HP25 keypad problems

[GreyUser]

(06-24-2020 06:18 AM)calc-calcs Wrote:  Thank you everyone for the very good information. Will reopen the calculator sometime this week as time permits and hope my keypad has the small holes behind the keys to apply deoxit red. Fortunately I currently have a small tube of it as do I use it with some frequency on switch contacts.

Will report back with hopefully positive results. Thanks again.

The chances of having a sealed keyboard is extremely low. They were used for a short period of time on new 20 series units, late in the Woodstock’s life cycle and AFAIK, it was only on the 29C.

I don’t know if they were ever used in the repair department. I suspect so but the number of units out there in the wild was probably very small. My own 25C went back to Corvallis more than once, getting a new keyboard in about 1983 and it was the original design with metal strips.

The sealed Woodstock keyboard was based on the new design for the 30 series units that did away with metal strips, replacing them with conductive plastic domes. In a rush to get to that new product to the market quickly, they did not wait for the usual accelerated life cycle testing results. With product having been shipping for some time, the results of those tests came in and HP knew they had a problem. The keyboards were failing much sooner and in greater numbers than was expected. This was due to the design having one half of the connections passing through the domes in the form of a flexible, conductive trace. These traces would crack with the flexing of normal use and break the connections to the keyboard scanner.

The result was a redesign for the Spice series with the round metal domes that were the basis of the HP41 keyboard. I don’t the history as to which was first, was the 41 being prototyped and the round domes were already in use or did the redesign result get recycled for use in the 41.

I think there is an article in archives somewhere from an HP employee that related the story of the keyboard design failure in their own words. Hopefully someone will remember it and can provide a link, it was an interesting read.