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Converting a 4:3 Screen to Multi-Format

Discussion in 'Home Cinema Building DIY' started by fortean, Dec 7, 2004.

  1. fortean

    fortean
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    I've just completed a retro fit, multi-format conversion, to my 7ft Owl 4:3 electric projector screen.

    A video of the screen working is available at http://www.4tn.net/homecinema/

    Cost of the conversion was under 20 GBP. I need to iron out a couple of difficulties and find a more suitable material before I post instructions.

    For this conversion I used black felt. I need a better masking material that is more dimensionally stable if anyone has a suggestion.
     
  2. fortean

    fortean
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    The first thing to do is determine the length of the masking material and the fixing position at the back of the screen.

    I did this with wrapping paper about the length of the screen height when fully down.

    With the screen fully down sellotape a couple of pieces of string to the back of the screen near the top. Leave about 12 inches of string hanging from above the sellotape and be sure that the bottom of the string hangs below the bottom of the screen.

    Now take your screen up about 18 inches or so. The string is pulled over the screen roll inside the case and will start to roll up. Next drop the screen a little at a time. The ends of the string above the sellotape should drop through the slot at the front of your screen. You should now be able to pull the string through until you have the end of it. Now roll your screen up again, making sure that the string doesn't disappear back inside, and remove the sellotape from the back of the screen.

    You now have two pieces of string going up and over the screen roll inside the case and out down the back. Tape your wrapping paper to the string and pull it through. It's easier to pull the paper through from the front to the back.

    Do not remove the string from the paper, this is your emergency paper retrieval system should things go wrong.

    You can now sellotape the paper to the back of the screen at approximately the halfway mark. Once you have done this you should close your screen fully. The paper will drop at the front as it rises at the back until the taped part is inside the case. It will then start to roll up with the screen. If you have used a long enough piece of wrapping paper it will be left hanging out the bottom. You should also still have the ends of the string hanging out at the back. Watch both the paper and the string as you roll up the screen.

    Now mark the paper level with the bar at the bottom of the screen. Open the screen to its half way point. Mark the paper again where it's level with the bottom of the screen. Now lower the screen fully to its normal 4:3 size. As you do this the paper will be moving up again. Mark the paper below the slot in the case.

    You now have 3 marks for top, middle and bottom. You need to get these marks to be the same for all three positions. This is done by trial and error by adjusting the bonding position of the paper at the back of the screen up or down a little at a time. If your top and bottom marks are 2 inches apart you should adjust the bonding point by 1 inch.

    Once you have marked the bonding position on the back of the screen and marked the mask length on the paper you can cut the paper down to the correct size. Now attach 2 pieces of string to the paper at the front of the screen and tie on some washers or similar to act as a counterweight whilst you test everything using the paper. If anything goes wrong you still have string attached to help retrieve the paper.

    The rest is down to you now. Choose a suitable masking material, I'm going to try velvet next, cut it down to size leaving an extra 1 or 2 inches for your mask counterweight bar and attach it to the screen where the paper was attached.

    This was the most difficult part for me as the material is the width of the screen. I used 4 lengths of string but because I was using felt it stretched as I pulled it through. When bonding to the back of the screen I recommend that you fold the edge to be bonded up so it is between the mask and screen. This means that as the mask is pulled down it pulls against a fold in the mask but when it is rolled up there is no fold in the masking fabric. Take care when bonding that you do not cause any distortion in the screen surface. You may notice when using the sellotape and paper that distortion occurs. This is because you will have stretched the sellotape as you applied it and it wants to shrink back again.


    Good luck and please post some results.

    The Diagram shows a representative side view of the screen closed, half way and open. The screen is black and the mask is red.
     
  3. fortean

    fortean
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    For anyone that is concerned about bonding the masking fabric to the back of the screen you can add a false back to the screen...

    Lower you screen to maximum.
    Bond a sheet of thin fabric, a little smaller than your screen width, to the back of your screen at the very top. This should be outside of the viewing area.
    Allow the fabric to settle overnight.
    Raise the screen almost all the way up.
    Bond the bottom of the fabric to the bottom of your screen, outside the viewing area again.
    When you lower the screen this new backing may sag a little but you can attach your mask to this instead of the screen.

    Be sure when you do this that as you raise the screen the first time there is sufficient space within the case for the addition thickness created by the false back and mask in the roll.
     

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