Late 2013 27" iMac won't power on at all

[timeshifter]

Last time the system worked it was plugged into a 6 month old UPS, on the battery side. Customer had an extended power outage. They didn't know to shut the computer down. When the power came back the following day everything else connected to the UPS and throughout the house worked fine.

I just installed a new replacement power supply. Same symptom. Press the power button and nothing happens at all. The diagnostic LEDs on the system board are dark. I did not reconnect the hard drive or the display cables. Also tried removing the RAM modules.

Apple replaced the motherboard about a year ago because the USB ports quit working.

My diagnosis is that the logic board or some component therein has failed.

Agree?

Is there something I've overlooked?

 

[sapphirescales]

The power supplies in iMac's aren't any better quality than the power supplies in a $500 Dell from Walmart. If a crappy power supply goes, it can take the motherboard with it. That's likely what happened here.

 

[cyabro]

Yup sounds like the logic board to me.
You may find the original PSU is fine too, since the symptoms are the same with a new PSU.