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I’m having a problem with a 5k iMac 27" (mid-2015). When I turn it on, the backlight comes on, but the screen is completely black and there’s no activity. An external monitor (plugged in with a Mini Displayport / Thunderbolt to HDMI cable) works, but only after I log in. I can do that by selecting the user and typing in my password blind, although it doesn’t work every time. So I restart it, wait a minute, try to login, wait a minute, then if it doesn’t come on I restart and try again. When it does work, the computer seems to be working fine. It connects to my wifi (again only after login) and everything is fine, except that the built-in screen is black. Some background: A few months ago I opened it up and swapped out the HDD for an SSD (with a kit from OWC). It worked afterward, but some of the backlights stopped working, giving a look like stripes of dimness at the bottom of the screen. I needed to order more adhesive strips before I could open it up and re-seat the ribbon cable, so I left it and never got around to it. It worked for several months. Then one day the screen went black. I turned it off and back on, and the backlight came up but that was all. I tried resetting the NVRAM, reseating the RAM chips, and then gave up and put it in the garage until I had time. Yesterday I got it out and plugged it in. To my surprise, the screen came on and it appeared to be working fine! I walked away to grab a keyboard, and when I got back the screen was black again and it hasn’t come on since. I plugged in an external monitor and after a lot of messing around figured out I that I can login blind, and once I do the monitor comes on. I’m assuming there is a hardware issue, probably connected to something I messed up when I swapped the drive. I’m ready to open it up again but I’m not sure what to look for. What are the likely causes of this issue? Update: I opened it and checked the backlight and data cable. The backlight cable slots into its connector very loosely, so it might have shifted when I closed up and that might account for the dimness I noticed. Anyway the backlight is working. The data cable appears to be connected correctly. I opened its little swing-clamp, slid it out parallel to the board, and everything looks fine. There’s no sign of damage or strain in the cable, its connector, or the receptacle on the board that I can see. I also removed the other end where it connects to the display and checked there with the same result. No visible problem. I reconnected it carefully, closed it up, and turned it on. Same result.
Before you open the system take a flashlight and press it on the display at a sharp angle are you able to see the faint image of your desktop and its icons (or in this case the login window if you just turned on the system) If you do your issue is within the backlight circuit. That could be the displays backlight LED’s, the connector and the cable which connects to the main logic board connector as well as the circuitry on the logic board.
If you not see an image thats point to the driver logic on the backside of the display, the connector and the cable which then runs to the logic board as well as the connector on the logic board.
Check your cables and make sure the connectors are not damaged. A common issue is when you disconnect the display you could have yanked the cables distorting the connectors or damaging the cables. Did you follow the guide and use the correct tools and technique? You could have also damaged the display in the process of taking it off as well. As far as swapping out the HDD for a SSD you don’t need any fan software for this series. From 2014 onward the OWC cable is not needed nor any software. Update (11/09/2020) I suspect you may have damaged the display in the process of removing it. I’ve seen people slice the ribbon cables from the LCD to the T-CON when people inserted a knife to deeply
So you’ll need to pop off the metal cover to see if you might have done this. In any case its time for a new display assembly iMac Intel 27" Retina 5K Display Display Replacement
