Lisa 2/10 keyboard not working - looking for troubleshooting advice

[xevro]

I have two Lisa 2/10 computers. One belongs to someone I'm helping with a repair, and the other is my own Lisa, which I bought in May this year. I don't remember testing my keyboard when I first bought it, so I'm not sure whether it was working back then because I had to do maintenance on the power supply which took some time.

While working on the Lisa I'm repairing, its power supply failed and shut the computer down unexpectedly. I've since repaired the power supply. I also installed a Sigma Seven Systems X/ProFile interface and got LOS 2.0 running. However, I discovered that the keyboard doesn't work. I tried connecting it to my own Lisa 2/10 as well, but it didn't work there either. This makes me suspect that the keyboard itself is faulty.

I've tried measuring the keyboard signals with an oscilloscope. When I press the keys, I can't see any noticeable signal being generated; I only see a signal around 900 Hz, which seems to be coming from the Lisa. I'm wondering what else I could check. I could ask the person I'm repairing the Lisa for to lend me one of his working keyboards, which would help determine whether the problem is with my keyboard or the computer.

I also read that the conductive silver pads inside the keyboard can deteriorate with age. If that's the problem, would I expect at least a few keys to still work, or could all the keys fail at once?

Finally, would it be safe and useful to test the keyboard by touching the contact points directly on the PCB with my finger, to see whether the Lisa registers a keypress?
Any advice on further troubleshooting would be greatly appreciated!