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Answer to: Solid black line on screen after carrying out this swapTry resetting the PRAM/NVRAM. It may not work, but all it takes is a reboot. https://support.apple.com/en-us/HT204063 Good luck!
Answer to: Changed HDD, Backlight Stopped WorkingI had a situation with a 24" iMac where after replacing the hard drive, my display did not work correctly. Tried everything, including reinstalling OS X. Nothing worked. Took it to the Genius Bar. The Genius reset the PRAM/NVRAM & corrected the issue. You might want to try resetting the NVRAM. https://support.apple.com/en-us/HT204063 Good luck! Hope it helps!
Answer to: How to fix "vertical lines" on my iMac 27 late 2009Have you tried resetting the P-RAM? Try holding down Command-Option-P-R during power up.