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I'm asking for a diagnose of what is probably the problem. Here goes ... laptop monitor (hp 6715s) went dark. I'm using also an external monitor with the laptop, and since it kept its picture, I'm guessing it's not the graphics card.

When the night came, and I turned on my desk lamp, I noticed that the laptop monitor is not really not showing anything, but is actually showing my desktop picture - only it's very very very dark.

You cannot see anything on it, unless the lamp is focused on it, then you can see the picture, only very dark.

Does anybody have any clue what it could be? I'm just looking for informed guessed here, so I have at least some idea of where the problem lies.

The closest repair shop is quite some distance away, so going for a bit of self-diagnose before thinking of taking it to them.

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backlight gone ? –  Sirex May 6 '12 at 23:03
    
@Sirex - From what I understand I shouldn't see anything on it then? –  ldigas May 6 '12 at 23:06
    
How to tell for certain? –  ldigas May 6 '12 at 23:06
    
show us a photo. –  Sirex May 6 '12 at 23:11
    
@Sirex - Photo of what? –  ldigas May 6 '12 at 23:12

3 Answers 3

Usually it is one of these faults:

  • worn out backlight, that cannot power on anymore with the supplied voltage
  • burnt inverter board, not powering the backlight anymore
  • broken cable in the hinge, not powering the inverter board anymore

But I have also seen laptops, where this was no hardware fault. Shutting down and powering on again sometimes helps. Also closing and opening the lid might help (in case of a stuck lid switch or a dizzy embedded controler)

Note, that a defunct backlight will not affect the lcd, so the picture will still be there, only with really low contrast. So your problem is a defunct backlight for sure. But this can be caused by more than one fault.

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Tried the shutting down method. Closing/opening/"shaking" also ... no luck. Is there a way to tell between the backlight and the inverter, which one could it be? –  ldigas May 6 '12 at 23:12
    
Without appropriate equipment and experience the only way I see is looking for visible marks on the inverter board and the ccfl. If there are burn marks on the inverter (mostly seen on the transformer) or it smells bad, it is likely the inverter. –  Gurken Papst May 6 '12 at 23:26

If you can see a faint picture on the LCD with the help of an external light (like you described), then that means that either the backlight or the inverter has failed. Or possibly one of the cables running through the display hinge has become damaged.

The first thing I'd try is disassemble the laptop (see here for the service manual) and check the cables between the motherboard, inverter and LCD. Make sure they're properly seated in their connectors. You could also try wiggling them where they pass through the display hinge - if that causes the backlight to flicker, it means the cable has been damaged.

If the cables seem fine, then you're looking at either a faulty inverter or a dead backlight. The best way to tell for sure which one needs replacing is to check the inverter output to make sure it passes power. Since the voltages involved can be in excess of 500 V, this is something best left to a professional. Also, the frequency of the alternating current that the inverter produces (usually around 30 kHz) is probably out of range for most hobbyist multimeters.

If that's not an option, then one thing that can point towards the more likely culprit is whether the problem started suddenly or gradually. If gradually, then it's probably the backlight; if suddenly, then the inverter. Note that this isn't 100% certain, though.

If you're unable to determine which part is faulty, then you'll just have to replace one or the other and see if it helps. Since the inverter is cheaper and easier to replace, I'd suggest starting with that. Worst case scenario is, you're out $20-30.

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Is there a way to tell between the two? –  ldigas May 6 '12 at 23:11
    
The best way I can think of is testing the inverter outputs with a multimeter. But that's probably something best left to a professional, as the voltage the inverters produce is over 500 V. Edit: not sure if hobbyist equipment can even measure it since the frequency is usually around 30 kHz or so. –  Indrek May 6 '12 at 23:15
    
The problem is that I'm currently somewhere where I can't get to a repairshop easily. There is more of a chance that I can order the repair parts of the net and try to install them myself. Weird situation, but that's how it is. So I'm trying to figure this out the best that I can. –  ldigas May 6 '12 at 23:19
    
Since the inverter is usually cheaper and easier to replace than the CCFL tube(s) in the backlight, you could try replacing that first. If it doesn't work, you're only out $20-30. –  Indrek May 6 '12 at 23:20
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The inverter doesn't produce the picture, it only provides power to the backlight. So yes, a dead inverter would still result in a dim image on the LCD. –  Indrek May 6 '12 at 23:23

I found a solution for dark laptop screen/supposed burned out back light, *without repair! : (Dell Latitude D620, for 2 1/2 years and w/ previous users). Tap/lightly bang on the laptop screen front and back, up/dwn, left and R, open and close it, repeat, and the light has remained on. Advised by internet users that the sensor on the laptop screen lid is ‘off’/altered and wasn’t detecting lid open for the light to stay on. :) P.S. I also had a notch/tip at the edge of my laptop top missing with wire exposed so pay attention to keeping that straight. I didn’t check inside the laptop, but I discovered and fixed this 50 mins ago.

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