mirrors go in the light engine, I
>think I could make a heck of a good guess.
>
>JURB

I agree that these filters probably won't last, but it at least proves
that they are the only problem with the projector. I think you can
get polarizers for theatre lights. They should be designed to take
some heat and might work.

On this projector, it has maximum transmission when the projector is
off, so I could find the correct orientation by rotating the new
filter while looking through an LCD panel. The design of the optics
make this easy, but it might be impossible of other projectors.
Andy Cuffe

acuffe(a)gmail.com
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