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Calculator blog

Musings and comments about our common interest


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Valentín Albillo's programs ROM (1)

Captura_de_Pantalla_2022-06-22_a_las_21.24.07I make reference to this post from Ángel Martin, the developer of so many ROMs that we love to use (the Sandmath series of ROMs have been a permanent plug in my HP41CL memory). It includes some of the programs and exercises created by Valentín. While we are always in search of interesting programs, I bet that you will like more the accompanying articles. These are also referenced in Ángel Martín's article.

As an appetizer of what you can see there, there is a chess program! Due to the size of the printout of a chess table and the memory requirements, Valentin decided to settle on a 5x5 net instead of the standard 8x8 - but it is extremely interesting! The post provides the the manual that describes how to play it. It does not check the legality of your moves - but it does calculate its moves! Do not expect a fantastic performance, but with 50 times the speed of the original, it may amount to something.

Normally, it would require either fill in the program by hand (listing provided) or by barcodes (it requires to have the old HP41 barcode), but you don't need that if you have an HP41CL: you can upload the module with the seral cable and your computer!!

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HP calculator repair

test_-_27From time to time we receive emails on whether we repair HP calculators. This makes me wonder on how well we communicate: this is what we do! So we have thought it out and decided that apparently it is not clear from our website. Then we have decided to open a new department on the website about repair.

We are specialists on HP41 and card reader repairs, but we also have a partner (Ignacio Sánchez) who is expert on the rest of calculator types: Woodstocks, Classics and Spices.

We think that newer calculators (Pioneers, Clamshells and the HP48 family are far more difficult to repair than the previous models, much more modular and where the problems are typically mechanical in nature. And for some models (HP50g, HP17bII, you are much better off if you just buy a replacement off an auction site.

We also sell parts for repair (now based on the HP41 family but also battery solutions for other ranges too) and we plan to offer calculator enhancements (HP41CL in the past, bu we will try the Panamatic and Teenix solutions that improve on the previous calculators with a number of interesting additional features.

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New website logo!

We have enlisted a designer to get a proper logo for The Calculator Store. A logo is required in many places as SEO enhancement - but you did not come here for the logo, but because of our common love for HP old calculators!

Well, here is the new logo in small and original sizes. What do you think about it? I think it joins simplicity with the minimum information to identify a HP calculator: the two shifted keys in blue and yellow, with the dark brown color typical of old HP models. Due to the size and shape, ir could be used with a hypothetical iphone/android app!







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HP12c out of stock

Apparently due to the semiconductor scarcity there are no more HP12c for the time being. We have been given end-of-year delivery dates - and we have just finished our last units!

A pity - it is the fastest and best thought out financial calculator available!


We'll inform you when these units are back in stock.

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Classic AAA charger - prototype 2

Here is a working prototype of the AAA classic battery holder. Many picures and few words to show you where we are:




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HP12c solver for polynomial expressions

HP12 Polynomial solving.

From Valentín Albillo’s “Serendipitous Solver” article. I have not read it from a long time ago, but I got struck by it sheer originality. The HP12c is maybe the perfect financial calculator, but it is quite limited when it comes to scientific purposes. It misses trigonometric functions, integration, solver…

Does it really miss a solver? It does miss a generic solver, but it does have a solver for the NPV calculation. What the calculator does it trying to solve the NPV equation for r: getting the internal rate of return (IRR)


NPV: Net Present value
CF(i): net cashflow at  i period
r : discounting rate

but if you replace  1/(1+r) = x, you have a polynomic function in x. You can now find the zeroes of the function. You just have to enter the coefficients as period cashflowsand the constant term, and solve for interest i. Careful with the monthly/yearly equivalences! . Then you just have to find X substituting again. Be careful of the discontinuities that may happen when i crosses zero. Now, the calculation will give you a zero; you need to play with the initial values of r to find others.

This of course might be done easier with the HP15c – where you just need to enter the polynomic expression as a RPN program and then apply the generic solver. No need to do substitution. 

