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MAGNAVOX TV MODEL MPTV68A (B) The TV loses picture after switch on. After several minutes, the time varies. If the TV is switched off for a while then swtched back on the TV will operate OK again for several minutes before losing picture.

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  • yakamo Jul 10, 2008

    MAGNAVOX TV MODEL MPTV 68A (B)
    SEEMS THE VOLTAGE THAT FEEDS ONE OF THE PICTURE TUBE HEATER CIRCUITS DISAPPEARS AFTER SEVERAL MINUTES WHICH CAUSES LOSS OF PICTURE.
    PERHAPS SOME ADVICE ON WHERE I CAN AQUIRE A CIRCUIT DIAGRAM WOULD BE BETTER. DO YOU KNOW WHERE I CAN GET ONE ??

  • yakamo Jul 10, 2008

    ONLY PICTURE, AUDIO IS OK.

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You should be checking the thermal fuse..
may be that is faulty inside

Posted on Jul 10, 2008

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Based upon your description, the mosty likely cause is a poor solder connection in the high voltage section not the heater circuit. If you can solder, you should be able to resolve this yourself. Carefully remove the main board that contains the flyback trnasformer (part with large red wire coming out of the top) and check the solder connections in that area. Resolder any that appear to be grainy or have rings. That may resolve the problem. If not, we must start looking for components that are somewhat sensitive to heat in that area. There are protection circuits that will shutdown the set if problems are detected in order to prevent further damage.

One other question: I am assuming that the entire set shuts down, is that correct? or does the sound continue?

Thanks,
Dan

Posted on Jul 10, 2008

  • ABRsvcs Jul 10, 2008

    Thanks for the update. WIth the sound still working, the set is NOT in protective shutdown. As I stated above, teh most likely scenerio is that there is a bad connection in the high voltage section. Also check the connections at the vertical IC. This will be a small flat IC attached to a heatsink that will be marked LA7837 or something similar. The loss of vertical signal can mimic HV loss as well.



    Keep us posted.

    Dan

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Switch on and then switch off , is bad regulation of the incoming voltage could cause it could be a cold or brittle joint,a blown fuse, fusible link or other component.it could be one of several boards.Unfortunately,the level of sophistication on these TVs make it virtually impossible to diagnose them further without taking them apart and probing around with a multimeter and other tools.
Find a repair man to avoid high voltage shock,.

Posted on Jul 10, 2008

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TV has internal overheating problem

Posted on Jul 10, 2008

  • ExpressFiX
    ExpressFiX Jul 10, 2008

    one of semiconductor is overheating so circuit cuts down the power.

  • ExpressFiX
    ExpressFiX Jul 10, 2008

    If you have experience on repairing circuits you might diagnose it, After opening the back cover there will be power circuit area check if hot areas, Also possible leakage of capacitors

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