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Thread: What's wrong with my 5151?

  1. #1

    Default What's wrong with my 5151?

    First start it looks perfect, after running for anywhere from 30 seconds to a few minutes the horizontal geometry goes cattywampus and the raster starts quivering. Please take a look at the youtube video.

    I've tried:
    full recap
    deoxit on all pots and the horizontal linearity adjustable inductor
    using a different display on the MDA to make sure the incoming sync is still good

  2. #2

    Default

    Hi,

    I think it could be the environment. Do you have something close to the monitor that could produce magnetic interference? Have you tested in another room? Maybe even the computer case can produce some interference.

    Also temperature is a factor. Maybe it is not heated correctly or you are testing at very low temperature. My tests at 0 degrees (C) do not go well.

    If you find that one of the pots (inside the case) can correct it even temporally then you will know that it could be the pot itself.
    Last edited by tonata; January 23rd, 2021 at 08:48 AM.

  3. #3

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    Quote Originally Posted by tonata View Post
    Hi,

    I think it could be the environment. Do you have something close to the monitor that could produce magnetic interference? Have you tested in another room? Maybe even the computer case can produce some interference.
    It's not. Use conditions have been the exact same for several years and the problem developed recently. It has also been getting steadily worse, used to take 20+ minutes of operation to occur

  4. #4
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    Hard to say--I wonder if there's a small arc developing on the HV side of the flyback transformer.

  5. #5

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    Quote Originally Posted by Chuck(G) View Post
    Hard to say--I wonder if there's a small arc developing on the HV side of the flyback transformer.
    I certainly hope not. The raster never really collapses like it would with a FBT failure. Just shakes a tiny bit at the edges

  6. #6
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    Default

    Have you try to clean the internal adjustment pots for screen size ?.
    Also if is not that the powersupply in the monitor may have an issue and the voltage is not estable.

  7. #7

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    Quote Originally Posted by soviet9922 View Post
    Have you try to clean the internal adjustment pots for screen size ?.
    Also if is not that the powersupply in the monitor may have an issue and the voltage is not estable.
    Yes all pots have been cleaned.

    I might have a clue about the power though. The B+ voltage is 14.7 when the monitor first comes on and is working, but it jumps up to 16V when the problem occurs. Replace the 7815?
    Last edited by maxtherabbit; January 24th, 2021 at 12:56 PM.

  8. #8

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    Guess I'll buy a 7815 and try it, dunno why else the output voltage would be jumping up

  9. #9

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    Well replacing the 7815 made no difference, I guess I'll try the MJ2955 power transistor next

  10. #10
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    Transistor failures are usually all or nothing--and the 2955/MJ2955/2N2955 is a good sturdy old-school PNP part, but anything is possible.

    Does the brightness change when the thing starts misbehaving?

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