HP42S



#2

I wondered if anyone has had any experience with the keyboard on an HP42S becoming unreliable. I have a few keys that require multiple presses to activate. Is it possible to disassemble the unit and clean the keyboard or does this indicate another issue. The unit has not been dropped nor had any liquid spilled on it.

{moderator transplant from HP Articles Forum}


#3

The short answer is that it is not practical to open the unit to repair or clean the keyboard. It is held together with at least 43 permanently heat fused plastic rivets. Cleaning can only be done with external methods.

The longer answer is that chances are your 42S keyboard is at end of life. They just don't last forever 'ya know? But honestly, if they are commonly used keys on the numeric pad or the basic four (+-x/), the thick film printed circuit that forms the contacts and the actuator just plain wear out after a bazillion cycles and there is nothing you can do but replace it.

This is especially true if you look in around the keys and can see a teal color. See linked photo
here

for example. The original keyboard circuit sheets used in the USA and early Singapore models used what I think was a silver bearing compound for the domed actuator sheet. Over time, the silver begins to flake off due to stress fractures and sooner or later the conductivity is gone and the key becomes flaky, requiring a harder than normal key press to make contact. The worst case is when the material from the snap dome deposits itself on the two switch contacts of the lower sheet creating a permanent short. When that happens, the unit is dead and will not power up. They changed the materials in later production models to a more carbon based material that is still subject to wear out, but it does not migrate or flake off like the original materials.

I have used a common electronic cleaner that is primarily isohexane to clean particularly dirty keys by squirting it in around the keytop with a needle-point dispenser bottle, it works for some keys, but not all. Denatured alcohol would do about the same, just don't use too much and keep it away from the LCD lens if it is a double window type. Just allow it to puddle around the key, let it wick into the key area, press a few times to drive it out of the dome area and let dry well. You can repeat at least two times. Oops, take the batteries out first before doing this.

If you have access to an ultrasonic cleaner, this may work as well, use at least two passes with distilled water (again, sans batteries) for about fifteen minutes each cycle with clean water each time and let dry for at least a day after shaking all the water out.

Try the solvent based method first, as it seems to work most of the time if the unit is going to heal itself. I save the ultrasonic method for the last ditch try before the junk box.

In my experience, you've got about a fifty-fifty shot to restore normal sensitivity.


#4

thanks randy. i saved that post

#5

Randy: Thanks for the help. It doesn't sound good since it does take more force to activate keys and the keys are commonly used ones. I appreciate the insight, though, and I'll give cleaning a try.


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