I actually have a similar model -- the 6714 -- and ran across the same issue when I first got it. When you say "sun" are you referring to the
F8 key? (ie are you pressing
Fn + F8 to try to brighten the screen?) Not sure how yours is labeled, but if it's like mine then it's backwards.
Assuming
Fn + F8 does
not work, try
Fn + Up* instead (unless, of course,
Fn + Up is what you already tried, in which case try
Fn + F8). Incidentally, their counterparts are (usually)
Fn + F7 and/or
Fn + Down.
Hope that helps...
* "Up" means the "Up arrow key" on the bottom-right of the keyboard.