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Calculator blog

Musings and comments about our common interest


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Battery bay repair piece

sketch_moduleThe latest battery piece is used to repair the typical corroded contacts in HP41c calculators. By far this is the most typical repair need in these calculators.

Some years ago, Diego Diaz created a foldable, adhesive circuit that could be used instead of the original cicuit. Initially, you would peel off the original circuit, remove the plastic rivets with a X-acto knife, and paste the new circuit once pre-folded. This was a fantastic solution!

This gave fantastic results - for a while. The adhesive, while strong, would give after several months of use - sometimes up to a couple of years. No matter how it pasted and pre-folded it, after a year of use the adhesive would fail and the outer part of the circuits would peel off. Good for your own calculator, but not good for somebody else's file. 

So we set to find a permanent solution. 

After some thinking, we addressed these failures. We decided to fix both sides with screws, with a total of 8 of them. We use also two 3D printed pieces to fix these circuit ends:

On the right you can see an exploded picture of the part before adding the circuit. This makes sure that it will never peel off! It has also tighter measures than the original, and gets secured by its shape too. We still recommend to put the back ports when closing the calculator, since it pushes the module in place.



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A new Zebra batch has arrived


zebralinWe have just received a new batch of zebra connectors for HP41c. The compound chosen is more flexible than the previous one. The thickness has been optimized to avoid gaps in the lower part of the calculator, while keeping good pressure between both circuits when pressed together.

To do so, we have a central conductive part surrounded by two softer isolating silicone sides.

You can see it in the pictures beside. 

In some cases (depending on you application and what the old zebras were) you may need or not the optional zebra holder.

The price for a pair has been lowered to 6€. 


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Flex circuit installation for the HP41c calculators

We offer two products to repair the frequent corrossion on the battery contacts. The most frequently sold is the complete assembly - it is ready to be put in the HP41c calculators as it comes. This is the version we recommend to our customers.


However, some of them are more adventurous or more daring, and that decide to go the extra mile and assemble it themselves. for them we have the naked Flex circuit. 

However, I'd like to give some comments from my experience of assembling more than 60 units myself:

  • If you paste the circuit onto the original plastic piece, be aware that sooner or later the contacts will peel off the plastic. There is a solution for that: using our set of 3D printed pieces, the circuit is secured mechanically to the support, with fixing pieces and a total of 8 screws per part.
  • You need first to remove the original circuit. You need to use a X-acto type of knife, and remove the rivets that fix the circuit to the plastic. I usually then cut the circuit by the contact area, and use the freed area to grab it and pull out the rest of the circuit.
  • Clean well the plastic piece from the grease that usually can be found inside the calculator. 
  • Pre-fold the flex circuit:
    • On a table, lay the circuit with the contacts side on top and the while paper down, leaving the funnelled part far from you.
    • image6-2
    • Fold upwards and away from you, and create the two bends that will be the part between the four module contact tabs. These folds should coincide with the short sides of the two circuit opening. Do not fold anything else yet.
    • image8-1
    • Turn the circuit face down. You have now the white part facing up.
    •  Partially peel the paper until the second bend. fold it there.
    • image9
    • Start adhering the circuit to the plastic from the extreme, in the side farther from the battery contacts. Ensure adherence everywhere with a non-sharp tool.
    • image10
    • Fold the circuit by hand on the top of the first tab, and push the bent part into the "valley" between both tabs. With the tool, press the bends you did at the beginning against the "corners" of the valley. The white part still adhered to the circuit should have helped you entering the circuit in the "valley". 
    • image11
    • image12
    • Now you can remove the paper altogether. 
    • image13
    • Bendthe circuit over the second set of tabs. Ensure adherence with your tool, and create the next bend.
    • image14
    • Now paste the battery contact part to the small contacts tabs. 
    • image15
    • Paste the rest of the circuit in between
    • image16
    • You have  finished! Now let's ensure it stays put: you can try with memory modules in all four tabs, to make sure the circuit is pressed against the plastic.
    • If you choose all our parts, including the base and the two fixing parts, you can further secure the circuit with a couple of pieces with screws:
    • image17
    • Leave overnight

And you're done!


