Trying to fix polarizer problem in EPSON EMP-835. Need help - AVS Forum
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post #1 of 5 Old 11-23-2010, 02:33 PM - Thread Starter
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Hi all,
I am new to projectors and need some information and help. The projector is a EPSON EMP-835(LCD).

Problem: I was seeing purple spots in the projected image from the projector. I took it apart and found the usual dust sticking on the polarizers. In this projector there are 2 polarizers before the LCD's for the green and blue LCD's.For these two LCD's, the first polarizer had the normal coloring on the filter,the polarizer closest to the LCD looked clear.with a clear film glued to it. The red path has only one polarizer before the LCD.

Most of them cleaned up very well with a lens cloth, until I came to the one closest to the blue LCD. When I was cleaning the clear one(the one closest to the blue LCD) the lens cloth scratched the clear surface very badly. I assumed this happened because this plarizer was damaged by the blue light UV causing it to be softer than the others. I did not use any chemicals, just my breath.

I assembled the projector back together and powered it up. Now the picture has less blue and is kind of yellow. I must have damaged the second blue polarizer(the clear one) with the scratches . I let it cool down, then I took it apart again and took out the second polarizer(the clear one). Reassembled the projector and powered up again. Now the picture has way too much blue. Makes sense, since I took out the second blue polarizer.

My questions:

1. Why is there 2 polarizers for the blue and green LCD's and not the red one? To filter out more of the respective colors to balance the red? Do all LCD projectors have 2 polarizers before the green and blue LCD's and only one for the red LCD? It seems like there are more polarizers in the newer designs to increase the contrast ratio.(any one has a link for a explanation?)

2. Is the clear polarizer for the blue LCD color dependent or just a general polarizer? I held up the clear polarizer to a light and saw no coloring through it.

3. Could another clear polarizer from another broken projector(assumming it fits) work as a replacement?

The picture was ok before except for the purple splotches. I have searched the web for any solutions and have not found much.

Any help or explanation would be appreciated from anyone.

Steve
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post #2 of 5 Old 11-23-2010, 07:29 PM
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hi steve, wrong section to post in but i do have some answers for you.

Those two filters you see in front of green and blue, one is polariser the other is a colour filter to 1. reduce the overall amount of green and blue and 2. to increase gamut by filtering out cyan and yellow frequencies.

no not all lcd pjs have two filters infront of green and blue. some such as the 8350 have a magenta coloured filter at the lamp instead.

each RGB channel only has one polariser, the clear one that you broke. you can replace it with any clear polariser of the same orientation.

by purple blotches you mean magenta? if so then its a lack of green in those dots. so maybe dust on the green lcd or filters is to blame.
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post #3 of 5 Old 11-24-2010, 02:22 PM - Thread Starter
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Where should I have posted instead? I am new to this forum. Thanks for your information.

As to your answer about using another polarizer. It is just a clear polarizer?
Is it to just make all of the blue(or any color light) go in one direction for more control by the LCD?

I noticed in looking at other LCD projectors that the contrast ratios were in the 100-400 range, then there are the ones like this one that has a contrast ratio of 700. What was done to improve the contrast ratio?

Is there a good site for this kind of information? I have look a lot and have not found much.

Steve
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post #4 of 5 Old 11-24-2010, 03:38 PM
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Lol not sure the best place but I don't think the 3000usd+ community knows much about diy repairs.

It's likely just a clear linear pol but I cant say without seeing it.

As for why it's there, it is THE reason LCD works. Incoming light is polarized. LCD alters the polarization in a pixel by pixel basis. Then output light is filtered with another polarizer, thus producing black and white pixels.

That's why when u took it out, the entire image became blue, as the lcd no longer had control without polarized input.

There is no particularly good site that consolidates all this. I learned as u did, from taking things apart, reading various articles, forums.

As for contrast ratio, it depends on the polarizer efficiency, the light collimation, the LCD efficiency, and others that I'm not yet aware of.

So where did u get this pj and why are u trying to diy it?
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post #5 of 5 Old 11-24-2010, 10:36 PM - Thread Starter
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Thanks for your infomation.

I sort of fixed the projector enough to use it. I figure I couldn't do any more damage by trying to eliminate the scratches. So I used alcohol and q-tips to wipe the surface of the damaged polarizer. One side has a plastic sheet glued
onto it and the other side did not. The damaged side is the one without the sheet.

I think that the scratches are from a coating that got soft and smeared. The alcohol removed some of the smeared film. I only cleaned it a little because I though if I cleaned it a lot, the polarization would be removed. It's not perfect but a lot better. I will try to removed the rest of the smeared film later. I hope that removing this film has no adversed effect.

I bought the projector through Ebay and did not want to pay a lot for a projector. I tend to try to fix things.

Steve
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