Unless you're familiar with working with mains power devices and the safety issues involved, I really wouldn't recommend trying to repair it.  I would look for an off the shelf power supply rated to 3 amps 7.5V DC regulated and test the Powerbook with that.  If you don't want to sink cash into it until you know if it's working, see if you can find a friend with a lab power supply that can be set to that voltage and supply enough current.