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TV with short circuit - how do I find the short?

Discussion in 'Electronic Repair' started by news.verizon.net, Oct 11, 2008.

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  1. A friend of mine gave me an old 27 inch TV that no longer works. Upon
    inspection of the insides, I noticed the "5A 125V GMA" fuse was blown. I
    purchased a 10 pack of these fuses, and when I replaced it the fuse blew as
    soon as the TV was plugged in again. I am assuming that there is a short
    circuit on the board somewhere. How would I go about finding the short? Is
    it possible the power cable or its connection to the circuit board is
    related to this short?

    Thanks in advance.
     

  2. Simple way? Put in a much larger time delay fuse then look for the smoke.
     
  3. Arfa Daily

    Arfa Daily Guest

    Does the set have a 'conventional' mechanical on / off switch as well as a
    standby switch ? If so, does the fuse blow when this switch is set to off ?
    If yes, you might have a short circuit noise filtering cap across the
    switch. If the fuse only blows when the main power switch is on, then it
    gets a bit more complicated. It could still be a noise filtering cap on the
    'back' side of the switch. It could also be a faulty posistor, which was
    quite common on a lot of sets for violent fuse blowing. However, if it's
    none of those things, it gets a lot more complicated as, if the set is
    anything up to say 10 years or so old, it will employ a switch mode power
    supply. These are dangerous to inexperienced hands, and notoriously fickle
    with their faults. Violent 'instant' fuse blowing is usually down to a short
    circuit chopper transistor or hybrid, or sometimes the bridge rectifier, but
    that is often not the end of the story. There may be assorted damaged
    resistors, diodes, control IC if it uses one, feedback opto, and the whole
    lot may have been caused in the first place by a faulty electrolytic in the
    primary side.

    If the fuse does not blow instantly (ie set needs to be taken out of
    standby) or violently, then the chances are that the fault is downstream of
    the power supply, most likely in the HOP stage somewhere. However, when this
    is the case, most well designed switchers will detect the excess load, and
    shut down to a 'safe' mode where they will squeal, tick or chuff, rather
    than blow fuses. A bit more info is needed really, before any 'reasoned'
    diagnoses can be made.

    Arfa
     
  4. If you have electronics experience, especially around high voltage and
    potentially lethal equipment, then go to the TV Repair Guide at the
    Web sites below, especially for the SAFETY information. A blown fuse
    can have many causes.

    --
    sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  5. Guest

    If you're not familiar with TV circuits I'm afraid that this is going
    to be a difficult undertaking for you. Whatever you do though, don't
    try the larger fuse trick. That advice was just plain stupid, and so
    is the person for giving it. Lenny. .
     
  6. Yep, it's likely to be a short, but not a simple thing like a paper
    clip that fell across the power wires.

    Most likely a capacitor or transistor shorted, one of the bigger ones,
    like the power supply switching-mode transistor or its drivers.
    When that shorts it can blow the horizontal output transistor, or vice-
    versa. Plus maybe weakening or blowing out nearby diodes and
    capacitors.

    Worse yet, the real cause of the first transistor blowing is often yet
    some OTHER part being a bit below par, something
    not easily diagnosed, as it can be something subtle like a capacitor
    having a somewhat high ESR. So even if you
    replace the bad transistors ($$$) the underlying cause is likely to
    blow them again in a millisecond or maybe a week.

    And you can't just replace one bad component at a time, as the other
    bad ones are likely to blow the new component toute-suite.

    To do this right you need a source of gentle line voltage, like a
    Variac, lots of spare parts, lots of patience.

    I'd recommend you make the best of a bad situation and throw the TV
    off a very tall building. Taking all appropriate safety precautions
    of course.
     
  7. PanHandler

    PanHandler Guest

    Electronics run on smoke. When the smoke gets out they stop running.
     
  8. Clint Sharp

    Clint Sharp Guest

    You are more likely to kill yourself messing around with the rectifier
    diodes, especially if the reservoir capacitor is still charged.
     
