Hello,
On the power supply card of the hp 3020 (left side inside the printer)
there is only one electric cable. Reference is J302 on bottom left
blue frame of this diagram :

http://perso.orange.fr/zygomaths/index.fr.html

But I don't know where to plug in it.

A lot of thanks for that one who give me help because it is the last
step I need to put back my printer together