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Rainbow on Panasonic TV

Discussion in 'Electronic Repair' started by distar97, Jul 13, 2007.

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  1. distar97

    distar97 Guest

    This is very strange. My buddy has a 36 inch Panasonic Model CT36SF36
    that suddenly
    started displaying a rainbow pattern of colors that looks like a
    bullseye. In other words there are circular bands of color
    superimposed over the screen. The colors are transparent. You can see
    a sharp picture that is mostly black and white, but sharp.

    I doubt this is a normal degaussing problem. The bands are too
    circular. There is a bit of
    pulling in of the picture at the corners, as if the power is a bit
    low.

    Of course, the sets normal controls have no effect on the problem. The
    set was not dropped
    or otherwise physically damaged. The audio is normal.

    This is a big heavy set, swapping out parts is not practical. Has
    anyone ever seen this
    problem?
     
  2. mc

    mc Guest

    I'd still approach it as a degaussing problem. Did somebody hold a strong
    magnet up to the screen?
     
  3. Jamie

    Jamie Guest

    sounds like a little AC 50/60 Hz getting in the supply system somewhere?

    Maybe the heater on the Tube has shorted to the system ?
    or bad cap in the system?
    etc..
    Just a guess..

    Either that, some one has a magnet or something like it near it.
     
  4. distar97

    distar97 Guest

    No, it wasn't external magnets, speakers or other outside magnetic
    source. (no kids in house or others that may have
    accidentally done something)
    It could be some internal failure such as tube heater, caps, or stray
    AC. I prefer to save those type things for last.

    Could there be a flaw in the degauss circuit? It looks too organized
    too be the typical magnetic problem. Round circular color bands in
    blue,red and green centered on the screen do not appear like other
    screens I've seen where kids were involved with magnets or nearby
    speakers.

    Perhaps I can post a picture of what I'm seeing. I'll try and get a
    decent image up this weekend. Check later.

    All opinions are useful to me, I'll want to consider everything before
    I eventually open this back breaker up.
    Thanks, Denis
     
  5. It's not stray magnetic fields or the tube heater (that probably runs
    from the flyback).

    And I assume it didn't fall on its face - which could distort the shadow
    mask to produce a similar result.

    But you said the picture looked stretched at the corners?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  6. distar97

    distar97 Guest

    Sam G. said:
    "And I assume it didn't fall on its face - which could distort the
    shadow
    mask to produce a similar result.
    But you said the picture looked stretched at the corners"?

    Sam, thanks for your response... No the set did not fall or suffer any
    kind of physical damage that would have
    damaged the mask.
    I don't have a color bar generator handy, but I noticed that
    programming that should have vivid color looks as if the color
    adjustment was turned down, sometimes as if it were a B&W show. But if
    you ignore the colors, the picture
    is still sharp. In fact my buddy joked the set is fine for a color-
    blind person.

    The picture is not stretched but is pulled in a little at the corners.
    Not much though. It varies from channel to channel. It reminds me of
    what happens during a power brown-out. It's a minor observation
    compared to the circular rainbow problem.
    If I can get good representative photos, I'll post them on my personal
    site for easy viewing.
    Thanks Denis
     
  7. clifto

    clifto Guest

    Always degauss anyway.

    Years back I went on a service call. The problem was a complete and utter
    mystery to me; the colors were random and rainbow-like all over the
    screen, but the purity was PERFECT. Gray scale was PERFECT. Red purity
    was perfect, as was green purity and blue purity. At this particular
    shop the rule was to call the head tech before giving up, so I called
    and the tech told me to degauss.

    It fixed the problem.

    THEN it occurred to me that there was a thunderstorm the previous day,
    and this customer was in the area that got hit hard. I asked her if she
    had experienced anything out of the ordinary and she told me that they
    took a lightning strike directly into the asphalt-paved lot next door.
    I still have no idea how the purity could be so perfect while the
    shadow mask was magnetized.
     
  8. R!

    R! Guest

    Check the magnetic field coil correction system, Larger sets greater
    than anout 32" usually have a coil on the inside, fed with a modified
    pulse with some dc to correct for the earth's magnetic field.

    If the circuit has failed there may be too much dc on this coil...

    Simply unplug the coil and demagnitise the CRT. If that corrects the
    problem you know for shure where to look. This is not the degaussing
    coil it is smaller and closer to the yoke (deflection coils).

    R!!
     
  9. Jerry G.

    Jerry G. Guest

    The screen may be magnetized. The internal degausser may not be
    working.

    If the fault is not the internal degalusser, it is possible that the
    shadow mask in the tube has become defective.

    Both of these types of faults are comon.


    Jerry G.
    ======
     
  10. Could that have been preexisting condition that you just noticed?

    In any case, as others have suggested, try degaussing the set before
    digging deeper.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  11. AJ

    AJ Guest

    Change the degauss thermistor, resolder the connections where the degauss
    harness plugs into the main chassis, check if there is a degauss relay in
    the circuit, if so change it also.
    Seen a lot of large screen Panasonics, Samsungs, Sonys, RCA, Etc with very
    similar problems due to failures in the degauss circuitry. Also seen many
    dead sets with open fuse devises because the degauss thermistor actually
    changes its resistance value, which eventually blows the main fuse because
    of he additional current demand.
     
  12. hr(bob)

    hr(bob) Guest

    Just take a strong magnet and wave it around the face of the tube
    moving back gradually. See what that does.

    H. R. (Bob) Hofmann
     
  13. distar97

    distar97 Guest

    Thanks to all for the advise. Good response. I'll check it out. In the
    meantime I took some pictures.
    You can see them on a page I set up off-board Go to:
    distar97 daht googlepages daht kom sellash tvrainbow (I hate robots,
    sorry) Note: do not include www

    Thanks to all
     
  14. b

    b Guest

    that is excellent advice from AJ, Don't rule out the possibility of a
    degauss problem. You will need to open 'er up and check the degauss
    cct so as to be able to eliminate this as the most likelly cause.

    -b.
     
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