I know this is a very old thread but I believe I have solved the mystery. I purchased a used 27 iMac 5k with 3 black lines on the right hoping I could fix it. I tried everything to figure out why it was bad, GPU, cables, cable connectors, etc. I've also read every question on this topic on this site. It was not until I removed the metal shield on the top rear of the LCD that I saw that it had several cable ribbons from the long PCB connected to the screen. At first I thought the leads were bad, but upon closer inspection I noticed three of the ribbon cables were cut. 3 black lines on the right. 3 ribbon cables cut on the right. It appears that when the last person tried to get into this, they may have used a sharp blade or razor to cut the adhesive. In doing so they sliced too deep and cut into the ribbons! I don't know if the ribbons can be replaced or the rear PCB. I have never seen it sold anywhere and the soldering is so fine it can't be done by hand. :(
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