Chosen Solution
I have just replace the internal drive with a SSD drive, on my iMac (27-inch, Late 2012) 3.4 GHz Intel Core i7. I have put it all back together as described in the ifixit guide. The computer starts up with no chime. but the screen is black. Once the iMac has started I can access this macs log-in screen via Apple Remote Desktop, but the fan is fairly noisy. I have removed the screen again and checked the 2 connections and all seems fit correctly. The screen shows no signs of a fault. The system also works when I use another screen via target screen mode [cmd F2] on another iMac connected via a thunderbolt cable. So I’m not sure what else I can do. This is the second iMac (27-inch, Late 2012), I have swapped the hard drive on, this previous one went without a hitch. Any ideas on what I can do? or what to check?
Sorry Neil for not responding sooner Did you tear or kink the ribbon cable? Do you have a replacement display to check things? If that didn’t solve it I think it was just just bad luck the backlight logic died on you.. Not much you can do other than replace the logic board or if you know someone who has the sills and tools to fix your logic board.
