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    CRT TV green color missing after sometime

    my samsung 29 inch CRT tv is missing the green color intermittently. green color comes when i turn it on. works well for some 10 mins then green color vanishes. i can see green patches, but not proper green picture still looks pinkish or purple when i change the video mode to standers, custom etc in mild it green color looks black.

    looks like the color goes on warmup just assumption, i recently changed the HOT and snubbing capacitors tv works well. only issue is the color. i think while repairing i might have fiddled some part which i should not have touched.

    checked all solder point for any dry joints and fixed some. any guidance will be helpful

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    Re: CRT TV green color missing after sometime

    Almost certainly this is a problem with components on the CRT base PCB. Check it for dry joints and intermittent potentiometers. It is also remotely possible the CRT socket itself isn't making good connection to it's pins and it goes open circuit as the glass warms up and expands.

    Brian.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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    Re: CRT TV green color missing after sometime

    It might also be RT problem.Emission of the CRT may be decreased by time.
    RGB Driver IC ( or discrete whatsoever..) may also broken..
    There are some other possibilities.Check the signal by an Oscilloscope with a standard Color Bar pattern is applied..



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    Re: CRT TV green color missing after sometime

    Begin at the beginning: check the small PCB mounted at the socket of the CRT. Is one of the transistors getting too hot?

    But still more elementary: remove the PCB from the CRT socket, check visually, and replace the board and see if the problem reappears.

    See if it is possible to interchange the RGB wires (say R and G) and see whether the problem shifts or stays the same.

    Intermittent problems are often difficult to locate exactly and you will need patience.



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    Re: CRT TV green color missing after sometime

    i don't have a CRO, but checked the voltage at the green cathode on the CRT socket it was reading 135 v when green color was coming along with other pins those were around same value.

    when the green color on screen is gone, it was reading 176v but other pins were unchanged.

    - - - Updated - - -

    i interchange the green and red at the cathode base feed from the RGB amplifier (STV5109) , it is red now. so now next step seems to be the feed to RGB if that is also faulty then it will be the chrome ic signal. will keep posted but what could effect something after few minutes of startup. i switch it off for few minutes again turn the tv on i see the color back. is it some capacitor or something ?



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    Re: CRT TV green color missing after sometime

    Hello yaaku,
    I'd say with high certainty you have a thermal problem, where a component is changing its value
    with heat.
    Brian's suggestion would be the first thing you need to do. Failing that, then you need to
    get the aid of a can of freeze spray to isolate the offending component.
    Failing using freezer spray, you can easily swap components from the red drive with the green one.
    You should eventually find the culprit.
    Try concentrating on equal value electrolytic capacitors, signal diodes, transistors and resistors.
    You should find that each gun will have their own independent but identical valued components,
    as this will make it much easier.
    If you find no faulty components on the CRT PCB, then you need to trace back to the chroma
    processing. Best thing is to get a datasheet of the chroma IC to give you an idea of the section
    on your main PCB where you need to tackle the fault.
    This is not a common fault with the other colours which will make trouble-shooting the fault a
    hell of a lot easier, therefore you need to look at identical parts for each of the components
    that share identical values.
    Please let us know how you get on.
    Regards,
    Relayer



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    Re: CRT TV green color missing after sometime

    i interchange the green and red at the cathode base feed from the RGB amplifier (STV5109) , it is red now...
    This suggests that the RGB signal coming to the CRT base driver is faulty. The circuit at the PCB base is ok.

    The high voltage (176v) is corresponding to blanking; the lower voltage (approx 130v) is likely ok.

    There is an high voltage transistor what may be the likely culprit; there will be three identical circuits for each of the RGB and it may be easy to identify the bad one.

    Also see that all the three transistors are fed with about 176v but one of them is likely to be bad. These transistors are somewhat bigger and easy to identify and locate.



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    Re: CRT TV green color missing after sometime

    I checked the line coming out of the chrome IC to STV5109, interchange red and green still it was an issue with the green so i decided it is this section between input and CRT cathodes.

    I found the schematic for STV5109 crt base board and doubted it was the green feedback network so i worked on it. but when i tried to test the feedback voltage with the multimeter prob found some issue.

    but it got resolved immediately while taking measurements, checked for any dry joints but could not find.

    It is working since then all well color all perfect. luckly it was so strange fix.

    may be the feedback diode got some reverse voltage while taking measurement and the semiconductor had aligned itself don't know some material science change might have happened but it is working fine now.



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    Re: CRT TV green color missing after sometime

    it got resolved immediately while taking measurements,...
    Common old-age problems...



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