Ok so i figured it out, as usual, after a lot of looking around and being pissed...
So the problem is that there is some kind of limitation for the size of opengl my graphic card can render (something like 2500x2500) i was trying to put my crt (1600x1200) at the left of my laptop screen (1400x1050), 1400+1600=3000>2500, here was the problem... Once i understood this it was easy to find a workaround: just put the crt above the laptop's screen.
(just posting this in case someone has the same issue...)
Happy penguins