HP 41c . don't touch if you don't know


Hello and greetings from Italy! happy to be back on this board!
I opened today a 41c fullnut calculator which I bought used on e-bay quite an year ago; it has a little problem in displayng the second and third characther from the left of the display: any segment not displayed properly, and changing while pressing on it; at the beginning I tryed to forget this problem and delay the fix of it. Yesterday unfortunately I opened the calculator, thinking to find a bad solder joint between the keyboard printed circuit board and the display board or simply with the purpose of trying to clean the contacts (the battery ones were very clean, I wanted to do something. Well, to replace the black paper cover I disconnected the CPU board form the keyboard.............after that the calculator doesn't work at all!! At the beginning I had a lightly bad working calculator and now the display doesn't show anything more
I thought that maybe the CPU board was not touching properly the keyboard printed circuit. there is a connector that never I saw before (two white soft plastic or rubber stripes), I tryed to clean it with alchool (the display at that time was still showing something, when moving the claculator:segments letters and numbers). then I thought to fix it with a small amount of superglue around the bottom screw holes, just to hold the two printed boards in touch, but as I told you The display doesn't still show anything

Anybody who can help me to fix this problem, or, at least to make a diagnostics?
PS Luiz, Alberto have you experience with this kind of calculator?


This is probably due to the CPU board not making good contact with the keyboard PCB board. From what you describe the connector strips are the elastomeric kind, which as far as I know were not originally used in the 41c model, but used in later CV and CX models. There is another type of connector which looks like a whole bunch of fine wires wrapped around a kind of flexible clear small plastic sausage, and both sides are joined together by thin clear plastic with two holes for the screw posts. So I'd day those strips have been replaced.

Does your 41c use two nuts on the screw posts to hold the pressure on the connectors ? It is hard to get the pressure correct with the screws, and sometime it might be worth replacing with a short length of soft plastic tubing.

Are all the screw posts in good condition, both top and bottom ones ? If the top screw posts are cracked or broken, this can affect the pressure required for the connection of the flexible PCB which carries power to the keyboard PCB.

Do you have any photos to share ?



Thank you John, first, for the quick answer!
Actually you got it! I mean of the 4 screws, two are loosing, better is to say that two posts (one on the left top, the other on the left bottom) are not really broken, but the screw doesn't go to the end, it turns without to reach the end. (Excuse for my poor english)

Now I'm at work, in the evening I will take pictures and I'll share them


Hello Aurelio

battery post repair link to HP Museum article

try the above link to read an article on battery post repair. You will see a method to repair stripped posts as well as cracked posts.

Hope it helps,


P.S, if the posts are not cracked then just add the shavings and glue.

Edited: 13 Dec 2010, 11:09 a.m.


Hello john,
here the pictures of my HP 41C


Edited: 13 Dec 2010, 6:44 p.m. after one or more responses were posted


It looks like you have at least two cracked posts from your pictures.

The one on the right:

The bottom two posts keep the pcb to keyboard connector compressed and attached. You can clean the zebra strips with alcohol and a lint free cloth. Also clean the contacts on the keyboard pca side and the logic pca side.

top right:

The top two posts connect the Module and power supply to the keyboard pca. The connection should also be cleaned on the keyboard pca and the logic pca.

Repair the posts as indicated in my previous posting, take a look at the link.

ALL these posts must be intact with threads for the calculator to maintain the contacts.

Cheers, Geoff

Edited: 13 Dec 2010, 6:21 p.m.


Thank-you Geoff for your help
I'll read carefully your istructions to fix the cracked posts,

I must find silicon tubes (for the card reader I used in the past o-rings, but I think that silicon tube is better, so that I have to find it anyway) and I have to find a plastic piece to shave; I have not any old memory module cover, so that I must find something working in place of that. As I can understand the tube clamps the post and shavings flake mixed with glue rebuild it!!!

So the connector is the mitic zebra one??

thanks again

Edited: 13 Dec 2010, 6:59 p.m.


any type of tubing or orings as long as they have a smaller inner diameter then the outer diameter of the post

any hard plastic, scrap from an old calculator or radio or any rigid plastic leftovers.

One of the connectors is the zebra one. The other resides in the module of the loosely attached to the case back. It is the part that contains the battery module contacts and the rom module plugs. You will notice that the contact that meets the keyboard pca is supported by a foam or silicon strip. the silicon strips are good, the foam strip decomposes with time. So fix all the posts and allow to dry at least 24 hours.

Clean the contacts on the module, keyboard pca and the zebra strips. Blow out the dust. Attach both halves of the calculator with the screws, (do not overtighten). If it works don't worry about the foam insert in the module.

let us know how it works and we can take it from there!

Cheers, Geoff


Hello! five minutes ago, I ended the rebuilding phase; now the posts are drying in peace!
Tomorrow evening the response................. (moved on the bottom of the post)

Edited: 17 Dec 2010, 12:32 a.m. after one or more responses were posted


Looks good so far, did you manage to introduced shaved plastic to the posts.

The glue (sparce amount) is to repair the crack and the shavings introduce new plastic into the tube to replace the plastic removed as the screw stripped the threads.

Make sure you let it dry completely as it looks like there is a liberal amount of glue in each post.

When dry. Just barely screw in a screw with the silicon tubing in place. Then, with a soldering iron (takes a few seconds for the soldering iron to warm the screw), warm the screw. Do not heat the screw so it melts the post, but just enough to soften your glue/heat shaving mixture. Then while the screw is hot, and applying slight downward pressure as you turn it. Screw it into the post about half the length of the screw. Let everything cool, then remove the screw. It will now be set for use with the case back.


