Several years ago someone just gave me this old iPhone 3GS as non working, I could not figure out what is the problem and kept it in drawer for years. But recently I decided to review this old guy and give it another life. So i purchased new battery, took screwdriver and starter the surgery.
Check this video for details step by step instruction:
And not my renewed iPhone 3Gs works like a charm ( after I restored firmware using iTunes on my PC )