That's ok now ! I've had to repair the iMac twice because the Power Supply Unit was also out. At the beginning of the problem, the power button didn't work, but sometimes it worked. For instance if the iMac was stopped for several hours, or days ! Then, it was more and more frequent. I have supposed that some large capacitors on the PSU were too old and wrong. Could it be the reason ?
Many thanks to Ifix-It Europe : they have sent a new PSU very quickly. This 2nd repair took me about 45'.
Super tutoriel ! Félicitations et merci à son auteur, sans lui je n'aurais pas entrepris de démonter l'iMac... et serait resté avec une machine en panne à remplacer.