Lets backup here a bit… There are three parts involved in the backlight
- Power supply
- LED driver logic on the logic board
- The display its self
If we think this through, the LED’s within the display are driven in sets, so if one LED had died then the other LED within the given set or even the other sets would be lit. So the display is the less likely the issue here.
That leaves us with either a power supply or LED driver logic on the logic board. So how can we isolate this down? Have you tried running the onboard diagnostics? Restart your system and press the D key. Your external monitor won’t display so you’ll need a flashlight aimed at the display so you can make out the screen and the steps. Did you get an error message? Let us know.
One of the things we have seen is the newer ‘Thin Series’ iMac power supplies tends to be more sensitive to power fluctuations. I do strongly recommend using a good fill-in UPS and use surge suppressors on all your gear so there is no unprotected connection to your system either USB/Thunderbolt ports or network via Ethernet.
So, replacing the power supply is quite easy, but fixing the backlight logic on the main logic board will take some skills. It could be as simple as a blown fuse.
One last point I’m assuming you haven’t opened the system if you had at some point, you could have a damaged display cable or the backlight power cable.
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