G8MNY  > TV       05.04.18 07:20l 99 Lines 3992 Bytes #999 (0) @ WW
BID : 15106_GB7CIP
Read: GAST
Subj: Odd projection TV fault
Sent: 180405/0613Z @:GB7CIP.#32.GBR.EURO #:15106 [Caterham Surrey GBR]
To  : TV@WW

By G8MNY                                      (New May 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

I was asked by friendly TV dealer if I wanted a free non working 52" diagonal
Sony widescreen rear projection LCD TV, that he had just replaced. These were
5k UKP retail only 4 years ago. The expensive Xenon arc lamps are the main
cause for failure. Having worked on a smaller one before, I picked up the huge
beast. (quite light actually & only 12" deep).

    Prismatic   Fresnel
      Screen    / Lens
            \  /
   Perspex||)<> \           The 3 screen parts are all together.
    Screen||)<>  \Large     The bulky screen & mirror all detach & then
          ||)<>   \Mirror   a picture can be projected on the ceiling.
          ||)<>    \Back    To get the large picture size 2 mirrors are used 
          ||)<>     \       as well as an expensive wide angle lens.
Wide Angle Lens-_
       Mirror \ ()Projector

          <--- 12"--->

I made a brief look at the lamp & it had not exploded or changed colour, so I
tested the electronics, the lamp inverter ran OK (scope nearby to see the 5kV
AC) for a few seconds but no light. So I re examined the lamp & spotted a
broken wire inside the lamp sealed reflector, inside the lamp carrier system...

  Screw Connection_
              .-~  x | < Broken wire
Screw       _/      |
Connection-[ [lamp] |
            ~\       |Front
     Parabolic'-_    |Glass
       Reflector ~'-.|

This hot steel wire goes through the glass reflector with a rivet to stop any
the heat cracking.

To repair it I used a steel PAPER CLIP that had been shaped by spiralling it
around another piece of thin wire. This was carefully wound through the rivet
hole to pick up the broken wire inside, then attached to the screw connector.
To my surprise this all worked & I saved the 200 UKP cost of a new lamp.

Unfortunately the picture was not perfect, there was a yellowish centre. So I
stripped the optics down to find the cause.

Below is a simplified sketch of the optics (much the same for LCD projectors),
there are further colour correction filters though.

The Optical block is 4 90 prisms making up a cube, with dichromate layers on
the internal surfaces to reflect & superimpose the 3 colour light paths into
the single projector lens.
                                  Lens _Optical block
             __             L pol  ___/  pol L
            /  \          / e  L \ / L  e \
           |Lamp|  Mirror/  n  C  X  C  n  \Mirror
            ----        /   s  D /_\ D  s   \
Columnating ----                Pol|||
    Lens 1  ----                  Lens
                       RED        GREEN      BLUE
            \    |        /          /         /
       Mirror\   |       /  YELLOW  /   BLUE  /Mirror
              \  |      /          /         /
            Colummating    Dichromate
               Lens 2       Mirrors

Unfortunately I found the Polariser (glued on the rotatable lens) in front of
the blue LCD tile has been UV damaged & has a dark centre. SONY obviously knew
there was a design problem here as the cooling air flow is much bigger for the
blue optics!

Nothing could be done for this problem, but most pictures (e.g. Faces, Football
Cricket) do not have a lot of blue in the picture centre, so the colour fault
is not as noticeable as U may think. The TV still does some blue, so blue skies
are OK, it is just white & greys that show up the problem. Other than that, it
is actually a brighter picture than my 1 year old 42" plasma!

Anyway I after all that time & effort, I sold the set OK, for more than a few
paper clips, as new one that size would still be 2-3k UKP!

Why Don't U send an interesting bul?

73 de John G8MNY @ GB7CIP

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