The screen on my laptop Compaq nc6120 gets dim as I plug the power adapter and it will be ok as I unplug the power cable. Would there be any idea that can help?
I think there is nothing wrong with your laptop, it’s just settings in the power management software. Usually you can adjust screen brightness separately for AC mode (when laptop runs from the power adapter) and battery mode (when laptop runs from the battery). Apparently, in your case the screen brightness has been reduced for the AC mode.
Go to the power management utility and turn the screen brightness all the way up for the AC mode. On some laptops (some Dells for example) you can change LCD screen brightness in the BIOS setup menu.