Calculator blog

Musings and comments about our common interest


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Continuation on new repair aid piece

I have now an old HP41c where the upper circuit is pressed down to the zebra connectors via nuts. Now, it is difficult to ensure an even pressure and I am having problems with some of the digits. But if we force the nuts, I may break the post - as it has happened in other cases. These are the situations our new piece (wait until we can put it on sale) is made for.

This piece is intended to be put on the lower posts (where it fits tight to aid in case of cracked posts) and gives the necessary mm. of thickness so that the back side of the original type can pass the pressure on the circuit so that it stands firm on the zebra connectors. This negates the use of nuts, which can be changed only a couple of times, lest they break the thread of the post (or the post directly, as it has happened to me a couple of times). I am testing different thicknesses to find which will be the final product.

Say goodbye to the nuts!

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A new product for HP41c repair

We have now prepared a couple of new products designed to solve more HP41CL repair problems. We will discuss today the first one.

Our bent to do c to CL conversions has led us to specialize on full nut repair, instead of half nuts. But some of the tools and methods are also valuable for half nuts too!

The first of the new items we have created is the adaptor to replace screwed nuts in the original HP41C. You may have seen one of this: the processor circuit is secured to the main board with metal nuts around the lower screw posts. The problem with these: it is very easy to destroy the thread, since he read was formed by the action of the nut on the first attachment, nearly 40 years ago!

In the later units, the processor was secured in place by the pressure of the back side through two cylinders collated to the case. If you have one of the old cases, and you have broken the nut thread, you need to find a solution to push the circuit down against the zebra connectors. This is such solution. 

We have created a nylon piece that fits tightly around the lower screw posts, with a small bezel to ease the port entrance, and occasionally making a broken post able again. This piece translates the pressure from the back side of the calculator down to the circuit. Don't need to find the right height, or to adjust the angle: it will work - every time.

The piece has sides! As the back side has an angle compared with the circuit surface, we needed to create this angle to the piece. Therefore it can only be put one way.

As the color here really doesn't matter, we have created the price in the cheapest, clearest color from 3D printing: white.

I hope that is piece helps some of you fixing your beloved calculator!

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Calculator repair - spare parts


I have been spending some time in trying to learn 3D printing creating software, in order to create calculator pieces. In Shapeways, there are already several parts for the HP41c and other HP calculators. Look for designer davef, who has designed several parts for our calculator: back port doors, side battery doors, battery covers and other HP71b items. However, these never have exactly the look and feel of the real thing - they could even be considered nicer, but not the real thing.

But there is no need for nice finish for internal parts! So I am looking forward to create a piece that can be used in the innards of the calculators. You need to destroy - or abandon repair - a calculator to repair another, and that’s a quite expensive way of doing it! The price in Europe for defective HP41c is around 75-85 EUR. So any effort to create parts that help keeping in shape our best tools is welcomed - but it sure takes time for someone not used to it!

By the way, this is the Shapeways' HP spare parts site:

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HP41c types of repair


We usually get questions about repairing defective HP41c. It is usually Ignacio who repairs them, since he is much more knowledgeable than any other I know; but there are many repairs that can be performed by anyone of you - if you just dared to open the calculator. Let’s discuss the different cases, and tell what is difficult to repair from what is easy.

Rust in the battery contacts. This is one of the repairs that I would consider easy, and that can be done by any of you. Please refer to this product in the website​

Calculator would not start, but the battery contacts are OK. First check battery polarity. Second, is your calculator a full nut (square screen corners) or half nut (round screen corners). If full nut (meaning it has two circuits, pressed one agains the other by the two lower screws that are hidden by the rubber pieces. This is by far the most frequent problem, and it is called “broken lower posts”. These can be easily repaired by yourself (unless someone else tried before and destroyed the assembly). This is many times one of the most satisfactory repair: a little glue, a little time waiting for it to cure, assemble it back, and voilà - a previously lost calculator is back at work

Screen problems. It can be easy or difficult. If there is a black part of the screen, consider it difficult - you’ll have to get a donor for the screen, and the soldering and desoldering involved is not for the faint-hearted. If there is a blurred set of segments, it may suffice with cleaning the PCB and making sure that the circuit connects well to the main board - that may solve the issue.

