Started to experience problem starting the 21.5 inch iMac (EMC 2389). Pressed the power button and no response. Tried pressing several more times without any response. Waited for some time and pressed the power button and iMac started. This behavior became worse over time. Began to have longer periods between when it would respond to power on.
Removed glass panel and display per iFixIt instructions. Plugged iMac in. No LEDs on. (NOTE: These LEDs are very small and located as a group in the lower center of the mobo about an inch or two above the memory module. Use a magnifying glass to see the LED numbers.)
Verified that AC was available per iFixIt diagnostic instructions.
We did note that the number 1 LED would be on momentarily for brief periods of time.
Repair per iFixIt instructions went very well. Installed replacement power supply. Plugged iMac in (without installing display at this point). Noted LED 1 was on.
Finished reinstalling display and glass panel. Device is fully functional once more.
I tried measuring voltage between chassis ground and the 12 solder connections of the connector on the power supply board. I didn't see any voltage in standby. Unless I made a mistake in my attempt to measure the supply voltage output, this might explain why nobody has submitted that info as yet.