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Sanyo dp26649 LCD tv no backlight

Discussion in 'Electronic Repair' started by boardjunkie1, Dec 26, 2012.

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  1. boardjunkie1

    boardjunkie1 Guest

    Got one of these given to my in a "dead" state. Pwr supply checks out
    OK, but there is nothing at the BL_ON pin when powered up. What
    conditions must be met before the proper voltage appears at the BL_ON

    I don't normally work on consumer stuff like this so my theory on LCD
    stuff is basically non existant. Display controller issue maybe?
  2. boardjunkie1

    boardjunkie1 Guest

    Just an update on this. I removed all of the inverter transformers and
    found one with a bad primary winding that wouldn't ring. I put one of
    the good ones back in and still get no BL_ON voltage (I'm assuming
    this should be a 5v logic hi). Sounds like the driver circuit for the
    failed transformer was damaged and still throws it into error mode.
    Sound right?
  3. John-Del

    John-Del Guest

    A bad inverter transformer won't cause a problem until the backlight starts, then causing an error shutdown. Some inverter transformers (particularlythe domino type) are easily damaged when removed from the board, so maybe you damaged the xfr when you pulled it. Any chance you're actually gettinga very brief backlight on command?

    When off, you should be getting a standby voltage of typically 5 or even 3.3 volts from the power supply to the main. When you hit the power button, you should get an "on" signal from the main back to the power supply to turn on the rest of the secondaries (such as 24 for the back light inverter board). At some point, you'll get a backlight on signal for the inverter as well. Depending on manufacturers strategy, different things need to be accomplished by the main before the backlight on command is sent.

    You can always jump the on command line to the 5v with a 470 ohm resistor and see what happens. YMMV...

    That's how the majority of these things are configured. I guess in a weirdsituation, a shorted inverter transformer, used in a TV that runs it's 24Vfrom the smps all the time could cause a problem on plug in, but I've never seen one configured that way.
  4. boardjunkie1

    boardjunkie1 Guest

    No, there's nothing at the enable line at power up. Since a primary
    winding was bad I believe the driver IC for that section is damaged
    causing the feedback to disable the backlight. No transformers were
    damaged when removed, I checked them all with my Sencore LC77...thats
    how I found the bad one. I can verify this by scoping the primary side
    drive when forced on.

    All other voltages are there. 24v,12v, 5v, and 5v stby.
  5. Guest

    I don't have an encyclopedic knowledge of all LCD TVs, but in my
    experience there usually was no feedback from the inverter to the main
    video board. The usual sequence goes something like this:

    When the TV is plugged in the standby power supply (5v stby) comes up,
    which starts the standby controller (often a part of the main control
    IC). This may then briefly turn on the main power supply, check for
    problems, then turn off the main power supply. The standby controller
    then monitors the remote sensor and control buttons for an 'ON'
    command. When one is received the main power supply is turned on, a
    self test is run and if everything is OK the inverter enable line is
    driven high. If the self test fails typically an LED is flashed to
    indicate an error.

    Typically the inverter is supplied by the manufacturer of the LCD
    panel, and several versions may be found on a given model. A quick
    search on eBay shows at least three different models of inverters,
    including one that was notorious for blowing drivers, and another that
    was notorious for failures of the transformers.

    An additional point - some models of Sanyo are known to have a 'lock
    up' mode. If a failure is detected, they will shut dwon and refuse to
    restart until a particular reset sequence is entered via the remote.

    One point you did not mention - does the power button on either the
    remote or the control panel switch the main power supply off and on?
    If it does, one technique I use is to hook a signal source (video
    game, DVD player) up to an input, turn the TV on, then use the input
    button to cycle through the inputs to see if sound is heard.

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