yellowish, greenish patches on tv in bright scenes

Discussion in 'Displays' started by StevenFC, Sep 4, 2003.

  1. StevenFC

    StevenFC Second Unit

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    I was just wondering if anyone knows what causes the yellowish, greenish patches on my tv when the scene is a bright one. "Paper Moon" has a few of those kind of scenes and it causes them to appear. My tv is only a few months old 27 inch Panasonic with a digital comb filter and component inputs. Other than those patches and a slight downward slant of widescreen images (which I guess is the reason it was a closeout item), it provides a great image.

    Does anyone know what causes those patches?
     
  2. DavidES

    DavidES Stunt Coordinator

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    Sounds like color purity problems. The de-guassing circuit may not be working right or the purity rings got bumped somehow on the yoke or see below.

    Did you just recently move the tv? Changing the alignment of tv with the Earth's magnetic field can cause purity problems solved by de-guassing. The tv's de-guassing coil is usually not strong enough to correct though requiring a tech with a coil specially designed for de-guassing.

    Did the tube suffer a mechanical shock? That can damage the shadow mask/aperture grill, sometimes permanently.

    Call in a tv tech to de-guass the tube and check your tv's de-guassing circuit.

    De-guassing is what is done to remove stray magnetism which will adversely deflect the electron beams causing them to strike the wrong phospers.

    If you have the skills and tools needed, you can read up here on how to check circuit yourself and make your own de-guassing coil if you have old busted tv sitting around.

    Here are some pics of common tv symptoms including purity. Click the pics. They tell you what to check.
     
  3. ChuckSolo

    ChuckSolo Screenwriter

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    I have the same issue on my Panasonic 36" TV that is about a year and a half old. The yellowish/greenish patches are only visible when a white or off white back ground is visible. Reducing the "Picture" level as well as the "Brightness" setting on the TV minimizes the problem, but does not eliminate it entirely. I just talked to a TV tech at one of our local TV repair shops and he seemes to think that it is indeed a degaussing issue. He is coming over tomorrow to try and fix the problem. I will let you know if he fixes the anomoly.
     
  4. StevenFC

    StevenFC Second Unit

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    Thank you very much gentlemen.


    Addendum:

    I just looked at my video settings and noticed that the contrast (or "picture" as panasonic calls it) was all the way up. So I turned it down and fiddled with the brightness and it seemed to help. I actually think that my "Paper Moon" DVD may just be a little uneven when it comes to backgrounds. Like the sky or the back wall of the inside of the cafe at the beginning lf the film. Kind of has a purpleish tint to it on one side.


    I think I need to get "Video Esssentials" But in the meantime I'll keep working with it.
     
  5. ChuckSolo

    ChuckSolo Screenwriter

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    Incidently Steve, both of my Panasonic Tvs had the "Picture" setting set at maximum as a default. I just bought a new 27" Panasonic for my bedroom and noticed the "Picture" mode was set at maximum also. Fortunately, Panasonic TVs retain the settings for each of the "Video" modes separately from the tuner. If you set the "Picture" and "Brightness" settings too low, you will note that the greenish/yellowish tint does indeed turn to a purplish shade. At least mine does, that is why adjusting those settings minimizes, but does not eliminate the problem.
     
  6. Reginald Trent

    Reginald Trent Screenwriter

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    So what cause this in a RPTV?
     
  7. Steve Berger

    Steve Berger Supporting Actor

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    There are two types of purity problems that can produce the kind of symptoms you describe. One of them is always present but is more noticeable on solid color backgrounds. The other only occurs when a bright image is on the screen (and tends to persist briefly for a period of time after the bright image is gone).

    The first is an actual purity adjustment problem (misaligned) or outside magnetic influence (speakers , transformers , etc) too near the set.

    The second is a condition caused by an intense bright image causing the shadow mask to warp slightly from the heat of the electron beam. Reducing settings is the only way to minimise this problem. There are several different types of shadow masks and various mechanical and electronic methods to compensate used by different companies. The cheaper the set (and manufacturer), the less effort to correct. The smaller , the less need to correct.

