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September 17, 2012 8:32 AM   Subscribe

Laptop techs of MeFi, I present you a challenge...This laptop refuses to turn on its backlight!

Greetings. I've got an HP DV4-1125nr laptop that boots up fine, NO BLINK on the panel to indicate any error. The backlight simply doesn't come on.

Initially I assumed backlight failure, replaced screen, no fix. Then I assumed inverter failure, replaced 2x, no fix. Interestingly, I have tested the inverter and it is throwing 19.5v+/- on the supply-in side, so I know it's powering the inverter. (I have actually tried 3 inverters, none power any of the 3 screens I have.)

The only possible remaining thing I can figure is the lid switch, however disconnecting it entirely does nothing, neither does a magnet on/off it while it's sitting there with the backlight off. IDK how to bypass it otherwise, suggestions welcome.

Any thoughts? Display is perfect otherwise, no artifacting or anything.

Cable inspection has shown no crimps, shorts, etc, I've never heard of the actual display connector having anything to do with the backlight.
posted by TomMelee to Computers & Internet (10 answers total)
You don't mention it, so I'll ask, even though with the thoroughness you describe I would think you must have: Have you checked the light bulb itself?
posted by cerebus19 at 8:44 AM on September 17, 2012

Yeah, that would be my thought - whatever LED or lamp is supposed to be lighting up the panel just - isn't. And this is why my diagnostic skills on laptops have never gotten any better - every fault tends to lead to a part that isn't cost-effective to replace.

If you can get that lighting gizmo as a replacement part, that's what I'd try next.
posted by randomkeystrike at 8:59 AM on September 17, 2012

Another dumb question: is your laptop plugged into the wall? I think mine, the backlight does not turn on if it is running off of the battery (to save power).
posted by jillithd at 9:14 AM on September 17, 2012

Hah. Yes. The bulb is an integral part of the screen (ok, well, it slides in) and we've got tried 3 screens so far, including one that we know for sure works, and no joy. And yes, it's plugged into the wall.

I am unfamiliar w/ the mechanism that turns the display on---I don't understand how the inverter is energized but the display is off---to me this is nonsensical. Either it's somehow throwing the *wrong* power (although the range is supposed to be around 20v, so...doesn't seem like it), or there's something stopping it in the middle, like maybe the display cable is saying "hey don't turn that lamp on weiner!"
posted by TomMelee at 10:49 AM on September 17, 2012

I had a backlight go out that was due to a fault in a cable going to the inverter. Even though the cable looked fine testing it with a multimeter showed one of the wires was open circuit. This cable was one going from the motherboard, through the hinge assembly, and up to the inverter board below the screen, so it was in a position to get repeatedly flexed over time, fatiguing the wires and leading to failure.

If you haven't replaced or at least tested the cable that would be my next step.
posted by 6550 at 11:06 AM on September 17, 2012

Thanks. That had crossed my mind. This particular laptop is what you might call an "r-title" machine, which is to say that some other tech at some point did some...er...rough work in it.

I'll try disassembling the cable first to look for wear and seeing what I can come up with. That's a good idea---thanks.
posted by TomMelee at 11:41 AM on September 17, 2012

I had this happen once- the bios was actually shutting the backlight off incorrectly. try a bios flash with the newest version for your laptop from the mfr. ; it may fix things.
posted by jenkinsEar at 5:49 PM on September 17, 2012

Holy wow. I will do that, thanks. If that works I will totally PayPal you a beer, lol. If not, I guess I'm ordering a display cable.
posted by TomMelee at 6:10 PM on September 17, 2012

I've never heard of the actual display connector having anything to do with the backlight.

That's how the power gets to the inverter.

Are you sure it is the backlight that is out, and not the display? Can you see the images in the LCD if you look at it crooked or shine a flashlight into it?

If you plug a monitor into it, do you get video?

Have you tried hitting the "switch output" key combo?
posted by gjc at 7:05 PM on September 17, 2012

I meant the flat ribbon cable that supplies the actual graphical image. I am aware that the display cable supplies power to the inverter via a separate connector.

Yes, I know the monitor is fine, we've tried 3 including one that is known to work, and none will light up. I can see the image on the screen with a flashlight. "Display is perfect otherwise, no artifacting or anything."

posted by TomMelee at 5:38 AM on September 18, 2012

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