Testing the Model M
So it's assembled - now to test it. I started up a small computer, checked that a keyboard worked, then replaced that with the model M. I typed a few keys, and they worked! I checked the v, b and space bar, and they all worked. Those were the keys that were failing before. So my repair of the conductive traces using graphite, worked.
Then I did a systematic test of all the keys. The q, w and several others on that row and the row above, didn't work. And a number of keys in the same rows, needed a very hard press to work.
So I'm thinking that the problem is with the plastic carrying the horizontal traces, and I'm going to have to open up the keyboard again to find out what the problem is. This will be a lot easier than my previous efforts, because all I have to do is undo 57 nuts - I shouldn't need to do any drilling. And I'm hoping that I won't need to remove the keycaps.