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What's next after bad caps LG W2252TQ-TF schematic?

Discussion in 'Electronic Repair' started by mike, May 18, 2013.

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  1. mike

    mike Guest

    I picked up a free LG W2252TQ-TF.
    lights up for a second then quits.
    Figgered bad caps, but I pulled and tested 'em all.
    All good. No bulges.

    Checked all the diodes. Other semiconductors aren't
    shorted. Two fets in the backlight supply probably
    have the highest stress level, but I thought I'd
    ask for ideas before trying to get them out.

    It's gonna be difficult to power it up disassembled.

    Ideas on other possible high failure rate parts?
    Schematic?

    Thanks, mike
     
  2. mike

    mike Guest

    Good call. One of the backlight secondaries is open.
    Thought I'd take a shot at trying to fix it.

    Hypothesizing that the connection to the pin might be open, I measured
    some capacitances to attempt to determine which end.

    The good secondary measures 2.68pF from primary to either end of the
    secondary.
    The bad secondary measures 2.68pF from primary to one end of the secondary
    and 10pF to the other end.

    Now, I'm confused. I expected to have the sum of the two measurements
    on the open secondary to add up to about 2.68pF.

    Another thing I'd never seen before was that each end of the winding has
    two turns of what looks like 5 or so wires twisted together headed for the
    connector pin. The wire to the pin is potted, so can't see the actual
    pin connection.

    Can't get my head around how or why they did that???

    The winding is in the clear, looks like a single layer, so
    if I can find the break, I can probably fix it.
     
  3. Guest

    Much good advice may be available at www.badcaps.net/forum Be
    prepared to provide good quality pictures.

    Try www.elektrotanya.com for a service manual.

    PlainBill
     
  4. mike

    mike Guest

    Thanks, guys.
    Found the service manual.
    Looks like I can get a transformer for $12 or so.
    I've got lots of time, so I think I'll take a whack at fixing
    the transformer.
    Be just my luck to buy a transformer and find other issues.
    Never can tell what you're gettin' at a garage sale.

    Thanks, mike
     
  5. mike

    mike Guest

    The plot thickens...

    LG W2252TQ-TF
    Powers up but no backlight.
    has four backlight lamps with two transformer secondaries.
    TMS93137CT is the transformer.
    One of the secondaries is open. The other one measures about 1.5K.
    I zapped it with a stun gun. The open secondary now measures 1.5K also.
    But the "fix" didn't survive the reinstallation of the transformer.
    It's open again.

    So, I ordered a new transformer. Got it today, and the SAME secondary
    is open.
    Looks new and has the right part number on it. The secondary winding is
    definitely
    there, just open. Same side as the original. Maybe some manufacturing
    defect.

    When I opened up the monitor, both my original secondaries measure ~1K,
    but after
    powering it up, the bad one is open again and no backlight at all.

    By now, you're asking, "is there a question in there?"...
    nope, just venting.
    If anybody has a suggestion, I'm all ears.
    Meanwhile contacting the vendor....

    Thanks, mike
     
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