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More HP15c

image0I am having a discussion with my son, an engineering freshman. He's just finished the first year, and he's very happy and thankful of having the HP Prime calculator. Probably it is the best calculator for a student nowadays. I have the feeling that the HP50g is the better calculator, due to its more "elegant" programming model; however, the Prime has more raw power and is easier to program is a procedural language similar to basic. (It is surprising what can be done with this language. You can take a look at the games that have been programmed with it in I hope that my children never see it or there goeas another distraction more)

My discussion with him versed on the fact that the HP15c is the best calculator to master "for the rest of his life". I see no point in having him using a HP41, even the cel: is a cumbersome machine that will occupy a lot of room in wherever he puts his things; The hp42s would be an alternative, but even that one is a little too big; but the HP15c is exactly the right size and has everything the prospective engineer needs - short of anything a computer spreadsheet or Mathematica or Mathlab can do. Plus, it is small enough to be really portable and carry it on your jacket pocket; and it keeps the utter elegance of the HP50g we were discussing about. 

I plan to have him learn how it really works. He'll have one as heritage, and he'll be thanking me for it in the long run. I guess that the Prime will not see much use after he has finished college!

I will give it to him with the leather case to increase the acceptance rate - by the way!

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This week I have been using the HP15c in my normal work. for that I have entered several financial programs including the Time Value of Money program. It is a long program but it makes use of many of the programming techniques in the HP15c.

The HP15c always surprises me regarding how cleverly distributed the keyboard is, and how well thought out is the function set. Most of the functions can be applied to complex arguments; also, compared with other HP calculators (including the advantage pack for the HP41c), the complex mode doubles the RPN stack and so programs can be applied to real or complex numbers, depending on which mode the calculator is; it allows to redefine the upper 5 keys; and you can combine complex numbers with the integrate and solver features.

A surprising capability on a calculator this size is the extended use of matrixes - you can invert matrixes and can also use them to solve linear systems - which is something you can do in an exam whithout anyone noticing that you're using your calculator. Also I liked how elegant is to solve an equation system: after entering the 3 matrix dimensions (coefficients, independent terms and results), you enter the values for each term in the matrix, and then you recall the independent term vector, you recall the coefficient matrix, and then you just press the divide key! then you have the result in the results matrix. So powerful, and yet so simple and elegant.

The calculator came initially with two manuals: one was the user manual, roughly equivalent to that of the other calculators (but with many more functions, of course); however, this was a machine devoted to scientists and engineers, and it should explain them with details how it works and what are the limits of operation and accuracy in the different functions and features. So, the machine came with an "Advanced Functions Handbook", pictured besides. 

The manual dealt with the following topics (reading from the index):

  • Using Solve effectively
  • Working with integrals
  • Calculating in complex mode
  • Using Matrix Operations
  • Accuracy of numerical operations

These subjects are dealt with detail and combining precision and clarity - a must read for any computer science student!

The previous anniversary version did not have this manual, only the User Manual. Let's hope that, if this calculator is ever resurrected, it comes with this manual too!


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New springs for HP41c, Woodstock battery case

IMG_4321So far we have been using springs with 11 mm height, made in ss304 stainless steel, 0.5mm thickness, with the right size for the HP41c and woodstock battery cases. However, the height and spring effect proved to be too much in many cases: the pressure required to open the machines was too much. Also we had to sell them in pairs for the HP41, since very often our spring was taller than the one to be replaced.

In the picture besides you can see the old spring (at the back) and the new spring (at the front)

So we have taken off a turn in the spring and reduced the height to 8mm. It still works well but it forces less the calculator - in particular the fragile upper posts of the calculator.

By default we will serve these lower height springs, but we still have some longer springs if you abosolutely need them.

We will equip the battery holders with these too - it will make your calculator last longer. In the case of the Woodstock, this is very apparent - the previous ones were a quite tight fit.

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I have created a "HP15c Corner" in our website. Something tells me that it will be useful some time soon. For the time being, we will put there all the HP15c or HP15c LE that we get for sale, and all parts and accessories related to them, including leather cases, battery covers, etc.

We'll also take care of the links to this wonderful calculator that we may find, as well as all the software for it that was available at the time.


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