    Warning: Old style manual degaussing coils can actually cause permanent purity and convergence errors on some new style picture tubes because of the way they are set up at the factory.

    On an RPTV it depends on the size (and color) of the error. Small ones could be burned phosphors ; moving ones could be impurities in the cooling fluid on a picture tube ;mirrors can crack ; front screens can warp ; registration (convergence) can go off ; a bug could die on the lens. Each of these things can produce a symptom that might be described as color splotches although that would not be a very accurate description for an RPTV.
     
  8. DavidES

    DavidES Stunt Coordinator

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  9. Steve Berger

    Steve Berger Supporting Actor

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    Some manufacturers make a special low-power degaussing wand for use on their sets. The usual practice (approved) in the shop , however , is to freeze spray the degaussing thermister which causes the circuit to run longer (but not harder).

    Usually the normal action of the circuit will clear up problems after several "turn on from cold " cycles. If the set starts out OK and picks up spots (which remain) after a short period then there is a magnetic source too close to the set. Degausser clears it then magnet remagnetizes it.

    During factory setup a computer operates a robot arm that hard magnetizes portions of the shadow mask to correct some purity problems. My guess would be that this allows them to use CRTs that otherwise have to be trashed as unadjustable.
     
  10. Allan Jayne

    Allan Jayne Cinematographer

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    Can you get the screen to go from an overall dark scene to an overall bright scene suddenly? (If you use AVIA or Video Essentials you should be able to.)

    Does the green and yellow patching appear instantly with the scene brightness change or gradually?

    For a bright scene, a lot of electron beam energy is being caught and absorbed by the shadow mask and as a result the mask heats up. A mechanical shock or jarring could have caused internal damage in the picture tube in turn causing the shadow mask to expand unevenly with respect to the screen surface. Then off color patches can appear that degaussing cannot fix. WHen you see the color patches appear a few seconds after you switch to a bright scene, that is the effect of shadow mask heating expansion you are seeing.

    Generaly, when the shadow mask expands, it should bulge towards the screen. If it moves from side to side, you get color purity patch problems.

    Video hints:
    http://members.aol.com/ajaynejr/video.htm
     
  11. Jeremy Scott

    Jeremy Scott Second Unit

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    My 36" JVC HDTV has a purity problem. I can somewhat correct it with the remote with the purity botton.

    Is this somethigng that a warranty would cover?

    Mine is more noticable on white screens witha light blue color i various corners.

    thanks.
     
  12. ChuckSolo

    ChuckSolo Screenwriter

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    The yellowish patches on my Panasonic 36" direct view are always in the same place and depending on the settings, contrast, brightness, etc., they go from the yellowish tint to a purplish blue. They are at the very top of the screen and in the right hand corner. Although I use a Pioneer center channel speaker that sits on top of the TV it is supposedly shielded for that purpose and the TV had the spots before I was using a center channel speaker at all. The TV was bought new and I first noticed it when a brightly lit sky scene was showing in a non letterboxed movie. Incidently, the repairman that was supposed to come out to my house to fix the issue on Saturday last never showed up. I got stood up.[​IMG]
     
  13. DavidES

    DavidES Stunt Coordinator

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    Chuck, use Avia geometry tests to see if the convergence (colored edges/fringes) is good. If not good, you can see if tech can improve it some. If good, then the shadow mask sounds damaged. Just set white level correctly.

    Jeremy, the color purity adjustments should be covered under warranty. Read your owner's manual on/near the back page for more info.

    Thanks Steve B. for the response. Very interesting changes in tv tech industry.
     
  14. ChuckSolo

    ChuckSolo Screenwriter

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    Thanks David. I need to purchase the Avia Software. Is their website the best place to do that?
     
  15. DavidES

    DavidES Stunt Coordinator

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  16. Jeremy Scott

    Jeremy Scott Second Unit

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    i realized today that if i turn my tv off for like 8+ hours, and turn it on that the corners are bad purity. if i leave it on for a few minutes, then turn it off for a few minutes and turn it back on, it looks better?

    anyone know what could cause this?

    Is it maybe cause it is not warm?
     

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