I also have the NP770Z5E. What helped me was to change the grub kernel
boot parameter to

acpi_osi=linux acpi_backlight=video

in /etc/default/grub (line GRUB_CMDLINE_LINUX_DEFAULT=) (I have Debian Linux). 
Hint was found at
https://wiki.archlinux.org/index.php/Backlight#Kernel_command-line_options

After command "update-grub" and reboot changing brightness was possible
again.

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https://bugs.launchpad.net/bugs/1485246

Title:
  Backlight issue - Samsung Chronos 7 (model - NP770Z5E)

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