Brightness issue

Hello everyone , I am new to your Linux forum and like many other people i have a brightness problem.
I have Ubuntu Mate 15.04 on my desktop pc and a very old Samsung monitor. (vga is AMD/ATI RV770 Radeon HD 4870)

I have xbacklight set to max from terminal and it says no output
I also have the brightness applet in panel but when I decrease or increase , there is no change . doesn’t work.
some menus and programs are ok with brightness , but some other are very dark. youtube videos are also very dark (flash and everything else is installed)

What about installing a vga driver ??? in additional drivers menu doesn’t give anything to install…
Any ideas what to try next?
thank you in advance

does the command “xbacklight -set x” result in “no output” or in “No outputs have backlight property”?