Won't start. Only first diagnostic LED.

My iMac 21,5 EMC 2389 got damaged by short circuit the LCD connector/cable by opening the iMac.

LB connector and LCD cable were damaged. Both replaced.

Also found fried L9000 coil on the back. Replaced with original component.

When I put the power cord into the mains (NOT pushing the power button) following happens:

-fan (low speed) and harddisk start

-ONLY first diagnostic LED on the logic board comes on.

-nothing else happens

I can shut the iMac down by holding the power button.

I can start the iMac pushing the power button. Start means fan and harddisk start and only first LED is on. Nothing else.

There must be atleast one faulty component (transistor?) who lets start the iMac without touching the power button. Which one is it?

I have the complete schematics (92 pages) and the board view file for this logic board LB 820- 2784 (K74 MB ASSY 3.6G 631-1430)

My questions:

Who has had the same experience?

What is the general reason why the other diagnostics LEDs dont come up?

Who can help me reading the schematics and trace the faulty component(s).

When found, I have the possibilty to have components replaced.

I think its easier to find the same LB used but its more fun to find the fault and have it repaired.