Question about LG DU-52SZ61D TV

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Very dim picture but bulb is good

This is a 52" DLP HDTV. The picture is clear but very dim, only visible at all when close up, except for an approximately 1" vertical strip on the right side, which is brighter but still not bright enough. The bulb is good, as I can see it lighting up through the lamp cover and I even tried a new bulb with same results.

The edge of the brighter strip is not clean and sharp but rather blurred and somewhat uneven. It looks like what you might imagine the picture might look like if a piece of cardboard or something was mostly blocking the light path. Could it be some sort of obstruction? Though dim, the picture is clear and the colors seem even, and the sound and everything else works fine on the set.

If it matters, this set is only about 3 years old, and the previous owner told me he had the light engine replaced once while it was under warranty. It was then working fine until recently when the cleaning lady supposedly "bumped" it. Whoever worked on it did a poor job, since one of the two screws in the lamp door was replaced with a sheet metal screw instead of the correct machine thread screw, and the phillips head of the other screw was badly rounded. Perhaps the owner did this replacing or checking the bulb or something. I'm hoping the repairman just did a poor job on reassembly and the "bumb" knocked something loose which is now blocking the optical path. I have no experience repairing DLP TV's so I'd like to know for sure or at least know what to look for before I tear into the set. Thanks in advance for any advice or information.


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  • onsitecomps Jul 23, 2008


    Thank you very much for your fast reply.

    I will double check when I get home in a little while, but in the meantime I can answer some of your questions. First let me state that while I have no experience repairing DLP's, I repair computers for a living and electronics is a hobby, mainly audio gear and a bit of pic based robotics and automation. I'm comfortable with a soldering iron and basic test equipment. I have a fairly decent quality digital multimeter with a thermal probe, and an oscilloscope, but no true signal generator or luminance meter. If all that is needed is a test pattern, I can hook my PC up to it though.

    I've studied various articles on DLP, such as the ones at, and from that and the explanation you gave me, I beleive I can state the following:

    1) The color wheel seems to be working, as you can see the video on the screen, though very dimly, and the color seems correct.

    2) I don't know if the bulb is at it's proper luminance, however looking through the lamp door it does seem to be shining brightly. Also I tried a new bulb and still had the same problem. This does not rule out a bad ballast though, I assume.

    3) I will check to be sure all the lenses are in place when I get home, but from what I remember there didn't appear to be an easy way to access them. I may have to pull out the entire engine assembly for this. (which I will do if needed). Without an exploded view diagram, any determination I make will be an educated guess however.

    4) Obstruction - This is what I suspect, and hope, is the problem. I will take some photos and/or video of the screen showing the problem when I get home so you can see what I'm talking about. The edge where the dimmer part meets the brighter strip on the edge is not a clean line, like you might expect with some sort of digital problem (bad processor or such), but rather is blurry and uneven, exactly like you might expect with an obstruction in the light path. Anyone whose ever watched an old reel to reel or just made shadow puppets on the wall will know what I mean.

    If this is not the problem, I found a website which sells a .pdf service manual for this set for I think $19. It should have the schematics and exploded view diagrams I may need.

    If it's a lens focusing or alignment issue I suppose I'll have to have a pro service it, as I imagine you need special tools and equipment.

    Thanks again for your prompt reply and guidance. I'll reply later this evening as soon as I check the things you suggested.


  • onsitecomps Jul 27, 2008

    OK, I found the problem. Just past the lamp is the condensing lens, which focuses the light through a small rectangular aperture plate (this is what it would be called in a cinema projector I believe). After passing through the plate, the light travels through an approximately 3cm long rectangular metal tunnel, which is lined with four glass mirrors running lengthwise, one for each side of the rectangle. This tunnel sits just in front of the color wheel, passing the light on through it and on its way to the rest of the optics, including the DMD.

    This tunnel is the problem. The top mirror came loose and one end of it slipped down to just above the bottom of the tunnel. This is causing most of the light beam to be blocked, except for a small sliver at the bottom and whatever light manages to filter through the foil in the mirror itself. This explains the screen image I described - a full color image across the whole screen, but very dim except for one brighter strip down the right edge of the screen, about an inch or so wide. The various turns and inversions the light makes passing through the lenses and mirrors of the optics account for the bright strip being on the right instead of the bottom of the screen.

