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?
Is this a good question?
7 Comments
@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?
by oldturkey03
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.
by Jack Voorbij
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....
by oldturkey03
Ok that's good to know, thanks. How would resetting the NVRAM suddenly cause the board to work though?
by Jack Voorbij
"resetting the NVRAM suddenly cause the board to work" you did not say that resetting the NVRAM resolved your issue.
by oldturkey03
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