Calculator blog

Musings and comments about our common interest


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Zebra may go alone


We have received some inquiries on whether the zebra PCB assembled requires the zebra-holder. The zebra holder piece was designed to hold the separate zebras you can find in older calculators: the golde ones, not the elastomeric ones. If you find these, it is very likely your calculator won't work when reassembling - you'll have to replace it.

When you are using our zebra, it can very well go alone, without any support. It is strong enough to keep shape no matter what:


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HP41c Repair book

We are in the final stages of writing a book on HP41 repair. We expect to have it finished by the end of September. In our experience you do not need to have any electronics knowledge to repair 80% of HP41's damages. The nature of many of these is either mechanic or bad connections due to corrosion. The focus of this book is on that 80%. It is written for those of you that wouldn't dare to repair your calculator before!

Here are some of the chapters:

  • HP41 models and versions
  • HP41 modes of failure - diagnostic
  • Opening and closing the calculator
  • Battery and ports replacement
  • Lower posts repair
  • Upper post repair
  • Calculator back screw support repair
  • Screen problems
  • Keyboard repair
  • Calculator testing

The target price will be 9,99€ for the kindle version a 2-3 € more for the print version, and will be sold through and on this website. I'll keep you posted through this channel. Do not hesitate to bring suggestions for additional content!

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New product: 10nF capacitor for HP41c

We have introduced a new product. This is only for those that want to use the HPCL circuit on an old HP41c (with serial number before 1954AXXXXX). The screen set requires that the processor have specific capacitance. This is provided by the processor that came with the machines, but not by later processors (in case the original became defective) or if you want to install a CL super-processor. So we have bought a pile of 10nF surface-soldered capacitors, to install between two parallel circuit lines. We will provide instructions if needed:


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Now on sales - the zebra replacement!

We have done more tests and now we have put it for sale: it works well. Now thinking about packaging to send them in the best way - so that the adhesive stays put until installation. Alternatives are mounting it on a lower post assembly, or another clipping method to maintain the pressure between up and down.

You can purchase it here:

Further pictures on the first production run:


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How to replace a HP41c screen


There are cases (black spot on the screen, screen only works for half the digits, broken screen) where the only solution is to change the screen. These are not produced anymore, so you will need to find a donor HP41c.

To replace the screen: First unsolder the two screens (from the donor and the damaged calculator) Use a small soldering gun and help lifting the pins with the tip of a plier, one by one. Important: unsolder the screen at the calculator side: it is much more resistant!!!

Then solder the donor screen back in place. Locate the screen in its hole first. then start soldering pins, with a very clean soldering gun. There is no need to add more solder. After melting the solder, leave the gun while maintaining pressure on the pin with a flat screwdriver tip, to ensure good, durable contact. Patience until finished. Then reassemble and try again. 

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Testing repaired HP41C

I have received today a fantastic tool for testing the HP41c/cv/cx calculators that I usually repair: clones of the HP service modules. 

HP released two versions of the service modules: one for HP41c and another for HP41cv/cx machines. You can read the details from the service manual:

HP41c Service manual


These modules greatly simplify testing, including memory, screen, keyboard, status, etc. We will discuss these modules and other diagnostic tools in further blog issues.

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Battery module for broken calculator backs


If you remember, we had ordered a new time of 3D printed battery modules, specially designed to be used with glue on broken calculators back sides:


This version is intended in case of both supports of screw head broken. For any other case, the standard piece would fit better. The repair is also definitive: if it doesn't work or gets corroded, the repair would be much harder!

Here is the body for the standard piece:


The pieces have arrived and we will do some tests during this week end. We'll come back with the results next week!

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HP41c repairs!


We've just received three calculators for repair: a CL that worked on assembly but not on delivery, a C and a CV. The C is easy to diagnose: corroded battery terminals. Let's see if it is the only thing needed when we open it. The CV has a beautiful battery terminal and looks solid, without any post broken - but it doesn't work!

It will be an interesting week end...

I'll try to take some pictures to illustrate further bolg issues - also with oscilloscope output views, to compare a new CL with older models, when it comes to wave frequency stability.

Please contact us at if you need your HP41c serviced!


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Back to the office!


We have returned from holidays with a new 3D design, although unrelated to calculators - a battery door for the ubiquitous Samsung TV remote!

We will have it in different colours - as a way to distinguish different remotes. It is still not created as a product, but it is on its way.

We have several products created as 3D: here is the list so far (not counting repair parts):

- Side port HP41c

- Back port HP41c

- Battery holder HP41c

- Battery holder Woodstock series (HP21, HP22, HP25, HP27 and HP29 calculators)- Shanling M0 DAP cover

- Samsung TV remote battery door (coming)

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Development priorities


We are preparing the development plans for 2020. Your requests would be listened to, but be aware that someone insisted heavily on creating the Woodstock battery bay and we didn't get an order from her finally! So we would like to have some quorum for the demands.

My list so far:

- USB connected HP41c adaptor (not battery - putting any kind of electronics inside would require huge certification effort (or alternatively huge risks for our very small company). It will be a 3D printed part with a connector. The jury is out as to whether to include a capacitor to hold the charge for longer.

- A permanently glued battery-ports connector, to hold better together with the back case.

- Many other small items for HP calculators: screws, spacers, screen plastic covers, etc.

- Finish a HP41c repair manual (we're already half way through)

Please also submit your requests to too!

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