Calculator blog

Musings and comments about our common interest


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Testing new repair method for HP41c lower posts

I am now testing a new method to repair the lower posts. Do not worry - it does not change substantially over previous methods - it is just a simplification.

I hate gluing. I have always hated it. I always made a mess of it when I was a child and I was assembling aircraft models. A disaster. And I have improved with my skill, but just a little, and I still hate it.

The piece we designed for the lower posts had a grip on the lower part:

Captura_de_pantalla_2024-05-21_a_las_10.05.52This grip has the thickness of the calculator keyboard circuit. If we drill the holes with a little circular motion so that there is a small room below the circuit for the grip to hold, and if we use our long lower screws (this does not work with original screws), and we press down when we screw the calculator together, then we mya not need to glue the piece down!! The piece was designed with a small inward angle - and the grip gets expanded when you introduce a long screw, pressing sideways and securing the part.

When gluing the piece down, you need to be extra careful to cover all small keyboard holes. If these are covered by glue, that key may not work and you can only disassemble the whole keyboard. If we do approve this method, the whole gluing process is not needed and there are some pieces (the keyboard support and the clamps) that are not needed anymore!!!

I also would have to slightly modify the upper post repair part to give it a stronger grip too. Let's see if this works.

You can test with your HP41 during repair - it is a non-destructive testing and you can always glue it if not satisfied!!

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Valentín Albillo's programs ROM (1)

Captura_de_Pantalla_2022-06-22_a_las_21.24.07I make reference to this post from Ángel Martin, the developer of so many ROMs that we love to use (the Sandmath series of ROMs have been a permanent plug in my HP41CL memory). It includes some of the programs and exercises created by Valentín. While we are always in search of interesting programs, I bet that you will like more the accompanying articles. These are also referenced in Ángel Martín's article.

As an appetizer of what you can see there, there is a chess program! Due to the size of the printout of a chess table and the memory requirements, Valentin decided to settle on a 5x5 net instead of the standard 8x8 - but it is extremely interesting! The post provides the the manual that describes how to play it. It does not check the legality of your moves - but it does calculate its moves! Do not expect a fantastic performance, but with 50 times the speed of the original, it may amount to something.

Normally, it would require either fill in the program by hand (listing provided) or by barcodes (it requires to have the old HP41 barcode), but you don't need that if you have an HP41CL: you can upload the module with the seral cable and your computer!!

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More about: HP41c, chess
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HP41c upper post repair


We have applied the same solution that we created for the lower posts to the upper posts: a grip that clips on the keyboard circuit, and strengthtens the grip with the introduction of the screws. 

In the case of the upper posts there was a requirement to drill anyway - so it doesn't change the original procedure. But the piece has the same 3-petal grip that we use for the lower post:


The connection strengthtens when the screw is put in, since it makes pressure to the sides of the post and makes sure it doesn't move. We plan to put it in production already!!

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More about: HP41c
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New HP41c repair kits!

We have received a new shipment of repair kits and we're going to produce the assembled repair kits off them. 

What is an assembled repair kit? As we know that many of you (myself included) have fear of folding and adhering the flexible repair kit to an old, damaged battery plastic body, Nacho does for us. So far, it consumes a damaged old battery body - so it has to be taken from calculator bodies damaged beyond repair.

Hopefully one day I will set myself to create a 3D body that can be used for the purpose - but I haven't got the time so far. T

he assembled repair kit is a drop-in part for damaged-corroded battery contacts in the hp41c family of advanced scientific calculators. It solves the main issue that plagues old 41c units.

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More about: HP41c
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The new HP41c from Swissmicro

These days we have changed our web server. This was the reason that we had the website down for half a day during the morning of last Wednesday. I hope that the increase speed can provide a better site navigation. I also apologize in case you wanted to enter and could not do it.

Coming back to business: I have received a mail from the friends at, the ones that provided the credit card-sized HP15c. I have a couple of units of an early version; later versions were much better made and good looking.

