Cables when extensively use tend to ... DIE ! Mac Laptops are no exception. I wanted to fix it, for a looong time.
Step 1: first : BRUTALITY !!!!!!
First locate the problem. For me the cable was dead on the thin cable at the beginning.
I didn't find any 'clever' way to open the transformer, maybe someone did an instructable called 'how to open a powerbook tranformer with ease', i didn't find it, sorry for this. So I opened it with a hammer and a few flat drivers with anger and sweating. Don't forget to unplug it before you start, don't forget to test it without the case (BE CAREFULL TEST IT WITOUT TOUCHING THE TRANSFORMER ITSELF, I GOT AN ELECTRIC SHOCK 220volts because i played with it while on).
After opening it, i discover that in the box, the cable was stopped by an ingenuous knot molded into a cube of plastic. This time delicately, not with a blade but with a sharp cutting pliers, I extracted the knot off the cube and unlocked the knot.