Welcome!
By freak coincidence, I was just reading about a similar problem in an old Amiga Format. Sounds like the keyboard membrane has a crack in it. It's reparable, but difficult. You'll need to open up the case, and take apart the keyboard. Whoever wrote the letter to the magazine said that the break in the circuit can be filled by some electrically conductive paint (auto parts stores tend to have this stuff).
Note that I've never actually disassembled an A500 before, so I don't know much about this repair job.
You'll probably also want a few other opinions on the problem before undertaking this ;-)
Good luck, and don't hesitate to ask for more help. Someone here will lend a hand.
