Dell Latitude D820 screen darkens when brightened too much.

UnknowVector

I come from the net ... My format, Vector.
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Got a Dell Latitude D820 here, and I'm having some trouble with the screen.

Background: It has a function key, and the combination of the function key with the up and down arrows adjusts screen brightness (no adjustments for brightness / contrast separately as far as I can tell, so the function key + arrow keys is either changing one of them and the other is constant, or changing both at once). Function key + left arrow automatically sets the brightness to something more or less reasonable.

Laptop is running on AC power.

Function key + F8 switches video output to the VGA port (I've got a TV here next to me, currently hooked up to that VGA port: that works, no problems). The alternative configuration is for the laptop to use its built in LCD display. (I can also configure the laptop to stretch its display onto both screens, but that doesn't factor into this problem as far as I can tell.)

Can't find any details on the built in LCD screen under the device manager or system information, sorry >.>.

Anyway, actual problem: The screen is a tad dark right now, so I try to increase it. The first step _up_ the screen appears to _darken_ (maybe a result of the "brightness" adjustment actually changing both brightness and contrast at once, combined with the lightening in this room and my own perception; possibly irrelevant). Next step up it brightens as expected. There's a fairly comfortable level a couple steps above that, but other users complain about it being too dark. A step above that and the LCD screen starts making a faint high pitch hum, after ~10 seconds the screen becomes very dark. Adjusting the brightness with the function key + arrow keys appears to do nothing (but actually does), if I toggle to the external monitor with function + F8, toggle back and then quickly using function + down arrow to decrease the brightness (or have already decreased the brightness while the screen was darkened, despite it appearing to have no effect) then the screen returns to normal. If you don't lower the brightness fast enough, the screen goes dark again.

If I restart while the monitor is darkened, the computer begins booting and the screen is bright enough to see, but the LCD hums, and eventually darkens again. The computer finishes booting and I can fix the screen with function + f8. So rebooting appears to have the same affect as toggling back and forth between the built in and the external screens, fixes the problem temporarily, and leaves a short opportunity to decrease the brightness so the screen doesn't darken again.

I'm thinking its some kind of power saving feature? Screen too bright -> lower the brightness, but why would it be on when the laptop is running on AC power, and why would it force the screen into such a dark state? Could it be a failing back-light? When it gets too bright it fracs up, and toggling back and forth between the built in LCD and the external monitor (or rebooting) turns it off and then back on, thus making it happy again for a short period of time?

Sorry for the supremely long post about such an incredibly mundane issue as a monitor >.>.
 
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