I think the odds of you being able to fix this thing are pretty slim. The only thing I ever try to do, and it has ALWAYS worked, is the following:
Oxidation will always eventually form on all of the contacts of the charger and the battery. This, in combination with dirt and misc. gunk will end up preventing the charge from passing between the charger and the battery or device to be charged. What you do it take a pencil eraser, or a white pen eraser is better, and firmly rub all of the contacts on the charger and on the battery or device. If you can't access one of them with the eraser then maybe use 220 or 150 grit sandpaper formed into a narrow sliver. This will clean your contacts and, in most cases fix your problem. Make sure to blow out any dust after you clean it.
How old are the batteries? Ni-Cads don't last forever. If the batteries are more than 3 years old, especially if they have been overly expended or improperly charged (frequent topping off or partial charges) then they could be croaked. I am only saying all of this because I have never seen a charger go bad. There are no moving parts. You should be able to order new batteries or battery packs.