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please help me fix SMPS

Discussion in 'Electronic Repair' started by Louie, Aug 13, 2003.

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  1. Louie

    Louie Guest

    DAMN i feel like i been to hell and back with this thing! i finally
    found a nice guy at a TV repair shop who was willing to give me some
    functional information, but he told me that the cost was $90 just to put
    my SMPS on the bench and tell me which components were bad.
    after surfing to the far corners of cyberspace, my only option is to
    repair the thing myself.
    if people could give me some tips it would really save what is left of
    my ass.
    so here's the deal:

    -switch mode, looks like a half bridge type unit, form a projection TV

    -customer did some electrical work and sent 220 volts up the cord, now
    when i plug it in the only sign of life is a periodical, barely audible,
    click like a microscopic spark discharge.

    -but i know that's not what it is, because the outputs on the supply are
    totally flat. in circuit completely plugged in. dead. 0 volts ac & dc

    -the source voltage, raw 110, full rectified, filtered, is fine.
    measured approx 180 steady DC, which

    -goes to a STK730-130 regulator and support circuitry.

    -take it out of circuit. check it out: no visible signs of damage
    anywhere. no fuses.

    from left to right: input section, 180 DC, checks out

    mid section, STK chip, other circuitry, 5 primary taps. here i see
    something, i don't get it.

    theres a single resistor connected to STK pin 5. that's the "TR1 gate"
    pin. it must be the start resistor, but no, the other end of the
    resistor goes to DC ground, so it can't be.....whatever, the point is,
    its measuring only 8 ohms even though its supposed to be
    (hmmmmm....1....0.....yellow. 5?....4!) uuuuuhh.....100k. but am i just
    measuring the resistor? maybe the STK is shorted inside. who knows where
    those electrons are off to?

    here's something else, while it was in circuit i found the STK ground
    pin and found that there wasn't any voltage present on any of the other
    pins, ac or dc, except pin 11, the "TR1 drain" pin. so the transistor
    must be shorted. i guess.

    right section, isolated by the transformer and a photocoupler, looks OK,
    diodes are OK, but how do you test a cap in circuit? or the transformer?

    and how do you trick it into thinking its in circuit so i can measure
    the outputs? its a projection TV, but i got no specs on voltage or
    wattage. and i can't get a doc for the STK730-130.

    you EE guys must have some fancy tricks n stuff to do this work.

    thanks anyone just for reading this message. hope someone can help.

    Louie
     
  2. Sofie

    Sofie Guest

    Louie:

    Make and Model number of the projection TV ? ?

    How do you know that the circuitry in the television is properly
    operating?.... maybe there is a fault in the television circuits that
    affects the SMPS..... it is always best to troubleshoot, test, and diagnose
    the COMPLETE television..... all together. Usually most SMPS units can not
    be easily tested and diagnosed without the rest of the television circuitry
    to provide the proper loads and other connections.. ...I am surprised that
    you found a repair shop that would be willing to look at just the SMPS.....
    maybe his $90 over the counter quote was designed to make you go away.
    ..
    When you say the photo-coupler "looks OK" ..... did you actually test it?
    Also you do need to check all the surrounding electrolytics for leakage or
    high esr... you will need an ESR meter to properly test them or plan on the
    alternate method of substitution and replacement of all the electrolytics.
    The resistors need to be checked for proper value.... if there is any doubt
    about the in-circuit readings.... unsolder and disconnect one leg and
    measure it again. SMPS transformers very rarely go bad so unless there is
    something obvious, get past that one. All the diodes need to be properly
    tested.... if there is any doubt about the in-circuit test results,
    unsolder and disconnect one leg and test again. The STK730-130 is a common
    part and is readily available.... at the very least, as long as you are
    certain that no other components are faulty in the SMPS and that the rest of
    the television circuitry is not faulty, try another chip.

    My advice to you is..... that your best option is to find a service shop
    with an EXPERIENCED and KNOWLEDGEABLE technician WITH the proper test
    equipment, tools and replacement parts that will do a home service call to
    safely and properly fix your projection TV...... after all, there may be
    more wrong than just the SMPS. At the very least, have the tech give you a
    repair cost estimate so you can make an intelligent repair decision with
    facts instead of internet guesses.
     
