Rehabilitating mushy keys

[David Griffith]

According to Randy of fixthatcalc.com, mushy keys on an HP calculator are often caused by exposure to high heat for an extended period of time. What happens is that the plastic flexing part gets weak. That's why he says it's generally not possible to fix mushy keys. I don't know exactly what the keys look like underneath, but here's how I fixed mushy keys on something else, I forget what. The keyboard had tilting keys much like an HP calculator. I applied a thin layer of epoxy resin to the bar of plastic leading from the frame that held the buttons together to the button itself. This stiffened the bar enough such that it would spring back when pressed. The result was a complete rehabilitation and the keys were snappy again. Are HP calculator keys constructed such that this trick would work?

[David Ramsey]

I don't know what he means by "the plastic flexing part". Classic HP calculators have individual keys held in place by a plastic frame; the "key click" action is provided by arced metal "bridges" under each key position. It's hard to describe them but a photo will help (image found via Google Images):

[David Griffith]

I see now that my idea would not work on HP calculators and my idea of why keys go mushy. So, just what is it that happens when a key goes mushy?

[David Ramsey]

Quote:

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I see now that my idea would not work on HP calculators and my idea of why keys go mushy. So, just what is it that happens when a key goes mushy?

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Two possibilities:

1. The metal arcs that provide the key snap feel have fatigued or cracked. The only repair possible is to replace the keyboard assembly.

2. Depending on the calculator, screws holding the keyboard assembly together have loosened, or in the case of heat-staked keyboards, the heat stakes have started to fail, although this is rare.

What calculator do you have with a mushy keyboard?

[David Griffith]

I have two 32siis with mushy 8 keys. The rest of the keys are fine though one of them has keys distinctly less responsive than the other. I understand that some units were like this brand new.