Chosen Solution
My Thunderbolt Display (A1407) stays black after connecting it to my iMac. After many times unplug the power cable and plug it in again, the picture came back. Unfortunately, it disappears after 30 minutes and does not come anymore. The picture was good, no mistakes. What could it be? Power cable to the iMac or power supply? Has anyone else had this problem and how did you fix it? What would be the most sensible to replace? Many Thanks
I’m thinking you have a bad Thunderbolt cable here. It’s the only logical thing that fits the fact your displays fan & MagSafe cord are powered. It turns out you can plug in a Thunderbolt cable into the external ports and the display into your Mac. See what happens then. Update (11/26/2017) mmm… You do have me scratching my head here. You have removed the usual suspects. At this point it needs to be something on the logic boards power circuity that needs attention. This is beyond a DIY as you need to have access to the schematics and understand how to troubleshoot at the component level. Even then you’ll need to find a replacement part and have micro-soldering skills. The other direction is to just replace the logic board while expensive it’s a lot easier! A1407 27" Thunderbolt Display - Logic board Apple P/N 820-2997 ($192.00)
I’ve never seen a power cord go bad like that, but since it’s the easiest thing to try, you may as well try another one. Another thing to check would be if the display appears on the iMac when plugged in. Last time I looked at one of these it was rather hard to tell that there was a display when using a flashlight on it until I started moving the mouse around. I realized that the Mac was detecting the display and outputting an image but the backlight wasn’t working. This particular monitor would slightly flicker when plugging it into the Mac. If you’re absolutely certain there’s no powering on, then it may be the power supply. Here’s a link to the Apple Technician Guide for this model.