Keyboard issues. Can be easy or difficult, but it is worth trying it yourself to check. There are different kind of issues: bad contacts and bad clicking action of the keys (some times can be cured with cleaning -easy- but other times the key dome is broken -difficult and a donor is required). In general, opening, cleaning and adjusting it is good practice and doesn’t damage anything.

Finally, if the repair didn’t work, or you feel you can’t do it yourself, you can always contact Ignacio through us:

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Yet another method for Pi - this time from the outside!

inscribed-circle-recurrance-1This week's challenge is about doing exactly what we did last week - but from the outside of the circumference. It is not as easy as it looks at first sight - and I am not alone in thinking that the inner polygon approach is easier than the outer approach. Let's first lay down the equations after seeing the enclosed drawing. From there, you need to draw the iterative formula, and then program it on the HP calculator of your choice.

(2a)    file:///media/disk/home/slawrence/website/physics_insights/physics/formulas/eqe_temp_image_qIaqRm.png

(2b)    file:///media/disk/home/slawrence/website/physics_insights/physics/formulas/eqe_temp_image_Fb0pVr.png

There you are! Let's try!




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The HP41c repair kit is back!

This is just to inform that we have received a new lot of HP41c battery bay repair kits!

The production lots take some time to produce, but still it is a magnificent product that allows your loved calculator to have a new lease of life. It also can give you the feeling of achievement when you are able to revive a 30-year old workhorse instead of disposing of it and buying one new - as it is the case with the rest of the electronics nowadays.



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HP 41c battery bay repair kit

HP 41C repair kit installed on battery connectorThe current batch of HP 41c battery connector repair kit are leaving us much faster than the first time. There was a comment in the web, whereby the user found that it was quite difficult to find the product in the search engines. I will now put it as first in the accessories list, so that it is easier to find it in the web; and also will announce it in google adwords.

If you have not seen it, I will make a brief description of it. If you have seen HP 41c/cv/cx in internet, you will find that a good number of them have rusted connectors. In many cases, this rusting impedes connection and therefore the calculator does not function. Many otherwise good HP 41c are not used and sold at discount prices, while the rest of the circuit does work (there are very few calculators that have a real circuit failure - it is either the connectors or the screw posts that are broken and do not provide enough pressure)

Diego Diaz designed a printed circuit in a foldable, adhesive material, that can be used to replace the original, rusted one. He has also prepared a short instruction sheet that make clear how to do it. So far, all users that have bought it from us have succeeded. The HP 41c repair kit is for everyone!

Be sure to download the instruction sheet that you can find in our website to make sure you don’t spoil your repair kit circuit.

You can see in the pictures that the result is impressive. However, I think that it is important to follow the instruction to “cook” it at 90º, so that the bends in the flexible circuit remain.

It is a pity that so many calculators are thrown out and discarded, when a fast, clean and elegant solution is available.

Should you think that it is still too much for you, you can try contacting us to do the repair for you at, if you are in Europe, or to if you are located in the US. They have fixed rates for repair, while we do prefer to see the calculator and then quote (in many cases a few pictures will suffice)

The HP 41c is the most successful calculator in HP’s history. While HP has never revealed the total number of 41s produced, it seems to be week over the million. Due to their robustness, many of the are still in use. And due to its easy programmability, they can still be used successfully for many applications where a graphic calculator is not required.

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HP 41C repair kit

It is curious how things change over time. In the past we have sold very few HP 41c battery bay repair kits. However, we have seen recently a mounting interest on these. So much so, that we ordered another small batch. Well, it is gone already, and therefore we are ordering another 15 units.

While clearly not a best seller, the HP41c repair kit shows that the HP41c is far from dead. There have been three lots of the mighty HP41CL, designed by Monte Dalrymple, each with more modules built in, and all three lots have been exhausted. Now, another lot, the 4th, has been produced and some units are coming our way. But many of the HP41c that we have seen recently, while magnificent from the keyboard and scree point of view, have that little rust in the battery bay that renders them uncollectible, and unmodifiable for HP41CL use.

