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Calculator blog

Musings and comments about our common interest


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New design in for testing


Up to now, we have created two parts (among others) to repair the back side of the HP41c calculator. One is the brokenback 3D part. It is used to repair the often broken screw head supports in the upper part of the calculator. The other is the battery-ports assembly. It fits on a normal calculator back and also on a brokenback-repaired calculator back. 

However, sometimes I would like to have more adhesive surface to the brokenback part. Sometimes the destroyed part of the screw head support is so big that there is limited spece for the adhesive to bond.

My friend Ignacio Sánchez suggested to make both parts a single one, that would be pasted on a back side that has been flattened and cleaned from debris. This would be glued in place, therefore making it a permanent repair,

Here is the current piece: you can see the holes in the screw area, that mimic those of the original part:



And here you can see the new design. It can, obviously, be improved by closing the hole areas not used by the existing tabs of the back side. This will be the next redesign ater testing.


Now the parts are being 3D-printed at our supplier. Both will coexist - I'd rather prefer to make a repair that I can undo, or re-repair without much hassle - but sometimes the glued parts is what the doctor orders. Also, 100% testing becomes fundamental to make sure that it works in your calculator! Stay tuned for the formal announcement when the tests have finished


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X-Functions and X-Memory


We have just received a X-functions and a X-memory modules. The later is quite difficult to find - I can attest it! While the X-Functions is more common, the X-memory module (which requires the X-Functions as well, unless you are using a CX) is quite rare and never lasted for a long time in our shop.

Let's see how long it takes to sell!

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New firmware for the HP Prime


HP is releasing a new firmware for the HP Prime calculator. There are versions for both HP Primes, version G1 and G2.

In principle the release documentation informs about quality and reliability improvements - not so much about any particular additional development to the capabilities of the machine.

You can find it in, in the HP Prime documentation pages:

You will need to install also the new connectivity kit to make sure it all works properly. You will need to place the upgrade files manually in the upgrades folder of the connectivity kit, since it is not yet installed in HP's FTP sites and therefore will not be found automatically by the "look for upgrade" feature of the connectivity kit. Upgrade at your own risk, then!



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Zebras for HP41c

The zebras we had produced to our specs to repair the HP41c, while good enough to ensure that the calculator functions, are not enough for high-current applications, like HP41CL ROM writing or magnetic card reading.


We are looking into an alternative solution - which can increase 20-fold the current delivery. Until then, we will stop the product line.

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HP Prime loading program procedure


I have been asked how to load programs in the HP Prime - apart from keying them in. in short steps:

  1. Download the HP connectivity kit (and HP Prime app, now that we're in business) from the site: HP Connectivity kit download


  • Install it in your computer. Be sure that you select the right version of your operating system. In my case it defaulted to Mac OS 10.1 - I changed it to the right MacOS version
  • Open it and connect your calculator to it. This connectivity kit also serves to update the calculator firmware, if it is not updated.


  • Go to, and navigate to the HP Prime programs that you want - there are many!


  • Choose the area and choose the program you want to download:


  • Click on "details" and on the file name:


  • Unzip the file in your computer
  • Read the .txt or the .pdf files - it is good practice!
  • Open the HP Prime. You will see applications and programs - it depends what you have downloaded
    • if the directory has the .hpappdir, drag it to the applications folder
    • hpcomm7
    • if it is a program, drag it to the programs folder:
  • And it is ready to run!! 


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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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Testing HP41C with oscilloscope

When repairing a HP41 machine, it is always interesting to know what it is happening inside - regardless of the screen being blank or not. Using an old memory module, we have connected the pins with a ribbon connector, and then this connected to the oscilloscope we can see several things: is there the positive voltage? when you press a key, does it appear on the interface?

The system is designed so that when a key is pressed, there are two lines ( ø1 and ø2) that output a synchronizing wave - delayed some time the one versus the second. This is a tell-tale that the circuit is working - regardless of what happens to the screen.

Here you can see in the case of a CL just installed (thanks Mr. Lerich). The circuit is outputting 360 Khz, much more stable than the usual 343-365 that you see on normal calculators. The ringing at the corners may be due to the fact that the cables were not terminated properly and therefore there were reflections. Also it can be seen the induction effect on the rises of one curve on the other, due to the parallel 150mm of the ribbon cable.


If we see it closer, we see the rise-time of the curve is equivalent to 25 MHZ - quite a high bandwidth, even despite the mentioned termination issues. Also the stability of the voltages seem very good. Both are testimony to Monte Dalrymple's good engineering!!


We need to get deeper in this method to be able to better debug and repair our calculators. Now we need to get a service module - or an image burned into an eprom of such.


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Feet arrived!

We have received the sheets of feet for the HP41c. Tested them and work much better than expected.

We will send the 100 unit sheet for 50€ plus shipping - half the price than the 4-unit set. Ideal for collectors and repairers. 

Please contact us at


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