Bringing a Game Gear back to life

I recently recovered a Sega Game Gear from a box full of junk: the seller told me that probably it wouldn't work, so he sold it for a very small amount of money, but sadly had no games to go with it.
As soon as I got home I tried powering it up using batteries: It turned on but it also left me with a black screen and a very dim backlight illumination (the black screen turned out to be normal on the original Game Gear units without BIOS).

My repaired Game Gear

Keep reading if You're interested in some nasty repairs ;-)