Our principle so far has been to convert only original units into HP 41cl; however, when i run out of pristine units, I used the Diego-designed flex circuit for my own HP 41CL. This unit is based on a otherwise perfect HP41C with tall keys. It has the best keyboard I have ever experienced in a calculator, and I spent a lot of time tweaking the bolt posts to make sure the tension was enough to produce a good electrical contact between the board and the processor, while at the same time the fantastic keyboard feel was not compromised on the two lower rows due to said pressure. It would have been a pity to have to throw it away for lack of a solution to its badly rusted battery bay.

I think that all of you know already this little repair kit. It consists of a flexible circuit board, that has a strong adhesive on the back side. When properly folded and applied to the plastic that forms the battery bay of the old HP41c, it completely and neatly substitutes the circuit that came with it. This circuit had the tendancy to rust, typically due to acid leakage from batteries left for a long time within the calculator in humid areas. This has caused many a calculator to be discarded as dead (the other main cause being the lower screw posts being broken in full-nut models, resulting in an apparently non-working calculator. A firm pressure on the lower part of the keyboard often results on the display coming back to life. Many a calculator has revived in my hand this way, followed by a visit to heart surgery with 95% chances of total recovery)

It would be false to tell you that installing this circuit sheet is easy. You need to be patient and follow the instructions to the letter. It is key to pre-fold it with bends as sharp as feasible - with care for not breaking any path. 


The second important point is to make sure clean well the original plastic base from all prevuous debris - in particular of the rest of the original rivets that kept the original circuit collated.


The third key point is to start collating the circuit from the middle part between the module contacts. This is the most fdifficult part to do well, so why not to start there.


But don't worry: if I have been able to do it, surely you can. I don't know any person less gifted for the task, and I succeeded - first time. But you need to be patient. 


I had several units in my hand before commencing. If I were you, I would buy two units instead of one. It would be a poity if you manage to break one, and then you need to wait for a week for the second attempt!

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New products

We have put in our store 5 new products, that will be the collectors' delight:

- two units of HP 48sx.
- two units of HP 41cv
- one boxed unit of HP 28s

All of them fully functional, all of them in very good shape. I got an additional HP 41c, but, as it is a fullnut unit, I am planning to keep it in order to turn it into a CL version (the fantastic Monte’s processor board, with all the known pacs in place and full peripheral compatibility.)

I have already stated that the HP 48sx is probably the most beautiful calculator ever done. Other comments to this would be welcomed, but at least it deserves to be in the top three.


What to say about the HP 41c family? If you want to connect your calculator to equipment, this is still your only way. Agreed that the HP-IL units are few and far between; agreed that there are faster interfaces; agreed that it does not make sense to use a one-line calculator when there are 8 lines graphical calculators; but still, this morning I was at work with a 41c and the pleasure of the keyboard, the ease of assigning functions to keys, the sharpness of the screen, and the whole beauty of the device, made me feel in heaven.


And then, there is the calculator that heralded the new age: The HP28s. It has collector interest, and its 4-line screen was an impressive improvement.


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PPC ROM Module

I have just received a copy of the PPC ROM and Manual for the HP 41c/cv/cx

While all ROMs enhance the function set of the HP41c series, there are several outstanding. So far, my preferred by far was the advantage PAC. It moved the HP41c to HP15c territory - but not quite. 


The PPC ROM is an unassuming piece of hardware - identical to any other ROM. However, your opinion changes when you see the manual accompanying it. It has close to 500 pages (492 in my edition), and it is packed with all kind of functions. I have yet to go through it, but it looks as an early example of collaborative development -the sort that is giving us the likes of the wp34s and other open source software - but 30 years ago!

I am so excited with what I see that I would jump to conclusions too quickly. I have to go though the introduction pages (where I see names that appear very often in the HP museum forum) and get submerged in this great work.

As I am now travelling I can only read it - but this week end I will play with the hp41c, the ROM and the manual.


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Más sobre: HP 41c, HP41c, ROM, ppc