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Update 2012-03-19: I never figured out what the problem was because the screen started to work again after reinstalling OS X 10.6 for a second time. I sold the Mac on eBay a few months afterward (I explained the full history ;).

Original Post: My Late-2008 Aluminum MacBook Unibody screen doesn't usually turn on. External displays work via the Mini-DisplayPort.

Sometimes when power is not connected, the lid is open, and I have the external display connected, the internal monitor will work! When it works, it stays on until reboot.

I've tried:

I've been told by an independent, authorized Mac repair company that the logic board needs to be replaced. That seems a bit excessive to me, although I've read many similar diagnoses online.

Any other ideas?

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most likely a hardware problem which needs replacing. I don't really think that there is a solution other than opening the case and messing around with it (which voids the warranty) –  user6124 Jun 4 '11 at 15:03
    
I assume you've got the brightness all the way up? –  Nathan Greenstein Jun 4 '11 at 15:22
    
Agree with you : logic board replacement seems overkill (for now). Have you tried the same thing on a fresh OS X Install? You could just try to boot from the OS X Install CD to see if the same problem appears. –  gcamp Jun 4 '11 at 15:40
    
Definately get a second opinion. On that model a trained tech should reseat all cables, isolate the issue to Display, inverter or GPU/MLB. It's highly unlikely to be a power issue, but I have seen cases where insufficient internal power caused the display not to light but externals to work. Most techs can know MLB or Display if they see the failure, but if they have to guess and it's working when they see the mac - it's a bit of guess work to know until you start the repair. –  bmike Jun 4 '11 at 17:58
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3 Answers 3

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I would put a lot of weight in a technician that sees many macs as they really don't have any reason to mislead you. You can also get a second opinion if you don't feel the one you got the first quote listened to your concerns or explained how he arrived at that guess.

Do make sure you look carefully - when the backlight goes out on a LCD - there is so little light coming through it is hard to tell if the panel is drawing fine and the light is missing or the panel isn't being driven.

Without opening the mac and trying to replace things (and especially when it's intermittent) it's hard to know for sure what one piece is failing. If you ask a tech to put percentage chance you will find most situations have multiple causes, but one is much more likely given experience and symptoms.

The next time it goes out - you can use a flashlight to narrow down if the panel itself is not getting signal or it's just the backlight by looking for the menu and other graphics on the screen getting drawn.

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I'll get a flashlight and check it out. Thanks. –  weberwithoneb Jun 6 '11 at 15:28
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I had a similar issue - if I had a monitor connected, my macbook screen would sometimes stay dark. I found out that that was only happening if I connected the external monitor while the laptop lid was closed. If I connected it while the laptop screen was on, it will stay on.

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bmike is correct about the backlight. A good method to test if there's any doubt is a flashlight shone on the screen to determine if video or backlight has failed. Another good test is if the issue changes with the monitor hinge being moved from an open-to-closed state.

Given that you have consistent video output on an external display, the suspect would first be your LCD panel over your logic board.

Also to bckbck: Opening your Mac does not void your warranty. That's why RAM and HDD are considered DIY parts on newer units, and Apple supplies installation manuals for users. You are free to do as you wish with your own hardware. Any modifications you make that damage the unit, however, are on you and not under warranty. That's as far as it goes.

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Interesting about the warranty. Thanks for the info. –  weberwithoneb Jun 6 '11 at 15:28
    
I doubt it would be the logic board. I would guess it's the display, the backlight, or the cables that connect them to the motherboard. Have you dropped it or has the hinge been forced open too far, or the like? At any rate, testing and repair by a professional is a good idea! I have not examined your computer, but I am an Apple Certified Macintosh Technician. –  Wheat Williams Sep 4 '11 at 3:37
    
Thanks Wheat. I haven't dropped it, but I've traveled a lot with it, so I may have dropped it in my bag without knowing. I had it tested by two separate certified technicians and both blamed the logic board. I suppose I might take it to the Apple Store again and just have them swap out the display. If it's not the display, I could just tell them to leave it be (I'm currently using it as a media center in my living room). –  weberwithoneb Oct 29 '11 at 13:29
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