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Calculator blog

Musings and comments about our common interest


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HP41c case sponges


Now that I am starting to get involved with cases, there is the HP41 case that we need to deal with. 

As you know, the case is bigger than what is needed for the naked HP41 calculator. It was designed so that it could include also the card reader. Apparently HP management thought at the time that everyone would have a card reader. Anyway, they forced the whole customer base to carry a case that was bigger than it needed to be. So be it.

In order for the calculator not to dance within the case, HP filled it with a sponge, foam or whatever with the lenght of the card reade, but much thinner as it was intended to be in the lower part of the case. These sponges got lost in the cases where the user actually had a card reader, or just lost - period.

We are ordering a run of 100 of such sponges, should you want to pad your case as it was intended. The main reason is that when preparing HP41CL machines I cannot afford to send an "incomplete" item.

I will also put them on sale. I have noticed that the yellow sponges are not available in many foam producers, so I will use white color instead. Also higher density than the original. 

Please email to if you are interested in one!

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Scratches in HP calculator screens


There is an ongoing discussion in about how to repair scratches in HP calculator plastic screens. I have fixed many, and it is clear for me what is the best solution: to use Polywatch - the plastic version (do not get the glass version - it may destroy your screen). 

You apply a drop of this substance on a soft cloth and rub for some time. Remove the white residue with glass cleaner liquid on another tissue - or even a kleenex paper to absorb it. You will see a brilliant screen! This can be used too on you Swatch or Casio watches too.




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Why have we started producing the Voyager leather cases? or look at HP's late cases disaster!


Ignacio Sánchez asks me why have we started with the Voyager cases, and not with the much more sought-after HP41 cases. These are commanding 30€+ prices in the market - and that's for a 40 year old ugly vinyl case!

The reason was twofold. On one hand, it was a simpler model to produce, and being no leather expert and having to rely on a factory, let's start with a cheap, simple thing before commiting a big amount in tooling (the HP41 case is sibstantially more difficult)

On the other hand, those who have recent Voyagers must be very upset with what they have. There are many cases that are disintegrating. Here below we will show two cases, one that came with an original HP15c LE, and another that came with a recent, Atmel-powered HP12 (while it appears holding a 15c LE inside too!)


The case for the HP12c was notably worse (and I did not treat them wrong at all!!!): I can't wait to change it for something new. This is something you cannot lay on your office table!


If you have an early HP12c model (the three batteries, even the single CR2032 battery type), you have a case that stands the test of time. You are not in this circumstance. You don't need the new leather cases. But if you have one of the HP15C LE or later HP12C units, you will NEED the Calculator Store's leather Voyager case!! We will be receiving the initial batch next week already. See below how they look - a pity that we cannot use the HP logo!!

Voyager Case

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More about: HP12c case
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Two old leather cases for HP calculators - they even had a part number!

One belongs to an HP12c, and the other came with a HP18B


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Leather cases for most significant calculators

We have planned to sell a range of calculator cases for several models - all of them made from Ubrique cow leather. You have seen the tests of the HP12c - which was the easiest of all possible cases. You can see the test below. A question would be which other colors may be interesting. We have seen samples of Burgundy (nice) and Brown (very, very nice). If the original black model sells well, then the brown is next, and the burgundy will follow.

The next model will probably be the Pioneer case, also in leather. It is just slightly longer than the HP12c's, but the same profile - and it also can be used with the newer HP17bII+ (tested with the originals). The factory has already the original samples and could start production if we see that the Voyagers sell well. The original 12c vinyl case had a better texture than the Pioneer cases - although there has been an evolution to a very plasticky version over time - I am thinking about the cases I receive with the HP42s. Also these "plastic" cases break much more often - it is difficult to find a really "complete" unit. However, finding a replacement is quite easy, easier in fact than with the Voyagers: you just need to buy a second hand HP10b off Ebay, and you have it. (you can even drop the calculator later :-)). 