  9. ian field

    ian field Guest

    This can be as simple as a short circuit diode in the bridge rectifier, or
    as complicated as any one or more of a number of parts in the SMPSU. In CRT
    TVs the degauss posistor can also cause fuse blowing.

    Note that the 'ground reference' rail in a SMPSU is actually a couple of
    hundred volts negative and can easily supply lethal current!

    One trick you can try instead of blowing more fuses and possibly making the
    damage worse, is wire a 60W mains voltage bulb across the fuse holder - this
    will limit the current to a value that avoids further damage and provides an
    opportunity to measure voltages (or at least see where there aren't any that
    should be). Note that once you have the bridge rectifier working it might be
    possible to store a hefty charge on the big electrolytic capacitor - that
    can give a very nasty bite!

    In theory you should use an isolating transformer to power the set while
    working on it, but the important thing is to *AVOID* touching anything
    earthed while inadvertently touching any live part!
     
  10. Steve Ashman

    Steve Ashman Guest

    Thanks for the advice, but I think I have made an informed decision to cease
    work on this TV as too many people have told me on the group and in real
    life that I am more than likely to end up touching something that could kill
    me. The set does not have a mechanical on off switch, but the fuse blows as
    soon as the TV is plugged in, before the standby switch is pressed. As I
    said, I am most likely discontinuing work on this TV. I found a similar one
    in working condition for 25 dollars on craigslist. Thanks again.
     
  11. Arfa Daily

    Arfa Daily Guest

    This is probably a wise decision. No one on here would seek to put you off
    for the sake of it, but everyone is telling the truth and looking out for
    your safety, when they warn about the dangers of TV sets in inexperienced
    hands. Much modern equipment makes use of switchmode power supplies, and
    these are truly potentially *very* dangerous, and require specialist
    workshop equipment such as an isolation transformer and variac, as well as a
    detailed knowledge of the workings of them, to even begin to be safe to work
    with. They can also suffer from problems and cascade failures, which can
    make the 'strongest' and most experienced of us weep ...

    If you are new to experimental electronics, don't let this put you off.
    There is much hobbyist fun to be safely had with low voltage and battery
    operated equipment.

    Arfa
     
  12. ian field

    ian field Guest

    ..............and never throw out wall wart PSUs when the kit they power gets
    replaced, they're great for home brew projects.
     
  13. Dave

    Dave Guest

    Or just use foil. He he he.
     
  14. ian field

    ian field Guest

    These days the turnover of new designs is so rapid for TVs & monitors etc
    that each new model is only in production for a few months before the next
    one comes along, by the time it wears out the manufacturer might as well
    have forgotten it ever existed! Very few manufacturers even bother to reply
    to requests for service information, the few that do will refer you to
    distributors - most of which only deal with trade.

    The OP said he found something similar in GWO for $25 - unless you know
    where to look a schematic can easily cost that much and may even turn out to
    be a very fuzzy scan of a creased and stained original.
     
  15. Arfa Daily

    Arfa Daily Guest

    Although this is fundamentally true for a lot of equipment, unless you have
    a detailed understanding of switchmode power supplies, the schematics will
    be of little use to you. There are many techniques used nowadays, for power
    saving and power factor correction, which really muddy the waters around the
    front end of a switcher. Even the way that the primary side high voltage
    rail is derived, may be at best not obvious, and often just downright
    confusing, if a PFC IC is used, and the supply is a 'universal' type with
    automatic input voltage sensing. Locating a suitable 'ground' point on the
    primary side can be a very dangerous undertaking, and often, the schematics
    can be misleading in this respect.

    Given this particular poster's declared skill level, a schematic might have
    helped to identify if there was a noise filtering cap at any appropriate
    point just inboard of the power lead, but beyond that, I think that he would
    probably have been on a loser with this particular repair, so posession of
    service info would have been irrelevant.

    But yes, in general, I would agree that if you have the skills to be working
    on a particular piece of equipment, schematics are certainly desirable.

    Arfa
     
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