Occasionally, you will run across a keyboard that's been blown by a reverse-connected cable, but it's rare. The RJ11 connector is wider than the one used in the Mac, so you have to shave it down to fit. If the original cable is still attached to that keyboard, it's even less likely that this is the problem.

Your post made me sit and think about all the old macs I've fixed, and what keyboard problems I've come across. I could only recall one totally dead keyboard (from a 128K), and it was fried by a can of Coke accidentally dumped into it. Washing didn't restore it to health, so some grand short blew something good.

Again, just do a careful visual inspection as a first step. Then, depending on how desperate or determined you are to do more, post back, and someone will undoubtedly chime in with additional things to try.