Three LEDs, no chime, odd static noise.
A friend of mine asked me to take a look at her old iMac (EMC2133). She said during a recent storm her neighborhood experienced a blackout. When this happened she wasn't home, her computer was turned off, and it was plugged into a (good quality) surge protector. When she returned home, her computer would no longer turn on.
I took off the front cover and had a peek. When plugged in, the first diagnostic LED lights up (as it should). Pushing the power button immediately lights up the second and third diagnostic LEDs. The fans kick in, the hard drive spins up, and the optical drive does its POST, but that's it. No startup chime, just a black screen.
The voltage on the power supply is in spec (measured w/multimeter), and connecting the iMac to a bench power supply doesn't yield any improvement so I'm pretty sure it's not the power supply.
One thing I did notice is, when the machine starts up there's a "staticy-clicky" noise coming from somewhere behind the motherboard, below the hard drive (almost in the exact center of the iMac). I made a video so you can hear it; it becomes really audible at 0:16. It is most definitely NOT the hard drive, optical drive, or fans making this noise. Video here: https://youtu.be/k6hUDmUKkWk
Apple says the lack of a fourth LED means that the display panel isn't responding. I tried disconnecting it to see if it would come up "headless" but it doesn't. I also tried hooking up an external monitor (both with the original panel in place, and with the original panel disconnected) but that didn't help. Is there any way to test this theory without swapping out the panel?
Otherwise, what might this be? Others have suggested a bad GPU but LED 3 "indicates that the computer and the video card are communicating" according to Apple so that would imply that the GPU is operating correctly. Or, am I misunderstanding that? Apple says "(LED 3) will be ON when the computer is communicating properly with the video card. If LEDs 1 and 2 are ON and you heard the startup sound, but LED 3 is OFF, then the video card might be installed incorrectly or need replacement." But I have all three LEDs; only LED4 remains off.
Is this a good question?