Repair of keyboard HP 55


I own a HP 55 calculator. This calculator works fine except for three keys, marked ., 1 and 2. These keys are functional but missing the click feeling.
Can this be repaired easily and do I need spare parts?


This calculator works fine except for three keys, marked ., 1 and 2.

One EE trainee who worked with us had an HP-49G+ on which these same keys were broken (he'd put pieces of papers around the key slots to hold them in place). Benford's law as observed in calculators? This was first noticed in logarithmic tables (first pages, the ones with numbers beginning with 1 and 2, were much more worn and smudged than later pages).

No keys in my HP-45 have lost that click feeling yet :-) However, on occasions keys '2' and '3' bounce if I don't press them firmly.

Can this be repaired easily and do I need spare parts?

I think can fix it for you, in case you're loath to open it yourself and try to discover what is wrong. Once I was able to fix a similar problem in an HP-15C, but that was a spare unit and I wouldn't mind if anything went wrong.



Edited: 18 Apr 2007, 6:01 p.m.


The loss of tactile feedback in LED classic machines is due to a failure of the key strip. In the photo below, the keyboard assembly is on the left, you can see the small projections where each key is. The flexing strips develop stress fractures at the base of each hinge point, when they fail, they work electrically but you loose the mechanical feedback.

The only source for a replacement strip is from another HP classic machine but it is not an easy task to remove and graft a replacement on to the original as they are spot welded in place. They can be soldered but the results are not always 100%, the best solution is to replace the entire assembly. Unfortunately, the keyboard of the 55 is unique due to the timer/pgrm/run switch.



My HP55 has the same problem with the [1] key. I noticed that there is a small crack at the side of the flex strip, and I hope it does not break definitelly. I use the calculator only when I want to check some particular operation or to test programs, procedures, specifically for it.

Hope you succeed.


Luiz (Brazil)

Edited: 18 Apr 2007, 8:20 p.m.



I have reasonable good experiences with rewelding the strip:
Loosen the strip and soldering it back to the welding points. The trick is in soldering the strip back with the right curvature. If done well, it will be as new, if not....

If the strip is broken however, you will have to salvage one from a other keyboard.

If you need more help, I am located in the Netherlands around Utrecht so we could call if needed.



I want to thank you all for your replies.

I will open the calculator and see what causes the problem. Hope I can fix it. If I succeed I will write an article about it.

Ronald, please click on my name and send me an email, your experience could be very helpfull.

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