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Dell Latitude no backlight even after screen replacement

By O negative
Dec 27, 2013
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  1. I have a Dell Latitude D820 that I dropped while in its carrying bag so it wasn't a real hard landing.

    Now the backlight won't come on. I swapped an entire screen assembly from an identical D820 and it still won't turn the backlight on. I swapped the screens again and the borrowed D820 still works fine and mine won't work. (I didn't bother to test my screen in the borrowed D820.)

    I can run an external CRT with mine and I can see the LCD display if I shine a light on it so I know the display is working and not cracked. The laptop won't drive the CRT unless it thinks the lid is down so, to use the keyboard, I removed the sense magnet from the lid and taped it to the place where it would be with the lid closed.

    I've noticed in the past that this D820 often stayed fully on, including the display it seemed, when I closed the lid and walked away for hours.

    (Note: When I tested the borrowed screen I had the sense magnet removed.)

    When the magnet is taped in place (simulating lid down) both the backlight and the display are off. I can't see the display even when a light is shined on it, but when the magnet is removed (simulating lid up) I get the display with no backlight.

    It seems like the display knows whether the lid is up or down, but the backlight seems to act like the lid is always down. Is this possible? What should I look for?
  2. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    Did you replace the backlight inverter?
  3. O negative

    O negative TS Rookie Topic Starter Posts: 35

    Nope. Where is it?

    Is it plausible that a mild shock would selectively kill an inverter? (And leave the hard drive unperturbed.) If I get a chance I might swap the screen cable. It gets flexed everytime the lid is opened/closed and there is a pinch point.

    I'm also wondering if there are two lid sensors: one for the display and one for the backlight. A shock could easily misalign something like that.
  4. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    The inverter board is a small long rectangle board usually mounted at the bottom of the display panel in the center of the lower LCD housing. It has one wire coming in from the Video Cable going from the LCD screen to the motherboard. 2 wires on the other side of the backlight inverter go to the backlight bulb on the LCD panel. If this board is partially disconnected or bad, you will have the symptoms you describe here
  5. O negative

    O negative TS Rookie Topic Starter Posts: 35

    Yeah, that's what I thought that board was for. I did swap the backlight inverter when I swapped the screen assembly so the inverter is fine. I wish I'd swapped the cable at the same time
  6. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    So you think that the video cable from the motherboard is bad now?
  7. O negative

    O negative TS Rookie Topic Starter Posts: 35

    That's the only thing I haven't replaced. (Well, OK, I haven't replaced the motherboard.) If I get impatient I can always peel off the shielding tape at the flex point of the cable. If there is a cable problem that's where it is most likely broken.
  8. O negative

    O negative TS Rookie Topic Starter Posts: 35

    I guess there could be a motherboard issue afterall. After googling this issue again I find that there are fuses on the motherboard that can blow when you have a display defect. I'll have to poke around when I get home. If someone knows where the display fuses are on a Dell Latitude D820 I'd love to hear about it
  9. O negative

    O negative TS Rookie Topic Starter Posts: 35

    Man! I can't find the letter "F" anywhere on the top side of this motherboard. I've even looked under the little modem daughterboard.

    If I have blown a fuse it's a good thing that I didn't try my screen in the borrowed unit or I'd have two crippled laptops.
  10. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    There are no replaceable fuses on the motherboard... You have to replace the main video cable
  11. O negative

    O negative TS Rookie Topic Starter Posts: 35

    I am relieved to hear that. Some Dells have those tiny fuses soldered to the board. I'll try swapping cables this New Year's Eve.
     
  12. O negative

    O negative TS Rookie Topic Starter Posts: 35

    Fixed it.
    It was a broken bulb. I would have diagnosed it sooner, but I was in a hurry to return the borrowed screen assembly and didn't completely power down to install it. I just went into stand-by and when that didn't work I did a restart. If you've been using an auxiliary monitor it will remember that unless you do a "cold boot".
    When I borrowed the screen again I was able to get it to work (with a cold boot) so I knew it was either the inverter or the bulb. To get the screen apart I had to sharpen (with a very small file) my jeweler's screwdriver to remove the tiny phillips screws holding the cover and bulb tray. I found that the bulb was broken at one end.
    When I dropped the laptop it landed on the handle of the carrying bag with the front edge up. (The bag actually fell out of a vehicle when I opened the door. It was sitting against the door, handle up, before I opened the door.) This pushed the inverter against the bulb tray right in the middle, bending it enough to break the bulb.
    I happened to have an HP Pavilion with a cracked screen--but working backlight--lying around and the bulb was the same length with the same connector. (Lead colors are pink and white instead of light blue and white.)
    I was living in Terry Gilliam's Brazil there for a while--nice to be back!
  13. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    Thanks for the update:)
  14. Azycray

    Azycray TS Rookie

    I found this page while looking for the answer to the same problem. My monitor was dark on my Dell D820, I replaced the inverter, no workey. Then I replaced the whole screen because after I took it apart to check the light bulb there was no way it was going back together again. The bulb did look like it was burnt out so I felt pretty good about my chances of the new/used screen, acquired on e-bay, solving the problem. But no, still no monitor. So what's left that could be causing the problem, any ideas?
  15. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    Does an external monitor work normally?
  16. Azycray

    Azycray TS Rookie

    No, it doesn't. I've ordered a mb from e-bay, hopefully this will be the last part to fix this gremlin.
  17. Tmagic650

    Tmagic650 TS Ambassador Posts: 20,827   +163

    Okay, that's where I get my motherboards from. Good luck with this...


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