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Zebras for the HP41c

IMG_2838The HP41C full nut requires two zebra contacts betwen the keyboard circuit and the main processor. The original zebras seldon get destroyed, but in severe corrosion cases, the zebra connectors are also affected. Then you need to change them.

In there were several threads about alternatives for the zebras. We have done something else: we have custom-ordered a couple of zebra connectors that work for our application. The length, width and height are based on the originals. 

Now, the original is two round copper cylingers joined by a plastic membrane. As we don't have this membrane, we have designed a "zebra-holder" that keeps the zebra pair in place. It is made in 3D printed nylon (with a HP printer, of all brands!). It replaces the old structure, so that it can be round the zebras to keep them firmly in place. The old structure can be removed with pliers and a little force. Be careful to avoid breaking the screw posts.

If the screw posts are damaged, broken, or the screw rotates freely in them, the required piece is a different one, the lower post repair, which replaces both the zebra holder and the screw post, and that is glued in place.


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A couple of audio-related products

As some of you know already, my other hobby is audio. And as a frequent voyager, audio means portable audio. 

We have created a couple of 3D pieces for problems that we have found in our portable devices.

We are lovers of the Stax Lambda electrostatic headphones. However, sometimes the side assembly breaks. We have designed a 3D replacement, that you can find in our audio area or in our webshop in eBay. It just plugs into the hole of the headband and allows some rotation on its axis, since it has been designed with the stem printed within the body - this you can only do with 3D printing!


This piece helps solving a problem for many users - I hope! I have found yet no other use for the "print inside" capability of the 3D printers.

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HP41CL with Time Module

For some time we have left aside the blog, due to lots of repairs and CL installations. Then comms are postponed and the urgent orders come forward

So we have not informed here that there is now the possibility of integrating a Time Module in the HP41CL. This saves the user a 120-140 € module, and a slot. 

All the rest of characteristics and features of the CL are still there, while the list of included modules is continuing to grow. Also many of the modules designed by Angel Martin have been reviewed, debugged and improved.



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HP_prime_front_pictureWe have received the new HP-PRIME G2/B1S (2AP18AA). This calculator is fully compatible with the original, yet twice as fast (while speed was never a problem with the original one). The chipset has been changed, and the new one has several advantages that the previous didn't have. To start with, reportedly you cannot brick the calculator anymore (and this is something that in the past even happened to me, when admittedly I was trying something I shouldn't have done)

In next blogs we will go through this model in mode detail.

The pictures we have are still of the older model. The new model has a more start key coloring scheme, making it easier to distinguish the second and third function of each key.


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New batch of springs

We have received a new batch of springs. Due to the quantity, we've been able to reduce the price from 4 to 3 € apiece. You can find them here


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Success of new HP41c repair methods

80806dea-8f86-418a-ac37-128e1b53086bWith the help of Ingancio Sánchez, we have begun analyzing the failure modes of the HP41c, and we have been developing repair techniques for most of them. In the next articles, we will present the failure modes and the possible techniques in each case. 

  • Corroded battery terminals
  • Corroded main circuit
  • overdone lower screw posts
  • overdone upper screw posts
  • Broken lower screw posts
  • Broken upper screw posts
  • Corroded or broken zebra connector
  • Broken screw supports on the back side or the case (usually the upper ones)
  • Missing back port
  • Missing side port
  • Corroded spring for battery holder
  • Broken or missing battery holder
  • In full nuts, overdone nuts to keep circuit pressure - an alternative to nuts.
  • Broken plastic window

In next instances, we will go one by one with each of the chapters. Who knows, it may end up as a book!

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Higher 3D printing prices

We have to announce that we're raising a couple of 3D printed pieces prices. This is due to the simultaneous price increases from all 3D printing houses here in Europe. In some cases these prices rises are all over the line, but in others just in the black special treatment that we're getting.

The prices increases are in the following pieces:

Side port


zebra holder

Other will have to follow as we place additional orders and we see its prices risen!

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