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"Snow" on TV during use

Discussion in 'Electronic Repair' started by Tim R., Dec 26, 2004.

  1. Tim R.

    Tim R. Guest

    I inherited Grandma's old Zenith TV set from the early 80s and for the most
    part it works like new. However, sometimes when you first turn the set on,
    or maybe after a few minutes of use, the picture will turn to snow and
    static on every channel. It will stay like this for anywhere from a few
    seconds to ten minutes, and then will snap out of it. After this the TV set
    will work fine, with no problems whatsoever, until you turn it off and let
    it sit for a while. Then it may or may not come back.

    I believe there is an intermittant component somewhere that is causing this;
    maybe a faulty varistor that won't work properly unless it's at a certain
    temp. My belief that this problem lies somewhere in the RF stages, since the
    display and audio are fine.

    Where would I begin to look for trouble on this set?

    I have extensive knowledge of vacuum tube devices, but transistors and
    smaller are unfamiliar territory to me.



    Regards,

    Tim
     
  2. CJT

    CJT Guest

    Take off the cover and turn the set on in a dark room. Watch for corona
    discharge around the high voltage section and picture tube.
     
  3. Tim R.

    Tim R. Guest

    Y'know, now that you mention it, the pitch of the flyback does change in
    frequency every once in a while when the set's first turned on, but I never
    thought anything of it.

    Regards,

    Tim
     
  4. rpbc

    rpbc Guest

    The pitch change means nothing but a slight loading of that circuit, or
    loss of incoming sync. If the flyback was not working you would have no
    raster, which you have because of snow, plus the high voltage is okay.
    The presence of snow, and white noise in the audio indicates everything
    after the tuner is working, that the IF circuits are working full gain
    with no AGC from incoming signal. You are probably right in your first
    thoughts, it is in the RF selector, the tuner. Check supply voltage to
    it, also, if possible, remove it as a unit and carefully solder all board
    connections with a quality iron... you might fix a bad solder joint, not
    an unusual fix for that kind of problem. If possible, flex the board when
    the problem is there and see if it produces any change. If so, go after
    with the iron.
     
  5. Tim R.

    Tim R. Guest

    Thank you.

    I'll do as you suggested. I've dealt with bad solder joints before and they
    are an often overlooked problem.


    ..com...
     
  6. CJT

    CJT Guest

    No guarantees, but that's where I'd start.
     
  7. CJT

    CJT Guest

    <snip>

    .... a possibility nobody has suggested.
     
  8. rpbc

    rpbc Guest

    I'm sorry, I thought I read the sound did not work and had white noise in
    it too. Since the sound works forget the tuner,it almost for sure is
    working properly.
     
  9. Tim R.

    Tim R. Guest

    Where should I look, then? A stage before the tuner, perhaps?

    Will also try the corona check when I have time to disconnect and remove the
    set, as it's a daily player and surrounded by stuff.


    ..com...
     
  10. There is no stage before the tuner.

    Tom
     
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