  3. Louie

    Louie Guest

    yeah! that's totally what i can do. i can sever a connection, test the
    parts on that path, and just resolder it.
    cool! that way i can eliminate most of the suspects. but how do you test
    an inductor?
    it shouldn't be that hard to solve this problem. its a typical SMPS, so
    unless there's something unnatural about this particular unit, i should
    be able to test and repair it in isolation. these things are not smart.
    there's no signals or anything that return from the TV. if i can just
    simulate a load, i'm there.
    and how could the TV be damaged. the output is electrically isolated
    like it always is in a SMPS.
    besides, i don't have a fat wad o' cash to spend. in fact, it would be
    cheaper to start swapping EVERYTHING on the board than to call a
    technician out.
    how much does an ESR meter cost? less than $200? because i could go out
    and buy one cheaper than calling a repair shop.
    the point is, years of experience has taught me, way too many times to
    count, that there are choices.
    1. if you got a boatload of money, you can open a phone book, write a
    check and forget about it.
    2. if you don't, you can chuck it; or
    3. you can motivate yur lazy butt and DIY. this, of course, is risky,
    especially with PSs, because if you're an idiot your likely to get a
    capload of juice right through your heart.

    thanks Sofie, now i can proceed
    Louie
     
  4. Still, you haven't stated what the make and model is. Repair professionals
    almost always think in terms of makes and models when troubleshooting, and
    certainly when corresponding with other techs. Somehow, the importance of
    providing this information is often underestimated by those outside the service
    industry.



    Alan Harriman


     
  5. Louie

    Louie Guest

    that's a good point, to be sure. having spent years as an auto mechanic
    and PC tech, i can vouch that many repair jobs have to start with the
    question of model, make and year. and its the same way with appliances
    of all types.
    but not ALL jobs require that question, however. overheating, noisy
    brakes, dead batery, boot failure, connection timeout, keyboard failure,
    i don't have to ask. emission problems, idiot lights, ignition
    problems, CPU or memory problems, i do.
    when it comes to appliances, model-specific diagnosis is much more
    prevalent, because they're less modular, but i've already done some
    diagnosis on the TV, which is a GE 46GW948 projection tv, and have
    determined with certainty that the PS, and perhaps the high voltage
    module also, is bad. i'm gonna fix it, but i'm not gonna just plug it in
    and switch on full voltge only to have it pop all over again.
    and yes, lots of customers invariably ask questions without having the
    requisite information. sometimes, if they can't tell me the MMY, i can't
    tell them anything.

    back to the bench,
    Louie
     
  6. john

    john Guest

    The chassis number is ?????
    CTC what ?

    kip
     
  7. David

    David Guest

    Finally almost , but not quite, enough information to be able to tell you
    with absolute certainty that you are in the totally wrong area. That
    so-called power supply board you are looking at ONLY supplies power to the
    convergence circuit, not the whole set. The set WILL run with that power
    supply out of service, but you will have no convergence correction. I have
    to assume it has the standard PTK195 type of chassis. Chassis designation
    is way more important than model number on tv sets. Yes, it is REQUIRED on
    all tv set repairs due to the extreme variance in circuit design to have all
    the information in order to provide more than the most general guidance of
    where to look next.

    You need to troubleshoot the MAIN power supply that is on the main board
    with everything else. Given the failure mode, it is highly likely you will
    need to order the full service manual and maybe the training manual on that
    chassis from Thomson
    (should only cost around $150 for both manuals on paper). The main power
    supply is a mostly discrete design and a complete understanding of how an
    SMPS works is almost a requirement before taking that one on.

    Hopefully you are only looking at some pretty basic front end failures, like
    bad filter capacitor and open surge resistor. Typically that is the only
    failure when 220V ac is applied to that particular chassis. Sometimes it
    will take out the main SMPS Mosfet and some other SMPS support circuitry.
    If you do require a SMPS repair you will need a variac, oscope, and service
    procedures specific from the manufature that includes the proper waveforms
    and voltage in both standby and run.

    David
     
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