Model A1419 / EMC 3070 / Mid 2017 / 3.4, 3.5 or 3.8 GHz Core i5 or 4.2 GHz Core i7 Kaby Lake Processor (ID iMac18,3) / Retina 5K display. Refer to the older iMac Intel 27" Retina 5K Display (Late 2014 & 2015) guides as the system is very similar.

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Backlighting issues and limited brightness in iMac 5k 2017 27"

Hi everybody,

I have serious issues with the backlighting after I followed the instructions to replace the internal hard disk by an SSD drive - everything else works fine and I have no other issues. Can anybody tell me please what could have gone wrong and if I could fix that myself?

Those are the major issues:

1) Clouding in the lower part of the screen - many bright spotlights with darker spots in between

2) Brightness will not increase if setting the brightness bar higher than 7 dots

3) Screen goes sometimes completely black. No software issue as I see in very bright environment light or with a flashlight that screen itself is showing a normal picture. When it goes black, I cannot do anything to get it back working. After a restart sometimes it's bright again, sometimes not.

Any help highly appreciated!

Thank you!

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2 Answers

Chosen Solution


what you see is stage lighting effect. This can be caused by a bad ‘fraid display cable. Since you just worked on your iMac I suggest that you double/triple check that your cables are all seated properly and that they are in good working order. It can also be caused by some failed capacitors on the “return line” from your backlight LED’s. Post some images of your board, specifically the area around the display connector, with your question. That way we can see what you see. Use this guide Adding images to an existing question for that

Of course, it is also possible that you have a bad LED strip in your display, but this is unlikely.

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Thanks a lot for the guide in the right direction, I feel a bit relieved that it can be solved.

I took some pictures when I first opened the iMac, so this picture is before connecting everything again. Maybe you can see something already.

At the moment I don't want to open it again as I am waiting for new tape strips to arrive to do everything in one step.

--> see picture of board in original post


@mmarkus really hard to make out the details. It's a bit shaky


Dear oldturkey03,

I opened it today again and I didn't see anything special about the display cable.

However I discovered the power cable was not really tightly fixed. In contrast to the display cable this has to be pushed a bit harder on the outer edges to fix it.

By doing that my Mac display perfectly works again!

I am so happy that I didn't destroy my brand-new 5k iMac!

Thanks for your support!


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The issue was caused by a loose power cable. When I fixed it by pushing it in with the outer edges and making sure it sits tight it perfectly works again!

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Markus will be eternally grateful.
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