HP-32E Advanced Scientific Calculator - my first Spice machine

[jebem]

Display and solder-less sandwiched PCA.
Component and keyboard sides.



The keyboard uses three plastic layers:

- Top layer – Five conductive trace lines with embedded clickable dome contacts and double paths for redundancy.

- Middle layer – Isolates top from bottom conductive trace lines

- Bottom layer – Part of the flex PCB with seven conductive trace lines with fixed base dome contacts and five additional trace lines to interconnect (via solder joints) with the five lines from the top layer.

The flex PCB is folded around a rigid metal/foam frame where one side support the keyboard and the other receives the IC’s and power supply interconnection using a solder-less sandwich approach.
More details here.

I was curious to learn more about this unusual solder-less sandwiched choice.
I found out that the top flex layer is not glued, so it is easy to carefully lift it to see what is underneath and do some maintenance if required.




Here we can see the top flex PCB metallized traces and dome.




A small partial schematics of the keyboard and display, obtained by direct observation.




To analyze the ENTER key issue, I have used a ohmmeter to check continuity of some of the working keys.
A good contact gives zero Ohm.

The inoperative ENTER key was giving infinity resistance.
Initially I was afraid that the trace lines were broken because the ENTER key is not in series with any other key in the top layer flex PCB.

But then HP have designed the circuit with full redundancy paths, so the probable cause would be something else.

And after lifting the top layer flex PCB I saw the cause: the base contact at the bottom flex PCB was heavily oxidized at the point of contact of the upper metal contact embedded in the plastic dome.
After cleaning it the good continuity was restored.



Next steps involves the battery compartment connectors repair, fixing the plastic posts, and then start the reassembling for testing.