Third in line is a case for the HP41. The original case is one of the ugliest I have seen. What were the HP guys thinking? The shape: I can understand that they thought that everybody would buy the card reader to have it included in the standard case. However, the color was ridiculous! Compare it with any other case built before, even for the humble Spices (hp31, 32, 33, 34, 37): much better!!

Here there are some questions to answer before ordering it, and I would like your input:

  • Sized for the calculator or for the calculator + card reader?
  • Color: Black? dark Brown? light brown (closest to the original)? Burgundy?


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First leather case tests for HP12c calculators!!

We have received the first tests of leather cases for the Voyager series. The texture is similar to the original - but in good quality leather. The price will be below 20€ each. What do you think?

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Wsih list for Moravia-Royal

Now that the calculator business is in the hands of Royanl and Moravia, let's see what could be our wishist, now that the HP35s has been discontinued. A customer called me on Friday, and he told me "I want a RPN scientific calculator" - then I realized that there is no scientific RPN calculator anymore!!!

Rumours are that the HP12c will be put again in production (despite being in some dealers' inventories, it has been discontinued for at least 4-5 years). This would open the door for other Voyagers calculators - and I am thinking about my long time love - the HP15c - again.

Among the units in production we have the HP17bII+ - which is an excellent product and the most powerful current financial calculator. However, it has the same body and key disposition of the HP42s - so that is another possibility for a scientific calculator (the HP42s has not been bettered in my opinion) - with limited development time (both models could eventually be run on emulation layers with the original firmware)

However, the calculator market is based on school needs. These are best covered with the 300s+ and 10s+, and then with the Prime as graphical calculator (which runs circles around most of the competition).

I will try to send these thoughts to Moravia, who has been for some years my distributor.

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Unknow module overlay


I have found this overlay on a calculator I bought last week . The calculator came without any modules, so no way for me to know what the module does, or what is in memory (it was sold as "for parts" but I managed to return it to life - this is one of the pleasures of our hobby!) Any help will be welcomed. It would be good if it is one of the moudles included in the HP41CL memory!!


I am also considering printing and selling overlays for different modules. Would any of you be interested in any of them

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The original HP15C: the best HP product ever?

This week we have sold an original HP15c in bos, and in pristine shape. While I was reviewing its compact box, the instructive manual and the machine itself, I kept thinking that it was the best hp product ever.

Capable of inverting an 8x8 matrix, solving equation systems with complex numbers, and having all keys with three different functions (and in some cases 4), it has the most elegant logic of all units I have seen. Nothing is superfluous, nothing repeated, everything has a purpose. And it is compact and beautiful - the ideal lab calculator. 

Here are some pictures of it:




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A new set of cables available.


I am putting on sale my own set of loudspeaker cables. The reason for selling them is that they are a tad too short - 1.7m when I need something in excess of 2.5m in my current set-up. 

The cables are based on Goertz MI2, but I have made some changes to it.

  • First, take it to and cryogenize it. (Deep Cryogenic treactment, DCT, is when temperatures go below -175ºC). It is the same company that did the service to Townshend Audio - the makers of the Townshend Isolda, a similar cable with very good reviews. This changes slightly conductivity, a lot ductibility (you can feel it by handling it), but it makes it much "airier" and "sweet" when playing music. It really makes a difference
  • Second, take the polycarbonate transparent sleeve (which ages quite badly) and replace it with a Nylon mesh - a much lower dielectric density.
  • Third, we take out the intermediate teflon film between both conductors - then the only dielectric is the extremely thin polyester terephtalate films on both conductors - therefore lowering even more the characteristic impedance.




























The Goertz cables are well suited to tube amps and specially to low-impedance loudspeakers, in particular those that have a difficult treble range: electrostatics, ribbons and Gallo CDT tweeters. The it is really magical: the characteristic impedance of the tube amp, the cable and the loudspeaker match perfectly and magic occurs. These loudspeaker cables have the best soundstage I have ever experienced; and they have a bass to die for.

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