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iMac Intel 27" EMC 2309 (Late 2009, Core 2 Duo 3.06 or 3.33 GHz) ID iMac10,1, EMC 2374 (Late 2009, Core i5 2.66 GHz or Core i7 2.8 GHz) ID iMac11,1

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LED backlight failing - need confirmation of problem.

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?

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@jackyv "tested the voltages coming out of each pin on the backlight board, they were all consistent with one another, so I eliminated that." How did you do that and what was the voltage?

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Test with a multi-meter on a/c voltage, check across each set (pin 1 and 2 control led 1, etc) and got 0.012 volts across each set +/- 0.001 of a volt. I don't know the specifications but if the board had failed it would seem to me that all of those would read 0 volts or they would be inconsistent.

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0.012 volts is not enough to drive your LED's. Your board is definitely not working as designed. That very low voltage could just be shunted from anywhere....

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Ok that's good to know, thanks. How would resetting the NVRAM suddenly cause the board to work though?

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"resetting the NVRAM suddenly cause the board to work" you did not say that resetting the NVRAM resolved your issue.

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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

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Hi Dan, thanks for your reply. That link to "choke coils" doesn't give a very good description of what to look for on the board. Also I require a better description of what "MOSFET's" are.

I have had the backlight board out of the iMac already, and had it checked and components replaced, as I said, but will happily check other components you have mentioned, should you be able to describe them to me.

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Did you click on the word 'Choke Coil' you'll get to the part description. Maybe its just best to replace the board Vs trying to patch it up. Do you see anything burn't? Maybe a good image of your board (both sides) posted here will allow us to see whats up.

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I was using my phone earlier, apologies. I'll look at them next time I'm near the computer. No, did not see any burnt components though. Had it inspected and repaired by a computer/pcb repair shop. I don't want to replace the board if it's not going to solve the issue, it seems unusual that resetting the NVRAM would solve a damaged component issue, even if temporarily, no?

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@danj Hi again mate, any further ideas on this issue?

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Jack Voorbij will be eternally grateful.
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