I have been doing some repairs lately, and I have seen some full-nut modules that did not want to work. In the past, I have always classified them as defective and go with another.
However, this time it was a CX module. This are far rarer and very expensive to get. What's more, I saw it working on-and-off before the repair. (It was a lower post repair), so it should work - but it was not working in the original repaired calculator. Others were working, thoug, and I also managed to make work the CX module on another not repaired body. What could be the problem?
Here is a picture of the lower post repair piece:
I have designed a couple of pads on the left and right of the screw posts to allow for more surface to "glue" onto the keyboard circuit. In principle this surface does not collide with any part of the circuit. In practice, it does: there are some solder points that are a little bit too high - and these press onto the pads and avoid a good contact with the zebra. I have discovered that last week and filing them away has allowed me to "save" this CX circuit - and probably many others goign forward.
The ones to file are the ones at the right and left of the connectors, and sometimes also the ones between both connector lines.
This may also happen on some cases with non-repaired units when using the zebras over the old support: I see the marks of the solder points on the surface of the film connecting the two parts of the zebra.
I am updating also the book with these comments.