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I have a 24 inch, mid-2007 iMac purchased used. I don't know the original owner and it's out of warranty.

On the display is a series of tan or yellowish horizontal lines - all an approximate inch apart, for a total of about eight or so. The spacing between is regular. They're wider (about an inch wide) at the left and right edges of the screen and taper to nothing at the middle of the screen. They are particularly noticeable on light-colored or white backgrounds, such as a Pages document, most web sites, etc.

I'm certainly open to disassembling the iMac and getting at whatever it may be (the onboard GPU?) that's causing the problem. In fact I have opened the machine for an HD replacement and fanned and cleaned, but to no result.

I also use smcFanControl to boost the fans, as I thought it may be poor cooling in the machine, but this has no effect.

Does any one have ideas or suggestions as to what is causing these discolored lines?

full display - tan lines

Here's a closeup: a.

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2 Answers 2

up vote 4 down vote accepted

This looks like a backlight uniformity issue.

This is where the screen isn't being evenly lit by the CCFL bulbs behind it. Some of them are outputting differing amounts of light and cause this perceived 'discolouration'. The lighter colours really do show it.

Unfortunately, this is one of the disadvantages of a CCFL backlight system as opposed to something like LED backlighting - backlight uniformity can become an issue, especially with time.

You'd be looking at a screen replacement - outside of warranty, that's gonna cost you.

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^ This. No amount of PRAM resets or cable reseats will rectify the issue, I'm afraid. –  Dan Barrett May 18 '12 at 10:26
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This does appear to be the problem, from what I can see online. Here's a guide from iFixit on replacing CCFL bulbs. I don't believe I'll endeavor on this but it's good to know. –  walker May 18 '12 at 18:16
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If you boot the Mac holding the option key, this will have the firmware drive the screen and isolate any software corruption or odd driver bugs in most cases and let you know it's a hardware issue.

Usually vertical lines are the LCD failing - then a cable issue - lastly the GPU.

Usually horizontal lines are a cabling issue - then a GPU issue - lastly a LCD issue.

That's not to say you even have a hardware failure. Do post a picture and we can refine things based on the actual failure.

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Thanks. I booted with Option pressed down and got the screen asking for which volume to mount from - the tan lines were still there. –  walker May 17 '12 at 22:36
    
@bmike Do you know where the CCFL backlights are in that model? Could it be related to them? –  CajunLuke May 17 '12 at 22:43
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