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Calculator blog

Musings and comments about our common interest


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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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New zebra development!


We have received the samples of a new design for zebras for the HP41 family of calculators. This design will solve the current issues that the elastomeric zebras have. (This part is referred to in the service manual as 1251-5731 )

Let's see it:


Initial tests seem very good - 100% connection on first try! Dimensions are perfect - what we need to find is the optimal fill for the zebra part: how thick and compliant does it need to be, and which material is the best for the application, since it has to be durable and cannot rot, corrode or dissolve. Initial tests show that it works even without filling - the compliance of the film is enough to ensure good connection. However, we will put something in, to ensure durability.

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Interesting video about HP graphical calculators


I have come across this video about HP graphical calculators, in It is worth taking a look at it if you want to enter into these calculators -which were the workhorse of all engineering students in the nineties. It is prepared by Stephen Mendes, and he promises several other videos!!


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Repair your calculator during the lock down!


Now we are locked down due to the Coronavirus global pandemics - and it is likely that we'll have to stay for at least a couple of weeks. Now is the time to start things that we didn't have time in the past to do.

DSC_7337One thing is to fix everything that doesn't work at home - and probably this includes your old calculator.

I have started to repair calculators only recently. The repairs performed were increasingly complicated, but at the end I got Ignacio Sanchez's help and we designed a series of parts that help us in these repairs. But the first you need to do is the diagnostic of the damage.

During the next days we will discuss how to repair yours. However, my goal for this instalment is to make you believe that your calculator can be repaired. In my experience , 90% of HP41c calculators can be repaired - and I'd say that 70% are really easy to repair. And this means that YOU can repair it. 

Even very destroyed calculators are repairable. Curiously, the worst calculators to repair do not show any damage. In many cases users put the wrong battery model and burn the calculator with much higher voltage than needed (for example, put four a23 batteries (12v * 4 = 48v) instead of the 6v that are produced by four 1.5v LR1 batteries. Then the screen is burned and then you need to wait for a donor calculator - something that doesn't happen quite often.

But for the rest of the repairs, there is a solution!

We'll look into it in the following blog issues.

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