My iMac does not start up, LED#1 not on

all the sudden my iMAc did show no reaction at all. No reaction to the pressing of the power on botton. No noise etc.

I brought my iMac for diagnostic to an authorized Apple dealer/repair center for diagnosis. Their result was that the logic board needs to be replaced. I ordered a used and tested logic board for my iMac model.

After replacing the logic board still the same symptoms. I checked the LEDs. Even #1 does not light up. Unfortunately I trusted the diagnosis of the apple center and did not check the LEDs with my old logic board.

my questions:

1.) Could be even the replacement logic board be defect? Would need to have the similar defect..

2.) Is it the PSU? Is it worth to go down that route as LED#1 does not light up

3.) Is there anything else than the PSU which could cause this, except for the replacement logic board having the same defect

thanks for your help Axel