The message was to announce the creation of the HP41c in exactly the same format. It can use alphanumerics, comes with a proper clock (so you don’t need the time module - it is included) and it even has overlays for stopwatch and others. I look forward to get some units. You can find it here:

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More about: HP41c
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Shady Calculator Stories

Some sad stories in Ebay.

As many of you, I have been purchasing items in Ebay for both my own collection and for the shop. Most of the times you get good material - never a bargain, but what it is advertised. Then you find some guys that are quite liberal with adjectives - in particular, “working condition” and “good shape”. But in most cases they try to help.

But then you have a third type: those that give you a crappy product and then play with you. The aim is to dance with you until the 45 days period since the purchase has elapsed. A recent case comes to my mind. I bought an HP41c calculator sold as working. It took some time to arrive, in a plastic envelope, without any protection, dirty, battered, and, of course, not working. I paid for it “working” price, and it was in such a poor state that I did not venture into repairing it - I could have not sold it in such a state. I arranged the return of the calculator, which took some other days, since the delivery address was wrong, and then wait…and wait… and wait for the refund. “Do you want it in your account or in Paypal” “if in Paypal, then I need to put funds in it”. And then silence… the 45 days had elapsed.

Just in case you find him: the name is Pablo Alvarez Llinas, and his Ebay user name is beaviisb. In today’s world, anonymity is impossible. In linked he appears as working for HP Delivery Services.

I have another case, concerning an HP85 - but that’ll be the object of another installment of “shady calculator stories”

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New items in the HP41c corner

We have just included a number of additional hp41c items in the website:

  • A HP-Il interface
  • A rare, double X-memory module in a single module body
  • A touchpad module to protect the HP41c keyboard

In the next days we will include other items:

  • a Math/Stat module, boxed, with manuals and overlays
  • A Home Management module, boxed, with manuals and overlays
  • A Time Module, with manual

This way, we’ll populate the “HP41c corner” of our website. The old HP41c continues to be the most interesting calculator ever made!

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Repair in old HPs - things to avoid

Due to our work in converting HP41c into HP41CL, we have had to buy many calculators in internet. This has resulted in a number of calculators that were in less than pristine state. In fact, many of them had been repaired, and had fallen in the same traps again and again (by the way, the same traps I fell myself when starting, until a friend took me out of that ignorance!)

The typical part that always breaks first in a HP41 is the lower screw posts. The second is -guess it?- the upper screw post, but this time not the support in the main body of the calculator, but the part that supports the head of the screw in the back side of the calculator.

The third part that tends to be damaged is the battery bay contacts - but we have discussed it already - see our Repair kit page for more detail and how to solve the problem.damage

And when the problem happens, everyone retorts to the typical glue these days: a Cyanoacrylate compound, that dissolves plastic and makes a mess of the rest (apart from not being too good in resistance once dried, due to the damaging of the plastic used by HP. Here is an example on our right: you can see the state in which the posts are now. 

In the versions of the HP41c that can be converted in HPCL, these posts are abosolutely critical to ensure that the board and the main body of the calculator make good contact. 

On the other hand, a calculator that seems to be completely broken can in many cases be repaired with just putting good attention in fixing the posts, so that even pressure is achieved.

It is good practice not to tighten too much the screws once repaired. No only due to the possibility of breaking them beyond repair (you need to be aware that these calculators are 30 years old already!), but because it affect the much-loved HP keyboard feel! The difference can be appreciated between keys in the middle of the keyboard and keys just above the posts. This is not too agreable!

Coming back to the repair. The key for avoiding the problem just cited is to use a "hybrid" type of glue. Here below you can see an example that works well:

aglue This product does not dissolve the plastic and creates a lasting bond. Test it!


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The HP41c repair kit stock finished in 4 hours

I had just received 10 more units of the flexible adhesive circuit used to repair the destroyed battery bays of many HP41c machines. I put it up in the website at 3h15. The 10 units had flown by 19h00 !

When talking with the producer, I asked him for 15 units more - but he only has 10 for me. I have ordered them immediately.

This is a fantastic part, that has helped to revive many calculators. It is a pity that so many HP41c, for many the best calculator ever, are in non-working condition due to rust contacts. This product is well conceived, elengant in its application, works well and lasts long.. 


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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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