A pair of 21S with sticky keys


I have been given a pair of HP21S calculators--algebraic Pioneers with a lot of statistical functions. The problem is that both have had something sticky spilled onto and into the keyboards--probaby sugared coffee or tea, or soda pop.

One of them works, mostly, but many the keys are sticky and a couple are intermittent. The other one is like the first, only worse.

As they are not RPN, I'm not really interested in them for myself, but hey, they're HPs. I know the Pioneers are difficult and risky to open, and I really don't want to get into that myself. I'm wondering if there's a way to flush them out and dry them without opening. If I can get them to work, I'm sure they'll make someone happy. If not, no great loss, and they could be used for parts.

Can anyone point me to online instructions for dealing with this kind of issue? Thanks!



Hi Peter,

I guess that if you don't really want to open up the Pioneers, then the only thing remaining is trying to clean them "from the outside".

My somewhat narrow experience in the matter dates from a few months ago. I had read that besides the usual boil-in-this boil-in-that directions, you could also clean up using an ultrasonic cleaner. One guy in particular was reporting that he always cleaned mobile phones (as a business reselling them) that way.

Having acquired an HP-48SX whose keyboard was constantly playing up, I thought I had nothing to lose; I bought an ultrasonic cleaner with a large enough tank to fully immerse the calculator, and put it through a few cycles with warm water and the built-in heater full on.

I actually overdid it a bit, because the paintwork on the bezel became dull, and flaked off where it had been impacted (the impacts sites were not visible beforehand) - BUT the keyboard worked again! :-)

Later on I tried the same on an HP-28S which was unbelievably dirty, grimmy, and seemingly melted in places, but working. I was less successful: I managed to clean it in a major way, but I've lost 6 pixel columns on the display.

So, a 50% success rate. If you've got nothing to lose, then I would suggest that you clean up the calculators as much as you can first by hand, and then cycle it a few times in an ultrasonic cleaner (assuming you can get access to one).

Take the batteries off first, obviously, and short the battery contacts briefly (or leave the calculator untouched for a while) to discharge any residual energy (the one used for preserving the memory while the batteries are off). After cleaning, leave somewhere warm for a while (like an airing cupboard, e.g. 50C/125F), a day or two, so that all moisture will evaporate.

The problem with sugary stickyness is that an ultrasonic cleaner (which dislodges dirt particles by cavitation) may not be able to shift it right away. The sticky stuff would need to disolve in the water first, so you may need to "soak".

This posting is longer than I thought - sorry. I hope it helps, or gives you ideas.




I used an ultra sonic cleaner myself but kept the calc's in such a way tha he display was never submerged. Also keep temp. below 25 degrees C and use only half the amount of advised soft cleaner addition. Try it for 2 minutes and rinse. If it is a sugar drink or thee wih sugar it should be clean. If not then I think the ultrasonic is not the right tool.

Carefully will do it 9 out 10 cases.

Good luck



I have cleaned a few HP calculator keypads with the following procedure. It has worked every time but once, when the contamination was not water soluble. It has never ruined a calculator.

Equipment and supplies:

Distilled water

Eye-dropper (or something else that can apply water in a precise location)

Wet/dry vacuum (the more airflow the better, and non scratching [plastic] crevice tool)


Remove batteries and discharge capacitor.

For each sticky key; place one drop of water down around the key with the eye dropper. As soon as the drop seeps around the key, turn the calculator keys-down, and work the key for about 10 presses. Use the wet/dry vacuum for about 10 seconds to suck all possible water from around the key (still hold the calc keys-down).

Test the key. If any stickiness remains, repeat.

If no stickeness remains, do it one more time for good measure.

Go to next key and repeat (If there are several keys, I will do a row at a time)

When done with all keys, use the wet/dry vac to remove all possible moisture for a minute or two around all the keys. I still leave the calc keys-down during this step.

Place in a low-humidity environment with airflow for a day (when my heater or A/C is on, I'll put it near the vent - either heating or air conditioning the air drys it out.)

Try out all the keys. If the contaminent is water soluble, I've never had to repeat the process, but it might be possible if you did not get it all out the first time.



Wow! Just way too complicated here for my way of thinking. This is not rocket science... it is just a dirty calculator. Having cleaned more than my fair share of HP's, I'll add my thoughts and experiences with a few dirty HP's over the years:

While you could use an ultrasonic cleaner, it is overkill for basic cleaning and might well do more damage to the bezel than is necessary than you need to get it functional.

For gooey keys, the toughest thing is to know what you're dealing with. Chances are it is sugar based which is easy... the tough part is being patient while you wait for it to dry... Coffee, tea, soda, all no problem since it is water soluble.

Start by getting the batteries out of the unit and *do not* let them near it for at least two days after the point were you think it might be dry. Seriously, I'm not kidding. Two days.

Once the batteries are out, just submerge it in some warm (100°f) distilled water. Let it soak for 2-3 minutes and then pull it out and press all the keys several times. Submerge again and repeat. Change the water and repeat. Once you have done that, shake well, shake again, dry the surface and place in a warm area to dry for, well, at least two days.

About a day into the process, give the keyboard a try. If it is still tacky, abandon all hope, it can't be helped this way... if it feels okay, return it to its warm place and let it really, really dry. After two days, put the batteries back in, replace the battery door and go back to work 'cause you're done.

Honestly, if it doesn't clean up with water, it's a lost cause. I've seen units sprayed with WD40 and other such nonsense... those make pretty paper weights...


Immersing the entire unit in water might be OK for the more modern LCD calculators, but I recently ruined an LED calculator following exactly the procedure that you describe above. I think the problem has to do with removing the water by shaking. Very fine (almost invisible) wires are generally used on the old LED displays to connect the diodes to the display driver circuitry. When shaken, the water can flow fast enough to drag and bend these wires such that they contact (and remain in contact) with other wires or objects. The result is a largely non-functional LED display.



I don't know if this is applicable, but I cleaned a Spice series keyboard by washing in alcohol, and moisture was retained between the two film layers that constitute the switch circuits. Attempts to removed the moisture by shaking were unsuccessful. I found that placing in an oven at 150 degrees F for about 3 hours cured the problem without damaging the materials, but this may be too hot for an LCD.


I don't know if this is applicable, but I cleaned a Spice series keyboard by washing in alcohol, and moisture was retained between the two film layers that constitute the switch circuits

Pioneer and 48 keyboards are similar to the layers of a Spice. That is why I recommend the long dry times - as it really does take that long for even a small amount of moisture to evaporate once it is inside the layers. If you put the batteries back into a wet Pioneer or 48, you risk damage to the keyboard by way of electrolysis. That's the real enemy, not the moisture itself.

FWIW, the vast majority of the original Spice keyboard failures are caused by fractures in the metal strips that run through the top snap dome sheet. HP knew they failed way too soon which is why they redesigned the keyboard using same keyboard technology used in the HP41. That keyboard pc board also eliminated the awful pressure fit construction as well - leaving users with a good unit with its only weaknesses being the battery contacts and unsealed LED array.


Immersing the entire unit in water might be OK for the more modern LCD calculators

Yes, never had a problem doing it... otherwise I would not recommend it. :)

When shaken, the water can flow fast enough to drag and bend these wires such that they contact (and remain in contact) with other wires or objects.

I seriously doubt it, more like the wire bond in question was already compromised. This is very common with older units that have been subjected to battery outgassing, especially HP Spice machines since nothing in these old chip on board parts is sealed.

Mechanical shock will cause the ball to un-bond, it usually happens on the LED die side. Just dropping a calculator is usually more than enough shock to find the weaknesses :(

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