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HP41cl, time module and battery pack

0IMG_1471Chances are you have used in your university days the HP41c family of calculators. It may have happened the same as it happened to me: I bought a replacement already in the new millennium, only to discover that it was incomplete: I wanted more and more modules, and many of them were unobtainable. More than that: it was painfully slow! Some of my programs were financial, like NPV and IRR, the latter of which iterates until it finds the rate of return that makes the NPV equal to zero. As soon as the cashflows used were more than five, it could take several unites to finish it. My HP50g was much better in that regard - but it had a much worse keyboard, and nothing of the sex appeal of its elder brother.

I then found several ways of improving its speed by “surgery”. The speed could be doubled - but I was looking for at least an order of magnitude improvement, not just doubling it!

Then I came to Systemyde’s approach. If you’re familiar with the HP41c, you will know that it was made in two types: the original one (where there were two circuits: one with the keyboard and connections to the battery, and another “pressed” against it, with the “brain” of the calculator), called “full nut” and a newer one, where all circuitry was routed in the same printed circuit, called “half nut”. Both were functionally the same, and both coexisted for a number of years, depending on the country of manufacturing.

Well: Systemyde designed a circuit that could be dropped in place of the original “brain” circuit, thanks to the fact that it was pressed, not soldered in place. This circuit would be made with the latest technology and would profit of both maximum memory and speed.

If the typical module is between 4k and 16k bytes, and current chips can easily handle 4 GB, that is, one million times bigger, why not store there all modules ever designed, and then the user could switch between it logically, i.e., plugging them in memory via a pointer instead of a physical module on the back?

So they did it. They created a number of other functions to handle that powerful brain, and gave the possibility of running at standard speed, at 50x, or several speeds in between.

Compatibility with old and new peripherals was important. They managed to ensure that all known peripherals work with it. Also all programs ever created for the HP41c are compatible too.

We at the Calculator Store have installed already a dozen of such modules, and it is a pleasure to see how well they work. The challenge is to find bodies in good shape to use them! We have to discard approximately 30% to 40% of the calculators we source - and that is reflected in the price too.

There are two HP41CL bodies with V4 circuit installed.

One is a very early tall keys machine, with satin keyboard surface, good screen and keyboard (with a slightly softer “2” than the rest - other than that, excellent) and serial number 2009A00017, which reads as “the 17th calculator made on week 9 of 2080, in the US”.

The other has what I consider the best screen I have ever seen in a HP41c-family machine. Most units start turning to a yellowish tone after a number of years. This tone slightly reduces the contrast but it is still very readable. The tall keys model above didn’t turn to yellow and still kept the grey color common to most well kept calculators. But not this one: it has a light bluish tone, which makes it the best readable, most contrasted display I have ever seen. It doesn’t even show the slight change of bright on the segments when they are off, so common in most displays

With both of these units, we offer the possibility of getting a refurbished rechargeable battery for 25 € more, or a Time Module for 120 €. Please contact us for more details at

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