Chosen Solution
Hi there, I bought this iMac a few months ago with an issue of it turning off right after start-up. Narrowed it down to be the backlight turning off. Flashed the hard drive, installed Sierra, tested it for several days under different loads, no problem. Sold it to my Dad for a few hundred bucks as he needed a new computer. Few weeks into him having it, the problem came back. So I ripped it apart, starting following the Apple technician guide for this issue. The iMac works on an external display, so I know it’s not the GPU. Replaced a faulty v-sync cable, still didn’t solve the issue. Removed the backlight board and took it to get tested, replaced a capacitor on the board and re-flowed some solder joints, still didn’t solve the issue. Tested the voltages coming out of each pin on the backlight board, they were all consistent with one another, so I eliminated that. Tested the other cables on the LCD panel itself, they came up fine. At my wits end, I reset the NVRAM. Computer fired up with the backlight. Ran it no problem since. The trick of dulling the brightness down seems to prolong its life. Is this a software issue or do I need to replace the logic board?
This model uses a backlight LED driver board. Here’s the IFIXIT guide to replace it: iMac Intel 27" EMC 2309 and 2374 LED Driver Board Replacement. Jump to Step11 you’ll need to inspect the Choke Coils to see if one or more are burnt. You may also need to replace one or more of the MOSFET’s (or just replace the assembly) Apple P/N: 661-5311 - Backlight Board
