24" display; Built-in iSight Camera, Microphone, and Speakers; 1920x1200 resolution. Released October 2008, identified by model number A1267.

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A1267 ACD 24" backlight issue

About two years ago I was playing a video game on my Mac Pro, although it was a low-end GPU, I was rendering the game on high graphical settings. The fans in the processor started revving up and making frighteningly loud noises. Then without warning the screen just shut off. From that moment I found that the display would not work unless I reduced the brightness below 10/15 increments. This got worse over time and now it only works on the lowest brightness setting. Shining a flashlight at the LED screen reveals a faint image when the display goes black. This suggests that the backlight is the issue. I carefully removed the glass, LCD screen and had a look at all the components inside. I'm no electrical engineer, but the Power Supply board and Logic board look fine to me (no swollen caps or burnt areas). I tested the voltage on the test points on the Logic board and it all reports fine.

From here, I don't know how to proceed troubleshooting this problem. It's always evading my attempt to fix it. The best way to troubleshoot this problem would be to temporarily swap identical model components to isolate what is the source of the problem. But I'm living in an area where nobody has a ACD. I've tried bartering with ACD sellers on online marketplaces to no luck. There are six main physical components inside the monitor that are culprits, replacing each on the off-chance would cost a collective ~€300 if I was unlucky.

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Images: http://imgur.com/a/09iOx

Update (01/20/2017)

Dan,

I've read the service manual carefully several times but I can find no solution and I've read every answer to the backlight issue on fixit but find no solution.

I've taken apart and reassembled every component inside the monitor with the exception of the main driver cable leading directly into the monitor. It was covered with weird silver tape (electrical tape?) that had to be peeled back. But I haven't had the guts to attempt to remove the cable from the slot, as it doesn't seem to want to come out. Does anyone know how to remove this cable?

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Images: http://imgur.com/a/i3JUs

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See if this helps: The service manual

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As explained in the original post I checked the voltage on all the test points on the logic board and they all report fine. Initially I presumed the issue was with the GPU, but after buying a new GPU I can confirm that is not the issue. I replaced the main power cable too, but no change. The firmware is updated, so I'm fairly confident it's not a software issue.

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Is there any help you can offer me?

As explained, the inner components pass the inspection test. I don't want to shell out €220 on a new logic board in the hope that that is the source of the problem. What if it isn't - I've wasted a lot of money.

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Did you download the file in the link above? Review the section on display for all of the details.

Can't offer you more without doing the tests as outlined in the doc. If you can't do it thats fine, you'll need to find someone in your area to help you.

Once you've isolated things you might be able to get someone to fix the logic board or you'll need to replace the part as needed.

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Yes I've read the service manual top to bottom several times and I'm all out of ideas. I've followed the chain of solutions but they've been no help. The Logic board looks fine on visual inspection, and the voltages are correct. The Power supply unit looks fine too. The cables are all seated correctely. The one cable I'm unsure of is the cable leading directly into the monitor. I've been too cautious to attempt to remove it by yanking it out. It doesn't seem to want to come out. I would like to replace that cable if I could but it's the most difficult individual component to purchase on an online marketplace. Not that it matter anyway because I'm too afraid to remove it and don't even know if it's meant to come out.

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I think you're digging in the wrong direction here.

If you review the troubleshooting section you'll see the issue is likely the backlight circuit on the main logic board. Did you test the connections to see if you see the required voltages as the guide explains?

I would start at the basics before digging into the LCD display unit.

FYI: The metal foil is for EMI emissions control. You'll need to get some fresh aluminum tape to replace it.

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@morgan

I had exactly the described problem 2 months back, I did some extensive research and tests with my oscilloscope.

I am not yet 100% where the problem lies but the monitor has worked perfectly for 3 weeks now.

I disassembled my display totally including the LG Panel and took some images. See the ifixit.com 27" What components need replacing on the Logic board?

I took apart the LED stripe as well and desoldered it. In addition to changing 8 capacitors in the power source and now the display works as a charm. I have also some pictures how to get the LED connector out, but that is really tedious, see the pics.

What components need replacing on the Logic board?

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Any pictures on how you got at the LED stripe after removing the LCD panel assembly from the monitor case would be really useful

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Well just open the link I posted you have a dozen of pictures there. And if you need, I have plenty more pictures. I just didnt want to post them all.

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