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Dead TV, Magnavox, vintage 1995

Discussion in 'Electronic Repair' started by mc, Dec 4, 2005.

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  1. mc

    mc Guest

    I work on test equipment and experimental equipment a lot, and am familiar
    with high-voltage precautions, but have not worked on a TV in 15 years.
    Accordingly, let me ask...

    The patient is a Magnavox 25-inch color TV from 1995. It went dead while in
    use the other day (no one was watching at that exact moment) and now shows
    absolutely no sign of power.

    Where the AC power comes in, there is a tiny fuse, which is not blown, but
    it's something like 1/10 amp and can't possibly be the main power inlet for
    the TV. There is also an Omron relay or circuit breaker (at a quick glance
    I couldn't tell which).

    What should I be looking for/at?

  2. Guest

    "What should I be looking for/at? "

    Standard tv troubleshooting:

    Standby power supplies, system control and clock and data lines, on/off
    control lines from system control, main power supply regualtor system,
    horizontal start up, horizontal drivers, horizontal output, flyback.
  3. Golf

    Golf Guest

    Check for B+ voltage at collector of the horizontal output transistor
    (mounted on large heat sink in the vacinity of flyback). If nothing
    present, trace power supply backwards from collector. May be an open
    pico fuse in this path. If you don't have voltage at secondary of B+
    transformer, check the voltage regulator and standby supply as
    suggested. A model # would help identify a possible common problem.
  4. Guest

  5. Ol' Duffer

    Ol' Duffer Guest

    Give us a model or chassis number. Fuse that small is probably
    standby power supply, necessary but not usually the problem.
  6. Golf

    Golf Guest

    I forgot, but check the main fuse (glass) for open. If open, perform a
    diode test on the horizontal output transistor.
  7. Guest

    Check the capacitor in the main power rectification for undischarge
    current.If so,the power supply is not funtioning,which you can check by
    reading the 110..145v at the secondary supply....
  8. mc

    mc Guest

    Magnavox 25CR12 C121. Thanks!
  9. Andy Cuffe

    Andy Cuffe Guest

    Check the electrolytics near the B+ regulator. There was a common
    chassis around that time that tended to lose a large cap there. I
    think it also resulted in a shorted HOT.
    Andy Cuffe

    <-- Use this address until 12/31/2005

    <-- Use this address after 12/31/2005
  10. Ol' Duffer

    Ol' Duffer Guest

    That model number does not compute for me, unless it is a
    euro/asia model and I do not have access to that information.
    Are you sure it isn't 25TR12?

    If so, it could be an X6 chassis that may be subject to the
    main B+ capacitor failure syndrome. 100uF 200V capacitor
    goes open due to surge/thermal damage. Once open, the supply
    loses regulation and climbs from its nominal 129V to something
    more like 145V, which makes the horizontal output transistor
    dissipate more heat. The HOT eventually shorts out, which
    usually blows the 2.5A pico fuse, occaisionally damages other
    stuff. The horizontal drive supply resistors also get hot
    enough to burn off their printing and crystallize their solder
    joints. I see these often enough to have it down to about a
    half hour.

    Other possibilities are B8 or P6 chassis which are switch mode,
    run-all-the-time supplies that are much kinder to capacitors.
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