My iMac does not start any led on the motherboard

Hello, my imac 21.5 inches of 2010 emc 2389 does not start anymore after a repair of the graphics card, it is not the first time that I make a repair on an imac but I am stuck ...

When I press the power button nothing happens and more amazing no diode LED is lit!

When I plug the cord the LED # 1 lights up briefly then goes out after 5 seconds it turns it on again then nothing.

No fan turns, no sounds nothing is nothingness ...

I tested my power button with the multimeter and it works.

I tried to reset the SMC several times but no change ...

I went around a lot of forums, and try a lot of things, I also have all disassemble and go back several times ...

I tested the multimeter the voltage output of the power supply, I have the tensions shown on the forums ... I even tested on the card side of the motherboard so a priori there is good juice ...

Especially since the power supply worked perfectly well before the repair of the graphics card.

I even try to remove all of the motherboard and plugged only the power supply but it is the same at the LEDs

If anyone has ever had this problem please share your experiences ...