How to diagnose if iMac won't boot up

Hi,

My IMAC won't boot up. Nothing happens when I click the turn on button.

I've viewed some similar questions, I still don't know where the diagnostic LEDs are.

It seems nothing is on on the logic board.

I'll appreciate if someone and point where the LEDs are in a image.

Or send me a Technician Guide.

Or advice how to test my PSU

Thanks very much!

------------------------------updated--------------------------

I guess I found where are the LEDs

So it seems none of them are lit.

From Apple service manual, it says if LED#1 is not lit, I need to replace the PSU

But I really doubt it because I check my pinout of the PSU:

From up to down is:

3.3V

0V

0V

0V

0V

0V

0V(gnd)

0V(gnd)

0V(gnd)

12V

12V

12V

I think at least the standby power is correct, because it writes on the front side of the PSU

And when I click the start button the 3.3V will be pulled down to 0V

So can I confirm that it is the logic board issue and I need to replace it?

Thanks!