Dead keys on C64

Well I landed an amazing C64 software score, with boxed sets of Ultima I, all the Zork series, Neuromancer, an editor assembler package, a koala pad, a bunch of joysticks, and a bunch of other stuff I can't remember. The boxes and manuals are in almost _pristine_ condition! Didn't pay a dime... one of the guys I work with was amazed that I liked this stuff and pulled it out of his basement to give to me for a beer. Well, OK; I paid a beer. ;-) Problem: I just noticed the C64 seems to have these dead keys '5, 7, 9, and 0'. Looks to me like a dead bit on the keyboard controller, but I must confess to ignorance on the internal details of how the C64 accepts key presses. Can anyone give an overview, and would anyone know of a fix? Thanks, --jmg