HP-41CX lock-ups


I have recently acquired a HP-41CX in mint condition. It was produced in 1988 and it really appears to never have been used before (it was sold as Mint in Box).

I have been experiencing several (about 5 times) lock-ups during the short time (4 days) I've used it. The calculator cannot be turned off anymore, or it shows strange characters on the display, or it just displays "MEMORY LOST" when turned on.

Is this normal for a HP-41? Could this be related to just pressing a few keys to fast in succession? At least on one occasion it happened right after pressing 2 keys very rapidly when entering a program.

Axel Poqué


Have you already tried fresh batteries ?


Yes, I had to, since they were the only thing not included.

I would have opened the calc already, to see if maybe the srew posts were damaged, but I do not want to risk damaging it, given its good condition. There are no loose parts inside, and it feels stable in my opinion.



Is it half-nut or full-nut? Half-nuts have a rounded border in the display. My full-nut CV did that, abd it turned out to have cracked lower screw posts--which makes the compression contact between the brains and everything else.

I hope someone else with more experience has better news. But even with cracked posts, you can fix it.




It's a half-nut, produced 1988.

Is there a possibility to find out IF the screw posts are cracked (maybe transportation damage?) WITHOUT opening it? (I really don't want to damage it, since it is really mint!) How stable is the case if the calc is still ok - is it possible to bend it slightly, or does it creak when you try it? (I can't try at the moment since I'm at work and the calc is at home)


Both my HP 41CVs behave strange when I haven't had batteries in them for a long time (more than 2 month).

One of them has a dimed display and shuts down after turning it on and off again a few times. It takes a week or so with batteries in it untill it operates propperly.

The otherone sometimes does not operate at all after puting batteries in. I then have to take them out again and give it another try after a week .

I got now idea what causes these strange behaviors.



Since the calc probably hasn't seen a battery for about 15 years, this could be the reason (although I guess that it has been tested by the seller). So you think the problem should go away over time?

Many thanks,


Well, I am not sure it will go away, as it sounds a bit more serious than the problems I have.
But I would wait a while before opening it, especially when you are worried about damaging something.
Just leave the batteries in it for a week. It's worth a try in my opinion.
In case it locks up, you could also discharge the internal backup capacitor by connecting the batterie-contacts with alumin(i)umfoil for a day or so.


and good luck!


Ok, I will do so. Thanks a lot!



Yes, next step after fresh batteries is bad contact. You can easily check this out in trying to turn it on while pushing with one hand the edges of the calculator close to the 1st row keys (LN, LOG, ...). If behaviour changes and improves while your're pressing then basd contact has been recognised.

If you hear something inside, it might be the fixing posts that are broken.

You will have to open it, but don't worry it's very easy.
1) take off the 4 rubber feet. Put them in a smaal plastic box so that no dust come on the glue.
2) unscrew the 4 screws. Make sure you recognize the short and long screws.
3) check if the posts are cracked. If yes try to recover all plastics parts
4) the calc should split into 2 parts with a center plastic part. The battery contacts and port contacts shold easily come out.
5) with a rubber, clean all contacts.
6) take a pin and put it under the flexible contacts. There is a kind of white foam that you could strecth by 1/2 - 1 mm. Make sure you dont scratch the flexible contacts.
7) glue up all the fixing posts with dedicated plastic glue (some people in the articles of this site gave exact name of components and brand). I use 2 component plastic glue that can be found il any DIY shop. Let it take the time to solodify correctly.
8) blow up all dust/dirtiness but make sure not to scratch anything.
9) mount your flexible back, make sure they are in good contact evrywhere with the PCB and the battery contacts
10) screw your calc, using the correct screws. you could put some very light grease on the screws as you will have to "dril" your fixing posts. PLEASE DO THIS GENTLY, AS CRACKING THE FIXING POSTS IS THE MAST THING YOU WANT : at the end, each 1/4 turn put the batteries on and turn your calc on : stop screwing when it turns on correctly


I think I will do this as a last resort, if the problem doesn't fix itself in time, as Harry suggested. I'll check for contact problems when I get home today.



My guess is the internal contacts need to be cleaned with 91% or stronger isopropyl alcohol. Halfnut machines only have the battery contact assembly. Fullnuts have a zebra strip connector under the CPU board. Both can get oxidized after a few years.

One way to check for cracked case problems is to look very carefully at the side seams of the case. Gently squeeze the case halves together near the rubber feet. There should not be any movement (or mayby a few 1/1000 of an inch).

Also remove the port covers from the top of the machine and GENTLY pull on the case halves. Again there should be no movement. Dont pull too hard or you may cause the problem you are attempting to find. If the case halves come apart any then one or both of the top screw bosses has broken.


Hello Axel
Incorrect voltage supply sometime bring this behavior.
What kind of battery do you use ?
One day, by mistake, I supplied my HP41cx with 9 volts. The display began to show strange characters. I disconected the supply immediately, but after to have put correct voltage, the calculator didn't want to switch on again. I was very angry again myself and believed to have killed it. One month later, I try again without much hope with a soft 4.8 V. Miracle, the calculator work again !
So good luck with yours, and please tell to the forum if you succeed to fix the problem.
All the best


Just standard 1.5 V type N/LR-1 alkaline cells, so that should be not the problem.

I just discovered that I can provoke the behaviour by pressing the case near the top row, so it seems an internal contact problem (as suggested by Thibaut.be and Dave Smith).
Now I only have to convinve myself to open the calculator...


Gop ahead, open it. It's very easy, if you're a little careful you won't do any harm.


BTW one can't sell a 41CX in mint condition with lose screw posts inside... You might want to try a replacement...

On the other hand once you've repaired it it will be quite invisible.



did you buy one of these NIB CX's either at ClassicCalculators.com , or alternatively, from december64 off eBay?

If yes, you could try to get a replacement from the seller.




Yep, bought it NIB from december64 on eBay.

It is not that there is any visible or otherwise noticable damage to the calculator, nothing loose inside, etc - it just exhibits random lockups a bit to often. If you think I could get a replacement for this...



Well, try it... they still have some as far as I know, and cracked screw posts is quite strange for a new calculator.


Well, John Innes (december64) has kindly agreed to send a replacement. He even offered to send me a replacement if I tried repairing it first and couldn't get it working. I declined since I didn't want to (possibly) ruin an otherwise excellent calculator. (Thanks a lot, John, I hope you don't mind me posting this here, I just wanted to give an update and praise the way you do your business.)

Axel Poqué

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