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Calculator blog

Musings and comments about our common interest


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An early HP41C converted to CL

I have prepared an early HP41C (serial number 1944A...). I write it here because initially I put it on sale with not good enough pictures, but now it comes in all its beauty.

It is so early a model that it has tall keys and the original, early metal plate keyboard surface (not textured plastic)
















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Error in book corrected

I have found an error in the HP41 repair book - in the part referring to the card reader repair.

It is about the order or the cables leading to the electric motor. The black cable is the one furthest from the calculator; the red one is the closed to the calculator. The book said the opposite (or could be understood as the oppositce - it is strange to me since I knew the right order!

Here is a picture illustrating what I say: it is the two cables on the left. The upper part is towards the calculator.


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HP Repair book


Finally it is here!! 133 pages of our repair experiences and methods. Thanks also to the help and advices of Ignacio Sánchez, who suggested many of the repair methods and helped testing them.

You can click on the link below:

HP Repair book in amazon

It is also available on all amazon country sites.

Here is the chapter list:

1. The HP41C family of calculators
Some History
Special features
Serial number and places of production
The HP-IL loop
2. Identify your HP41
C, CV and CX
Full nut and half nut
Tall keys and normal keys
Screen revisions
Special models
3. Tools and Parts
Tools - Required
Tools – nice to have but not required
Contact enhancers
Conductive inks
Other tools
Spare Parts – replacement parts
4. Modes of Failure (or typical problems in HP41c Calculators)
Types of failures
5. Opening and Closing the calculator
6. Battery and ports module replacement
7. Zebra replacement
Installation instructions
8. Broken lower post repair
Material needed
Use longer screws.
Cyanoacrylate over the inner part of the post.
Broken post repair
9. Upper Post repair
10. Broken screw head supports
Instructions: first method: back repair piece.
Alternative Method: integral piece.
10. Corroded Circuit Traces
11. Screen Problems
Dirty screen
Screen repair
Damaged Screen Protection
12. Damaged Keyboard
13. Calculator Testing
Test with a Service Module (C or CV/CX)
Test without Service module
14. New: CL module installation
15. Peripheral repairs and other items.
Hp 82104A Card Reader
Vinyl Case
Missing port covers
Battery holder
16. Conclusion
Appendix 1: The Calculator Store Spare Parts
Appendix 2:HP41C List of parts
Appendix 3: Full-nut circuits
Appendix 4: list of software modules
References and interesting websites.
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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 products in development - part 1

We are finalizing the design of three different products. As they are not fully prepared, we'll show them in 3D printing form. We will present it during three blog installments.



  1. A second alternative for the battery-ports module. The normal one has a couple of holes to house the little domes where the screws heads are supported. These are usually broken, and we have developed another piece (the brokenback piece) that is used to repair-replace these domes. However, this is a double task in a way, and Ignacio Sánchez thought about joining both into one, and gluing it to the back of the calculator. This makes the repair permanent - which in a way is risky. With this new design, you need to grind off the rest of the two domes, and just glue the battery module on top. Resistance is excellent, since there is a much wider surface to adhere. You can see it in the drawing below: the left design is the new one, with a flat surface that allows for full contact with the base. Initial tests have proven successful.



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Just received more zebra circuits

Until now we have been using a test order of zebra PCB circuits ( Zebra PCB version 3)


We have just received the definitive version (v4) which has a slightly modified dimension to have a better fit (we're talking tenths of mm) and a more precise alignment of tracks.

All orders received this week end are already supplied with the new model

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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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More pictures of the new zebra prototype


You can see now some more pictures of the new zebra model:


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More videos on graphical calculators


In continuation of a previous blog entry about a new series of educational videos on HP graphical calculators, here are chapters 2 and 3:

Chapter 2:


And here is chapter 3:



By the way, we have received two second hand HP50g calculators in very good shape: a HP50g black and another HP50g blue

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Más sobre: HP 50g
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