Benq FP547 (or Q5X3), power on but no image and backlight

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  • Chungalin
    Badcaps Veteran
    • Jul 2014
    • 422
    • Spain
    #1

    Benq FP547 (or Q5X3), power on but no image and backlight

    This old 15" monitor detected VGA signal and powered on accordingly, but there was no backlight and no image was formed on LCD. I thought that it was a typical inverter problem, but the absence of image on LCD misleaded me at first.

    No bulging caps and ESR OK. I get schematics. There's 16V for inverter, 3.3V too, but not the 5V for panel. Then I discover that PF751 is actually a fuse! It really looks like a ferrite / coil! But after unsoldering I can read "bel MS 2A" underneath. It's a 2A / 125V fuse and it's open.

    DC-DC IC781 MC34063ACN seems fine. I just detect broken solder joints on 2 inverter transistors (Q759, Q760, 2SC5706), due to repeated heating-cooling cycles over years. Ressolder all 4 transistors on inverter, replace fuse with a 2A / 230V one and it works again.
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