HP41c Zebra - last tests

[Jose Gonzalez Divasson]

Hi,

We have been using elastomeric zebras for the connection between keyboard circuit and processor of the HP41c. These worked more or less well - until some more current was required (updating a CL, using a card reader). The higher resistance (a factor of 20 more than the original part 1251-5731) was the cause.

Following the work of Mark Fleming, we designed a smaller part that so far works very well. We are now trying to find what is the best fill for the connector tubes, to provide support with flexibility, but good connection is achieved even with nothing inside: the nice "MEMORY LOST" message appears immediately. You can see the pictures in the attached files.

   

As soon as we have settled on the fill, we'll publish it!

   
   

[twoweims]

Thank you! I can’t wait to buy some.
A new 41CL board has just arrived in the mail.

[mfleming]

(05-14-2020 06:36 PM)Jose Gonzalez Divasson Wrote:  As soon as we have settled on the fill, we'll publish it!

Nicely done supporting structure! A suggestion for fill would be round medium-hard Buna-N cord. It was the best Goldilocks solution among candidates of different shape, material, and compressibility.

[Jose Gonzalez Divasson]

Thanks for the tip!! I was trying silicone tubes so far, but yours is more elegant.

2mm is slightly on the high side, since the circuit is 0.13 mm x 2 = 0.26 and then we get 2.26 mm. Original is 2.1mm, "giving" down to 1.7mm when pressed

I'll try with 1.78mm found in RS Spain

[mfleming]

A hard cord material would not always conform to an irregular pcb surface, or if the two pcb boards were not exactly parallel. The result would often be intermittent or lost connection for some contacts. A soft material worked very well, but would not spring back after a long period in compression. If you opened and reassembled the calculator a second time, it wouldn't work! Medium-hard rated cord seemed to be "just right."

Most interested to see the final outcome!

[Jose Gonzalez Divasson]

Buna-N cord it is. Perfect connection. I have just assembled two calculators (one of them a CL) and it has worked well from the beginning. Less trial than the elastomeric zebras, or the wire-D gold units: it just works. It has the advantage of being a single piece and not needing additional support.

   

   

   

[mfleming]

Looks like what HP would have settled on if they had continued to make the 41. Congratulations!

[jjohnson873]

Is this new zebra board available yet? If so, is there a price?

[twoweims]

(05-19-2020 01:58 AM)jjohnson873 Wrote:  Is this new zebra board available yet? If so, is there a price?

https://www.thecalculatorstore.com/epage...a%20PCB%22

I just ordered some.