how do i open hp 21?


i dropped some water on the right keys, used a dryer, haven't tested it yet. how do i open it? is there some rubber under the keys preventing liquid to run down to the pcb (electronic green board)? thanks


shaked so a little bit water came out and used a hairdryer, haven't tested it yet. what's the best thing to do? it's my dads. thanks


Just make sure you removed the batteries and let it dry a couple of days in a warm place. If it is only water it should work fine when you put the batteries back




> what's the best thing to do?

Whatever you do, do it quickly! Otherwise your Hp-21 may end up like this one (S/N 1509A00321 ) that I bought recently:

From the outside it looked quite nice, the keyboard felt strangely stiff though, But inside it is completely corroded, probably due to battery leakge + water - or maybe even salt water.

Cheers, Max

(Password hp21)


Hi, Matthijs;

Arnaud's suggestion is a very good one, leave it without the batteries for a while till it dries out. You can also clean the contacts to avoid residual after the water evaporates by using destilled water, so you might need to open it, too. The image below is the inside view of a very similar case, the HP25 case:

This is the front view. As you can see the LED display on the top of the image, there are two holes right below it, close to the battery compartment. These are the holes with screws, and if you look at the back of the calcualtors, the two top rubber feet hide them. You should remove the rubber feet and unscrew them. These are the only screws keeping the half cases together.

So, remove the batteris, remove the rubber feet, remove the screws and gently separate the two halves, keyboard and back case, top part first. They separate slightly, then you gently slide the keyboard till it can be detached form the back case. After that, you'll see that the keyboard is connected to the mainboard through many connectors, as well as the display. You can separate each one and clean the mainboard plus the keyboard separately. DON'T TRY TO CLEAN THE LED BAR! It is easily dammaged and there is no replacement.

If you believe you cannot suuceed, ask for help. The HP21 is a rare calculator and it would be sad loosing it...


Luiz (Brazil)


Which HP21? Assuming you mean the Woodstock one, then :

Remove the battery pack

Pull out the 2 rubber feet at the display end of the machine and undo the screws under them

Separate the case at the display end (e.g. by pressing on the charger connector pins) until said pins are free from the case.

Slide the bottom (white) part of the case towards the display end relative to the keyboard section, so that the clip at the front (0-key) end comes free

Remove the bottom case

Unplug the logic board from the keyboard

Unplug the display from the logic board


what i did: i used the hairdryer about 10 times in total each about 45 seconds. and let it dry for about 14/15 hours (the night), i hope that's enough. i tested it and it works (all keys, tested mainly the following keys: 2,3,5,6,.,DSP because they could have been affected by the water drops), or can i somehow short circuit without knowing it?

i don't know what will happen after a long time so if you could post a image of the backside of the front maybe i can judge the risk of corrosion.

i don't know how it looks like, or is there a pcb/green board directly at the backside with no protection/rubber between it?

and is there a way to remove the rubber feet gently?

what about distilled water? should i put that affected part in a bath of distilled water and let it dry? or use a cloth (which may have ions/electrons like non distilled water).


There is no way for me to post a picture. I don't have a digital camera (and won't buy one until the results are better than those from my existing medium/large format film cameras!). I also don't have a scanner....

However, if you look at the HP21 page on this site, then click on 'technology and packaging' there are several links giving pictures of dismantled Woodstocks. The HP27 one shows the back of the keyboard assembly. While the logic PCB layout and chips are different in the different machines (and the 27 is the oddest of the lot, being NMOS rather than PMOS), the physical construction and keyboard are the same in all machines.

As regards removing the feet, they're pushed in, not glued. I normally just flick them out with a screwdriver.

I _would_ take the machine apart (don't forget it's static-sensitive) and make sure there's no evidence of corrosion.


Hi Mathijs,

From your name I assume you are Dutch?

I personally would put the keyboard in an ultrsonic cleaner and after that blow it dry with some airpressure. In that case all water or dust whatever will be away.

The rubber feets you can lift out easily bij pressing a needle under a slope in them and than lift them out.

Let me know if you need local help.


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