LCD backlight not working

Discussion in 'Build Help' started by Patrick Notton, May 3, 2016.

  1. Patrick Notton

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    Working on my second build and the LCD backlight just doesn't light up - the display does display the right information but oh so dark.
    Checked the connections and adjusted the contrast to actually see the screen but I can't see why the backlight wont turn on.

    Current build:
    no backlight.jpg

    1st build working like a charm:
    with backlight.jpg

    Anyone have an idea how to troubleshoot this? I'm hoping that this is just a minor glitch and not a faulty controller.
     
  2. Old_Tafr

    Old_Tafr Well-Known Member

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    If you still have the first BB you built? then swap the panel over.
     
  3. Dr Jeep

    Dr Jeep Well-Known Member

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    The backlight supply is on the last two pins (pin 19 and 20), so check voltage on there, it's derived from the board supply rail, so other than the LED on the LCD panel itself or a dry joint/bad pcb track the only other possible fault is an open circuit current limit resistor (RBL 0), no other components are in circuit.

    Make sure you are measuring pins 19 and 20 and not 1 and 2 (those are the LCD supply pins not the BL supply pins)

    http://reprap.org/mediawiki/images/5/51/RRD_FULL_GRAPHIC_SMART_CONTROLER_SCHEMATIC.pdf

    Either way it's a fault of some kind on the display panel.
     
  4. Henry feldman

    Henry feldman Well-Known Member

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    Isn't there a "contrast" setting for the LCD which can make it seem off?
     
  5. Patrick Notton

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    Thanks you for pointing me to the schematics.
    Checked the voltage across the pins and sure enough it was reading 0. Accidentally touched the side of the resistor when doing this and the backlight 'magically' worked... Quick solder job to fix the contact on the resistor and we have light!

    :) glad it was a simple fix.
     

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