Thanks to all for the help on this project.
The mixer is dead quiet now after replacing the caps on the rails located on the chassis (it's a big .1 uf and some .022uf).
After replacing those the hum disappeared.
As other suggested, I do have a ground loop in the patchbay too, which I haven't find a solution yet. I've read a few post and articles about the subject. It seems that there is no one solution fits all answers. If anybody has any tip / good practice...
while continuing the restoration of the Studer, I get a pop every time I put a channel in solo or in mute. I know there is a circuit in charge of that and the fault is probably around there, any specific clue on where to look? (like I said before, the console is all recapped on electro and tantalum).