LG 42PC3 buffer output doesn't follow YSUS

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  • legg
    Member
    • Apr 2018
    • 25
    • canada

    #1

    LG 42PC3 buffer output doesn't follow YSUS

    LG42PC3D -UD originally with green lamp but no display or audio.
    Owner says was working, then went 'pop'.
    All 5V caps bulging. replaced.
    All PSU output voltages within spec but -Vy is low on YSUS.
    One of three parallel fets between SUS_OUT and SUS_DN is zenering,
    eventually shorts DG. Replaced all 3 - -Vy now OK.

    Y scan buffer test point does not follow YSUS output below -50V

    Note: This unit seems to expect YSUS output to park at -Vy in between
    drive signal pattern periods. A replacement drive signal board confirms this.
    This shows up in a training manual for early 60PC1D, as being normal. Later
    models will park at 0V . . .
    Also, YSUS waveforms sometimes show a double setup ramp before the
    single setdown ramp at pattern termination. This is only shown in training
    manuals for dual-scan types. It might be because I'm not supplying a white-
    screen input signal (no input source is present).

    A second buffer board produced first screen illumination, for a short period,
    before R15 popped, leaving only a top horizontal band illuminated (greenish
    random). R14 is 47R 2W smd - looks to be in an RCD snubber network
    configuration. Replacing the C and popped R allowed the same behavior to
    be repeated, with popped R15 again.

    I replaced the 74AC540S supplying signals to the buffer board and a second
    replacement buffer board gave full screen illumination without anything
    popping - but only running it long enough to get waveforms and images.
    There is a random greenish pattern, with a redish-yellow vertical band shifting
    randomly back and forth across the screen.

    The only similar random illumination images I've seen in plasma screen
    diagnostic literature refer generally to either loose connectors or corrupted
    memory.

    Reprogramming seems unlikely for this model - distributed for 6 months only,
    in 2006, even though it was subject to an industry-wide bug fault requiring
    widespread reprogramming, at that time. You'd expect programming tools to
    still be floating around somewhere.

    This unit was a revision within a revision - no PFC in power supply as with 42PC1x.
    Its YSUS board had 74AC540S and 74AC541D (different body sizes) on
    the same board
    . . . .

    Anyone with ideas what's next?

    Y buffer test point still does not follow YSUS output lower than -50V

    RL
    Attached Files
    Last edited by legg; 11-18-2021, 12:02 PM. Reason: added typical wfm from training

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