    The question now is how to properly repair this problem. Super glue comes to mind, but I imagine a 120 watt lamp being focused down such a small tunnel creates a good deal of heat, and I'm not sure how super glue will stand up to it. It may just come loose again, or worse the glue might fume up and foul the mirrors or color wheel. Do you know of any easily obtainable adhesive suitable for this purpose? It has to be able to bond glass to metal and withstand the heat involved. It wouldn't need to be very strong at least.

    I was considering using thermal epoxy - the stuff often used to permanently affix heatsinks to various semi-conductors. This stuff could easily handle the heat and shouldn't fume up, but it's generally rather thick, like a paste or even putty, and would be rather difficult to apply thinly and evenly down the length of the tunnel.

    I wonder how difficult it would be to find a replacemt part for this tunnel should the need arise. The exploded view diagram and parts list in the service manual I bought doesn't even list or show it at all.

    Anyway, thanks very much for your time and assistance, Shuttle83. I really appreciate it. :)




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Hello onsitecomps/David
With all of this bumping and knocking, still the problem you are having should not be occurring. This is due to the fact that the light engine is basically protected from that kind of activity. Now I feel that I have to explain how the light engine works, because there are a few things I am going to ask you and I want you to know why I am asking you these specific questions.
First of all a light engine works in the following manner:
A digitized video signal is sent into the signal and control bus of a DLP processor the signal is used to activate the transistors that control the mirrors that reflect light from the light source (the bulb) to the screen (more explained in next sentence) of the TV. The light from the mirrors then is reflected into a color wheel which spins at a rate which allows the individual monochromatic lights from the mirrors to be reflected with its proper color sequence (this is what creates the color picture from the monochromatic light from the DLP processor). This is a high level description, yet it is accurate for the questions I am about to ask.
1) Is the color wheel moving? (you can tell because it will make a fan like noise when it is turning)
2) Is the bulb at its proper luminance? (you can tell by using a light meter)
3) Are all the lenses in place? (you can tell by opening the light engine and inspecting the lenses)
4) Is the light from the bulb striking the DLP processor mirrors? (This sounds like what is not happening in your case, this or the processor is bad, being blocked by an obstruction, or out of alignment)
I believe that the last question is what is happening in your case, please correct me if you think that my assessment might be incorrect.
As for doing any of the work yourself, I would advise against this because this is what may have happened in the first place with the first owner. (who knows for sure). I would suggest getting this repaired by a qualified TV repair shop since the placement of these parts and the handling is very critical. If you do plan on doing this yourself you will need the following items:
1) voltmeter (to measure the voltage levels on the DLP processor, and the other points on the circuit board)
2) schematic diagram with service manual (To see where parts are located and to indicate where test points are)
3) light level power meter or you can use a temperature meter and do the conversion formula for watts to lumens (heat energy to light energy)   4) Ohmmeter (to measure resistances of components and circuit devices)
5) Signal generator (to create/generate a reference signal source with which to test against)
Now, if you know what these pieces of test equipment are and how to use them great have at it, if not, my original statement hold true: get a qualified TV repair person to take care of this issue.
I hope this helps,
Thank you,

Posted on Jul 23, 2008

  • Henry McKelvey
    Henry McKelvey Jul 25, 2008

    Hello onsitecomps,

    I posted this also on the other posting you had sent out

    I was working on a projection TV the other day and I am wondering if this set has filter mirrors? Filter mirrors are mirrors that filter out all colors but one and the use the combination of the colors to produce a luminance light( white light ) which is projected along with the RGB lights. This might be the problem that is occurring here if the mirror that reflects the luminance light is damaged it would still show the color picture but without luminance. This is just a thought that hit me.

    I hope this is of help to you,

    Thank you,


  • Henry McKelvey
    Henry McKelvey Jul 27, 2008

    Hello onsitecomps,

    There are heat resistant adhesives you can use to do this, believe it or not this post has helped me to. A set came into the shop yesterday with a problem similar to yours, and I went straight to the mirror array and found a loose mirror I went out to home depot and got some mixable epoxy, formed two small balls just to hold the mirror in place I have been subjecting it to heat for a few hours now and it is bonding well. In any event I was glad to be of some help, but you did most of this yourself congratulations on that.

    Thank you, for letting me be of assistance




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