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Calculator blog

Musings and comments about our common interest


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EOL HP300s+ !!

300sI have received news that the HP300s+ is EOL (End-Of-Life) and we cannot place any more orders.

No one is really mourning - this was another brand's model labelled with HP's badge - but neither in philosophy or hardware it was a HP unit at heart. It did not even have RPN. On the other had, it had the looks, at least in the "+" version (the original was really ugly). It was the second cheapest calculator in HP's line-up. Only completists may be interested in getting it.

It is a good calculator, do not misunderstand me; but as many others in the market. 

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A half-nut "C" HP41 unit !

The HP41 model moved from Full-nut (separated keyboard and processor) to half-nut (unified circuit for keyboard and processor) around 1985. This increased reliability (very few half-nut require repair beyond the ubiquitous battery connector), but many felt that the screen was not as good as the previous model.

In 1985, there was no point in producing the C model: it was too limited and integration made easier to produce just the CV model, with 4 times the memory included. Thus, I had not seen any C model in half-nut guise - until this week.

Also the back of the calculator belongs to a much earlier calculator. We think that it may have been an official HP repair over an old calculator, where the back side was reused and a hal-nut frontside with C innards was fitted. 

The calculator is now in the hands of a famous collector.


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The HP19BII back door


The HP19bII hits a sweet spot in me. It was the calculator that was recommended when I took my MBA after engineering school. (The HP17bII was the other, cheaper alternative). It had everything the HP17bII had, plus 3 lines on screen and a more populated scientific menu: it included everything an engineer could need, short of matrix algebra: several conversion types, probability, trigonometrics and hyperbolics, etc.

The problem of the breed (common to the other clamshell units, the HP28 family) was the design of the battery door. The batteries placed a lot of stress on the door, and this force was supported by a limited area. When the calculator fell on the floor, the door usually broke down. Also, the plastic used was quite brittle compared with other models, making it more fragile over time.

At some point in time, there was a redesign to bring the battery door from the side to the back, solving the universal problem of the side door. In my opinion, also the plastic is a different formulation, softer and more flexible. Looks darker, too. The keys are black instead of grey:












And here you can see the back of the calculators (anf the different plastic color):


We have solde several units, but this one is for me! (By the way: I received one without the door, so we designed a 3D-printed door for the back door HP19bII - please contact for info if you want!

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We have received a couple of 220v charger for the HP41 family of calculators. They can be obtained with and without case:



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Printer paper for HP calculator printers

We have received some units of printer paper. While the boxes are not in good shape, the printer rolls are!


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Ideas for a HP41c module

tall_keys_-_5Some of our readers know that the other of my passions is audio. And more and more the DIY variety - so there are many calculations there. 

I would be very thankful if you could provide me with formulae or programs for audio design, or direct me to sites that have them. Some of the items I'd like to cover and integrate in the audio module:

  • Thiele/small parameter calculation for loudspeakers
  • Sound pressure -loudspeaker efficiency - amplifier power relations
  • Filter component calculation
  • Room resonance frequencies
  • Amplifier feedback component calculation
  • Triode model for a local operating point.

You get the point. Please contact us at with equations that you may want to have included in the module! This should come accompanied with some manual too.



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Woodstock battery bay

5E6A51FF-8398-4A13-8B6B-D6A334BE1B74Now we have two battery holders: one for the HP41 family and another for the woodstock family. However, the one for the woodstock family is different. The HP41c always had battery holders that could work with separate batteries (type N - never put A24 types or you will destroy your unit! I have received many with this problem), apart from the accu-pack that HP marketed; but the Woodstock family, as far as I know, used a battery pack only - there was no way to use normal batteries - unless you performed surgery in your pack. So ours is an easy way to stop using the original pack and use cheap, commonplace AA batteries.

For the sake of making things safe, we printed the + and - sign on the pack, so that you know what should be the polarity of the battieries inserted. These come in an out quite easily due to the elasticity of the polymer at this particular thickness. It took us several different thickess 3D-prints until we found that point.

The only thing that I sorely miss is the beige color. When we designed the part, there was beige as an option - but it was taken out of the options when it was time to start the tests. Damn!


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HP4c feet for free on orders >40€


IMG_2797When I started repairing calculators I took good care of separating the original feet without damaging them, and putting them aside to reassemble after the repair. Once we ordered our own feet, throiwing away the original feet is the very first thing we do. It is a small cost compared with the image of quality and comfort the new feet give.

We ordered feet with 1.5mm thickness. We have seen original feet of all thickness, but we wanted something substantial, so that it really keeps the calculator still on the table, and makes it difficult to displace it. Also we only ship sets of 4, because replacing just one doesn't look good and there might be height differences!

Once put the new feet, the feeling of having finished the repair or assembly is complete!

So we have decided to add a free set of 4 feet to any order over 40€ in repair materials, including shipping, starting today ! You will see it if you're subscribed to our newsletter.

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17bii_6bigFor me, the HP17BII+ is probably the best hidden secret of calculator world today. I don't know why it isn't sold much, much more. It has the feature set of the original HP17BII (minus a couple of only-programmable functions for the solver that I have never used), plus a new menu for currency translation (very useful when you're analyzing a foreign company financial statements), and a much, much (did I say much) better screen. It also has much more memory that you can use  to store many solver formulas.

And this is the beauty of this calculator: the solver. While the mighty HP42s had vectors and matrixes, it lacked a simple, programmable solver that solves your day-to-day problems in a much more flexible way than the typical program. And it can do loops and ifs, if you want to spend time; but that's not its purpose.

It also has a lot of menus made for the financial user - and a math menu that frankly it would be enough for the scientist - if only it had trigonometrics too.

Let's see the menus for a while:

FIN: finance. Time Value of Money, interest compounding, cashflow analysis, bond calculations and depreciation

BUS: useful % calculations: change, total, markup on prices and margin on sales. Very easy to use and fast!

SUM: statistical calculations on one or two variables, keeping all the data sets (different from the 12c and other units that have statistics). You can go to any sample and change it. Eventually you could plot it if you had the printer.

TIME: this calculator keeps a clock and can do all kind of time calculations, and keep appointments and alarms. Like the HP41C Time Module - but much better and simple to operate.

SOLVE: here is where all your formulae are. Extremely powerful - if you know your business and set to make it easy. (Calculate prices that go according to a formula involving market commodity prices, currencies, etc. is a very simple thing with the solver)

CURRX: currency calculations. You can have many pairs of currencies defined. Once updated the exchange rate, you can easily make all calculations.

On top of all that, it looks the part too: the metal surface is quite resistant, and the plastic back extremely so. It could print, too - only if you found an infrared printer, which are out-of-market nowadays.

And the price is only 77,90€ - much cheaper than 12c Platinum!

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Classic charger substitution

Some customers have asked for a solution for the Classic battery. What has been suggested is something similar to the Woodstock battery holder: a 3D piece that can handle 3 normal batteries (still to be decided if we can fit AA or it'll have to be AAA) and that integrates the contacts with the calculator. All in the HP original black and with the usual HP classic fixings. 

Anyone interested? Just to know whether to devote design time to it or not!

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