Q: iMac 2007 Intel won't turn on

To start, this isn't my computer. I'm a tech working on this computer.

It is an iMac late 2007 (one of the first Intels). The computer will not turn on at all. No startup chime, nothing. Just the light on the front indicating power is on and the fans are running. I've tried the steps outlined here: If your Mac won't turn on - Apple Support, and I still get nothing. After resetting the SMC I got a chime, but no boot, and only once. This clearly means that it is a hardware failure somewhere. My question is where?

In similar cases I've seen online, others have simply taken it to the apple store and they replaced it entirely, which means to me it's got a good chance of being the mainboard. Manufacturers tend to replace the whole system if that fails. Before I order a new board for my customer though, I wanted to see if this diagnosis is even close to accurate.

I know that power problems can cause all sorts of strange behavior, but I tested the power and read it at 12V, rated for 12.1V, so it should be within tolerance unless it just can't handle the load. I didn't see any physically visible problems with the power supply, like a bulging capacitor. Capacitor aging can be a problem here though, because it is 8 years old after all.

Any ideas, or a validation that it is the mainboard?