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TV startup problem

Discussion in 'Electronic Repair' started by Fannie, Jul 12, 2004.

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  1. Fannie

    Fannie Guest

    My sister inlaw gave me her non-working 27" TV and
    I'd like to get it going to replace my smaller one.

    It's a(n) MTC (manufacturer) model: MTV-2800S

    A button is pressed to, I guess, turn on the unit maybe
    to precharge the picuture tube and then another button
    is pressed to actually turn it on and off. A little red LED
    comes on, on the front, when the first button is pressed.

    When the second button is pressed (after being turned off
    for awhile) I hear a thunk from the internal speakers and what
    sounds like static from the picture tube firing up..... but then
    nothing. No picture, no sound and the filaments on the ass
    end of the picture tube don't light up and the speakers make
    a sound like it's lost power. The little red LED on the front
    stays lit.

    That only happens when first turned on after being off for awhile.

    I'm told that if I turn it off and on about 100 times it might actually
    come on and stay on. I've yet to confirm that.

    Should I search for a problem in the power supply I wonder?
    I'm a home-electronic-fix-it-for-myself-kinda-guy. Any advice,
    help, or suggestions would be greatly appreciated.

    Thanks, loads.

    - Chris
  2. Fannie

    Fannie Guest

    Call me stupid (hey, stupid!) ---
    two fuses in the power supply?!
    One in the front part, easily seen...
    one in the back, kinda buried...
    and .... blown.

    Replaced it. Turned TV on. It held.
    Now I'm hearing a whine or a shrill
    or a buzz coming from the area of the
    large transformer in the power supply.

    I think (correct me if I'm wrong) that
    it isn't an indication of a failure in that
    transformer but rather maybe an electrolytic
    somewhere in the power supply circuit?

    - Chris
  3. Jamie

    Jamie Guest

    depends on what transformer you are talking about ?
    if your referring to an upper class tv with a
    switching supply then you may have a short.
    normally a bad E-Cap in the power section throws
    the over load how ever, you could have a bad
    flyback circuit. (Transistor,Coil,Rectifier etc).
  4. Fannie

    Fannie Guest

    I'm leaning towards a bad E-Cap in the power section.
    Without a schematic I'm pulling the largest ones and/or
    testing/replacing. The whole power supply section seems
    to be on its own removable PC board so I (guess) I will
    stay on that part and concentrate the E-Cap search.

    Fingers crossed hoping it's that simple. LOL

    - Chris
  5. If you change the E-cap and it still does not work, check the circuit that
    drive the switching transistor.
    Check the diodes and caps.
  6. Fannie

    Fannie Guest

    Have changed the E-Cap and it has not fixed.
    In-circuit check of diodes seem fine.
    All large caps have been checked/replaced.

    Am now moving on to switching transistor circuit.
    Can't be much else left. :)

    Despite the squeeling transformer in the power supply
    and the front LED lighting up... there's little or no heat
    being generated from much of the power supply
    circuitry. Even those large stone resistors are cold.

    I'm about to check the high current switch transistor
    and regulator transistor. I'll post the findings.

  7. When you testing the diode, it is better to it take off. I had experiance
    when I tried to fix my freind's telly. I check the diode in-circuit and
    found it seems ok, but actually it was open circuit. The diode was used to
    start / give power to the switching circuit.

    If you have CRO, check the base of the switching transistor. Usually
    squeeling tranformer happens because:
    a. switching frequency is wrong
    b. the switching circuit try to start itself but fail, due to incorrect
    starting voltage.

    Good luck !
  8. Art

    Art Guest

    Very probable excessive load on the SMPS such as a shorted horizontal
    output, bad LOPT, Etc..
  9. Fannie

    Fannie Guest

    Yes, that's what it appears to be. There indeed is a short in
    the HOT. I'm waiting now to pick up a replacement transistor.
    It sure is gonna be nice to get this TV up and running. ;-)
  10. john

    john Guest

    Before you replace the HOT find out why it blew.

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