Chosen Solution
I have a Apple iMac “Core 2 Duo” 2.8 24-Inch (Early 2008) that won’t boot. I don’t think it’s the power supply, because I get a brief whirring of fans (and about two seconds of the optical drive spinning up) before it fails to boot. I don’t get a chime, or any backlight on the screen. My guess is the logic board, but I’m wanting to confirm that before I shell out the money for a new one…
Brief fan spinning is usually indicative of the switch mode power supply protection circuitry kicking in. The actual malfunctioning component could still be power supply related and not necessarily restricted to the logic board(s).