So I've had this old terminal for a while. It needed some logic fixes, but the keyboard was also not working well. Several keys would jam, and I had no idea why. The key switches used are either Hi-Tek (one wide contact and one with 4 'fingers'), or perhaps an early Stackpole clone. The plungers are black.
So today I spent some time looking into this issue. I noticed that the plungers that go into the square hole were often torn at the top. Many of the plungers have two torn corners. The plastic seems pretty weak, and the keys go into these plungers exerting force on them. So after years of use they tear. This makes them a little wider, and now when you push them all the way down, they may get stuck into the square hole in the keyboard 'waffle'.
However, I noticed that this was not true for *all* the keys with torn plungers. In fact, even with the plungers (and spring) from a key that jammed elsewhere, they still wouldn't jam. So clearly the key holes themselves also had something to do with it. Apparently there's a slight tolerance involved. So I took my trusty rotary tool with a filing bit, and lightly filed out the corners of the key holes with the jamming keys. Turns out this works perfectly, and I now have a keyboard without any jamming keys. There don't seem to be any negative side effects. The plungers go in deep enough that this doesn't give them any extra play. The keys themselves may be a little loose in their plungers, but this was already the case before the filing.
To demonstrate, I made a little video showcasing the problem and the solution. Hope this will help someone in the future:
