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Does anyone know when in 76 HP switched from metal strip keyboard contacts to plastic membrane?

I think it was in 76 sometime. Just curious as it seems like a very high rate of membrane styles I come across FAIL.

membrane style
[Image: Keyboard2.jpg]

metal strip style
[Image: Keyboard3.jpg]
(01-19-2015 03:30 AM)aj04062 Wrote: [ -> ]Does anyone know when in 76 HP switched from metal strip keyboard contacts to plastic membrane?

I cannot answer your question. But I have some other keyboards with different dates printed on it. Perhaps this can give you some information. Others don't show any date.

And I know that the keyboard rows and columns were routed differently on the main board to the ACT processor pins in HP-21 and HP-25, so they behave different, HP-22/HP-25C has the same layout as HP-25. The membrane style keyboard in the third picture I found in two of my HP-29Cs.

I have even more (different date) keyboards, but couldn't add all images as attachment.

That's interesting. I wouldn't have thought they had different routings between models!
I just replaced a faulty 29C board with one from a 21. Seems to work fine.
Just happened to my HP-25 that key "5" didn't respond anymore. I disassembled it and was confronted with the fact that you cannot remove the keyboard from the contact layer without destroying the plastic posts. I was lucky to find a metal-strip contact layer so I could just insert a tiny wire in the hole underneath the faulty key and clean the contact area. Now it works perfect again.

How is it with the membrane-style keyboards if you happen to have non-responding keys? Any chance to clean them?

I don't know. That is part of my dilemma with these. I have been able to get some working a bit better by wicking alcohol in. However, if the membrane is failing, there is no cure.
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