1 clamp the post with the tubing
2 add glue and shavings.
3 with tubing in place,
a gently warm the screw (not hot).
b place the warm crew tip in the post and screw it half way.
c this has the effect of tapping (adding threads) the post.
4 let cool.
5 remove screw.
6 let assembly sit and cure completely.
7 remove silicon. (if it sticks, use a razor blade to remove it.

It is essential that the post be completely cured before you tighten the screws. To tighten the screws:

1 place the screw in position.
2 reverse the screw direction (unscrew) until you feel the screw
seat in the threads.
3 screw in the tightening direction.
4 screw until it just stops and maybe 1/8th of a turn more.

Cheers, Geoff

Edited: 16 Dec 2010, 6:17 p.m.


... fortunately lots of good advice from the proffesionals here :)


ciao Aurelio

welcome back

If the Alberto you mentioned was me, here I'm.

Personally I don't like too much the 41 serie, even if I own

the C (fullnut), CV (halfnut), CX (halfnut) and Blanknut.

reason is, when it comes to repair, it's always trouble ....

I have been helped from a very skilled italian collector (Ignazio thanks thanks thanks)

and I have did something myself as well.

When I have had the zebra connector problem, this is what I did :

I usually take a lot of pride in restoring in a nice way, but in the case of the zebra connector .... well, this way works 100%, Geoff forgive me for I have done ...

I also have had a battery connector to restore, and this is what I have done, once again is not "cool" but it works :


I think you have had the best suggestion from one of the most skilled in among the forum, Geoff is relly a master for all of us, but in case is should not work out, well, this is a good option too.

If you mess up with the display, well, you should talk to Ignazio.

ciao a tutti, BTW, what part of Italy are you from ? I'm from Milano


Actually, THAT TAKES GUTS as they say in the spagetti westerns!!!!

Haven't had to do that yet but, there is always a first time and if it resurects an HP, do it. But maybe try the easier route first.

Cheers, Geoff


Ciao Alberto, hello Geoff and thank you both for the help!
Yes, Alberto, you are the one I meant in the opening post, I thank you and Luiz again, for the help in fixing a 34C problem. Actually that calculator is stolen in a drawer, waiting for my care but I think to give it soon a new life (!). Actually I prefer older LED calculators to the coconuts, but I love my 41, expecially the 41 (CX) that works very well (halfnut).
My first one was a 25c in 1977, still it works very well altough not properly in mint condition (school time was harder than 1000 surveies!!!) Today in the bottom of my briefcase lies a 49+g one, it's really smart and powerful, but compared with its honorable and older grandmas' it looks like a children toy!

Back to the case:
I'll try first to apply the Geoff method, then I will consider your advice too "iron solder adventure. Actually I'm not a master in soldering art so that I hope to solve fixing the posts. I live in Savona, if you plane a journey to the sea, give me a call!

Today I bought silicon tubing and liquid cement (the last time I used it was in the assemblyng of an aircraft 30 years ago !)
I'm going to start the job!!


Edited: 15 Dec 2010, 4:35 p.m.


Hello! five minutes ago, I ended the rebuilding phase; now the posts are drying in peace!
Tomorrow evening the response.................

Here I post the picture of the work performed, waiting, please, for you opinionions :

In the following picture (only after the camera mgnifying I realized it)you can see that the glue came out of the post. I hope this will not attack the post to the silicon tubing (!)



Good evening (afternoon overseas)!
This first inning was very bad and unsuccesfully, 'cause the rebuild of the two left posts (boottom and top)falided!!
maybe lack of flakes (not enough I mean)maybe I did not press them properly inside the hole, really I don't know, so that I've just tryed the second chance, now drying!!


Make sure you let the post completely dry.

The tubing seals the and clamps the posts but also does not allow the curing to ocurr quickly. That plus the amount of glue you are appear to be using may prolong the curing process. As the exposed surface of the glue dries it seals the undried glue. This extends the curing process.

Give it sufficient time, maybe two days (48hrs). If you are like me I must have it fixed prior to the repair ;-)

So be patient.

This system fails if the glue and shavings have not bonded correctly to the inside of the post.

Cheers, Geoff


This system fails if the glue and shavings have not bonded correctly to the inside of the post.

Tank-you, Geoff, this time I filled the posts with flakes, more carefully
I remember when I was a smoker: with the same care with tobacco into my pipe.........I will be so patient as I was smoking it years ago!!!


unsuccesfull, again!!!Now I can say,at least, that one of the posts is completely cracked!

Edited: 23 Dec 2010, 2:10 p.m.


The problem is most likely the pink elastomeric connectors.

They where replaced with the continuous round gold contacts as they proved to be a common failure point. HP always discarded and replaced with the improved part during service. I've seen many a 41 with them that worked until the logic board was removed and re-inserted.


The newly cracked post looks like the glue is not melting the plastic. The added plastic and the post and the remaining glue all seem to be staying separate.

The glue should be a plastic solvent based glue that welds by dissolving the plastic, not a cyanoacrylate and not a two-part resin and hardener (known as epoxy or perhaps araldite).

However, if you have used a cyanoacrylate glue, the solvent glue will not work very well on the old glue covered pieces and a two-part might be necessary. It is tricky to use as there is not much surface area to glue on the old posts. I have better luck with the slow cure types (hours not minutes) and the more liquid instead of paste or putty types. I usually buy the "J-B Weld" brand. It cures hard enough that it can be drilled, filed, etc.



Hi Sylvan,
I used a "Revell" brand liquid cement, that actually is "a plastic solvent based glue that welds by dissolving the plastic".
I checked on the web and found "J-B Weld" brand products, but I've not seen any liquid cement or plastic model cement, only two components epossydic glue or other kinds for motors or marine pourposes........

Last time I left the post drying at least 72 hrs!

Edited: 26 Dec 2010, 4:25 a.m.

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