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Zenith SM276759 - n00b to repairs..

Discussion in 'Electronic Repair' started by [email protected], Apr 25, 2007.

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  1. Guest

    Hi - please don't flame me *too* much, as I'm still teaching myself on
    the fly - and have done a good bit of mostly confusing research. All
    I have available to use is a multimeter, which I've done continuity
    tests with on most of the major resistors and capacitors. Uncertain
    about how to test caps properly without replacements or the like.

    TV is a 27" built around 1994.

    Has screen issues - is solid green with rescan lines (I think they're
    called) running down it.

    On screen display and menu shows up still, ironically, but anything
    else only provides the faintest ghosting.

    Messed with the color pots - got expected result; more green red or
    blue and whatnot, no change to rescan.

    Adjusting G2 all the way down gives extremely dark, extremely
    oversaturated image.

    Would appreciate whatever idiot-level help I can get; this stuff is a
    lot like Linux; you've gotta know 15 things just to learn one new
    aspect.
     
  2. 1994 to 1997 Zenith were famous for having picture tubes that shorted and
    what you describe is that symptom
     
  3. I'll flame you as much as I goddamn well please.
    If your teaching yourself electronics, then get a Radio Shack
    10930923409234 in 1 electronics kit.

    Don't just start fucking around inside a T.V., there are high voltages
    inside especially around the tube which will throw your balls to the
    wall.
    Many Tee Vees don't last 13 years so consider yourself and your dick
    lucky.
    Now you'll have to throw away the T.V.. It's a bitch to get those
    color pot settings back they way there were.
     
  4. Guest

    Heh...neither polite nor useful. Figures.

    Not teaching myself electronics so much as teaching myself
    troubleshooting of complex electrical systems. Big difference - and
    yes, I am aware of the voltages involved.

    Is not rocket science to avoid the problem areas and use prudent
    caution whilst poking around the annoying little bugger.

    I marked the pots; there is no issue there. Could've adjusted those
    back by sight.

    So - eh. You should probably work on your diction and delivery.
    Aren't gonna make many friends howling like this.

    --------------------

    Anyway, I looked into the shorted tube thing and am not sure how to
    proceed. Presumably one can bypass the flyback with an isolation
    transformer (haven't figured that one out yet but I have a rough idea)
    and fix the annoyance.

    I have the vague suspicion that there's a dead resistor right by the
    flyback - multimeter doesn't twitch trying to put current through it.
    Would a dead circuit cause something similar to a shorted out tube?
    It's a fairly high octane one; about 560M. I figure it's gotta be
    some sort of side-supply going to the CRT.

    Appreciate the...well....*constructive* help. And I'll be careful.
    Have a good bit of experience, just not with TV's.

    Be well, all.
     
  5. You presume wrong.
     
  6. Andy Cuffe

    Andy Cuffe Guest

    Honestly, I wouldn't touch sets with those CRT 8 years ago and I'm
    sure they haven't improved with age.

    There should be a date on the CRT. If it's the original 1994 CRT,
    there's no point in continuing unless you can replace the CRT.

    The shorts in those CRTs are usually cathode to G1, not heater to
    cathode (so an isolation transformer won't fix it). Even if you could
    get rid of the short, the emission is probably too low to produce a
    good picture.

    Don't let the set run with a bright raster because it will blow the
    power supply. Turn the screen control on the flyback down if you must
    run it.

    These sets used to come in dead with blown power supplies. After
    fixing the power supply, you'd get anything from a poor to fair
    quality picture, but eventually the CRT would short again and blow the
    power supply.
    Andy Cuffe

     
  7. Guest

    Appreciate the clarification. Sounds like this thing is fubar - will
    likely strip it for components I can use elsewhere and donate it to
    the local garbage workers' job security fund. :p

    Considering I got it for free, it's more than been worth acquiring, if
    only for experience.
     
  8. Jeroni Paul

    Jeroni Paul Guest

    If it has a consistent short it may be blown by discharging a charged
    capacitor.
    Shorts inside CRT gun can be found with multimeter unless they are
    intermittent, just check continuity between all pins, only heater pins
    should read some continuity. Then look inside the glass and follow the
    connections to identify what is what. Some CRT manuals with pictures
    on the net may help you learn the parts that make up a CRT gun. I've
    seen some tubes have two pins connected inside for no apparent reason.

    On the other hand you may swap the connection to the bad color with
    another working one, there may be series resistors that will make that
    easy. You've to identify the RGB signal pins on the CRT socket. Then
    if problem persists on the same color you know it is the CRT.
     
  9. Andy Cuffe

    Andy Cuffe Guest

    That's normally true, but considering the quality of these CRTs, it's
    pretty pointless. These CRTs have a manufacturing defect that causes
    them to short and lose emission. Even if you could make it as good as
    new it would only last a year or two.
    Andy Cuffe

     
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