On boot: backlight lights up, then goes dark after half a second
I’m trying to repair an iMac 21.5” (Mid 2010) EMC 2389. It has two symptoms:
(1) When I press the power button, it chimes a few seconds later, then the backlight lights up on a grey display, then the backlight goes dark immediately (a half-second later). If I shine a flashlight at the display, I can see the Apple logo as it’s trying to boot.
(2) When the iMac tries to boot, it shuts down after about ten seconds.
I’m guessing (2) is because of a bad hard drive, so I’d like to find a solution to (1). Here are some more details:
(1a) If I hold Option as I turn it on and then shine a flashlight at the screen, I can see a hard drive icon as it waits for me to select what to boot from.
(1b) If I connect an external display, I don’t get any signal on it This is probably okay as it doesn’t reach the point at which it tries to initialize the graphics chip.
I thought this would be a simple issue of the LED power board being dead. So I purchased and installed a replacement LED power board, but I get the same symptoms.
Anyone have any suggestions for other things I could try?
It chimes once when it boots, before the backlight flashes on then off.
If I hold Option when it boots, it will sit at the boot volume selection screen (as verified with a flashlight) forever without turning off. That’s why I think that the turning off when booting is some problem with the internal drive, and if I were to boot from an external drive it would probably boot all the way - but probably leave the backlight off.
Anyone have any suggestions for what could be wrong with the backlight, and how to fix it?
- If I hold down Command-R during boot, the iMac boots into Recovery mode and gives me the usual prompt to restore from backup, reinstall macOS, run Disk Utility, &c. (I can see all this with a flashlight pointed at the iMac's display.) At this point, my experience on other Macs tells me that it should have enabled an external display; but with this iMac, the external display remains dark and doesn't see a signal.
- There are only three times when the iMac's display backlight turns on for half a second: when it first boots, when it initializes the graphics chip (I call tell because this happens partway through booting into Recovery mode), and when it wakes the display from sleep (after I let Recovery sit for a while and then I press a key on the keyboard).
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