A replacement for the HP-41 zebra connector

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Hi everyone,

I've been working on a replacement for the zebra connector between the 41 CPU board and the keyboard PCB. I've had no luck finding a source of replacement elastomer conductive strips, at least not in hobbyist volumes. I've also been unsatisfied with alternatives to the stock connector. After trying a couple of ideas I've hit on something that works for the calculators in my collection. I'm using a flex circuit to bring signals from the keyboard PCB contacts to the CPU PCB contacts.

       

The first picture shows the flex circuit on the trace side next to a pair of zebra connectors for scale. Note the flex contacts are gold plated and the traces between them are covered with a plastic layer to prevent shorts. The second picture shows a pair of rubber cords, held in place with adhesive tape, that serve to press the flex contacts against their PCB contact counterparts when a case is closed and screwed tight.

       

These other two pictures show the flex circuit after it has been folded over to fit the lower posts in the calculator body. The flex can take a crease to keep it folded so long as the crease is not so sharp as to damage traces. The final photo shows the flex in place before placing the CPU board on top. Closing the case puts pressure on the CPU board and presses the flex contacts against the boards, just like the stock zebra connector.

I experimented with a number of material types and shapes before settling on what works very consistently for me. I only have three calculators to work with though, all the same CV model and all produced within a two year span. There could be some difference in models or production method changes that might affect the working of the replacement.

What I'd like to do is get a few people to try the replacement on a calculator that has a missing or flaky zebra connector. The idea would be for me to get some feedback on ease of installation, any fitment problems you might have, and most important any connectivity problems. The calculator you repair should be exercised about once a day to see if any problems pop up. I usually run through all the alpha characters and do a few calculations while holding a calculator in hand on or the desk each day. With a module plugged in, a CAT 2 exercises the I/O bus. That's all I would ask of any volunteer.

Hopefully this sort of "field trial" would prove the viability of this replacement and serve as input for a final design update. I have enough remaining material for eight people. I'd also like to get a idea of the level of interest from everyone in a zebra replacement. More volume obviously would mean lower cost.

PM me if you'd like to give the replacement a try and are willing to provide some feedback on your results. Let me know the model and year of the calculator you would use. I'm quite sure this will prove to be a successful substitute for an aging part that is no longer produced!

~Mark