Won't Boot: How to diagnose if Logic board or PSU at fault?
Model = A1311
EMC = 2428
iMac 21.5-inch Mid 2011, 2.5 GHz Quad-Core i5.
Symptoms: Has suffered no trauma (No shock or water) but suddenly no chime on boot. Screen remains black - no light. The only thing that moves is the DVD drive. After inserting a boot DVD, it 'scratches' for a while before ejecting it (And it would only eject it after I had dislodged the drive from the iMac casing. I guess there was too much resistance in the casing's 'rubber dust curtain' for the DVD to push through). Neither the fans nor HDD spin. Inserting a boot USB memory stick does nothing - It doesn't even light up. Attaching an external FireWire-powered boot drive does spin, but it won't boot.
Attempts to reset the SMC (Remove power cord and hold the power button) and PRAM did nothing.
On removing the screen I can see the 4 diagnostic LEDs numbered right-to-left 501, 502, 503, 504. Only the right-most LED (LED501) lights, and it remains lit as long as the power cord is plugged in. Hitting the power button makes no difference to these lights
I have removed and inspected the power supply unit (PSU) but see nothing suspicious about it. I couldn't see any blown capacitors.
Strangely, I can't find an Apple technical manual anywhere for this Mac so hope someone 'out there' can better advise me.
I suppose the symptoms suggest either the main logic board or PSU are at fault but I prefer to be sure before buying a replacement part.
I am comfortable with testing the the PSU (Dangers understood) or logic board (There seem to be lots of square terminals on it for testing and reseting purposes) but there's no point in trying without a guide.
Can anyone please advise? Refer attached photos for clarification.