I have an older Toshiba Satellite c655-s5049, after some research I decided to upgrade the CPU. I replaced the Celeron 900 with a Core 2 T7300. I also put a second 2gb ram stick in. I put the ram in, and everything worked fin, so I then put the CPU in. With the new CPU I get display when it is unplugged and running off of battery, but when I plug it in the display is so dark that I need a flash light to see it. I've checked the brightness settings in the power settings, and all appears well. I'm at a bit of a loss here.
If all else is working ok, I would just try a software fix, like I linked to above that can override your display settings, and lighten up the display.
I gave this a try, and a minor degree it worked. I went from having a near pitch black screen to almost pitch black screen. As far as troubleshooting goes, I'm sure that means something. I just don't know what!
As for the BIOS, I made sure I had the most current version on the Toshiba website (Listed for this specific model).