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Calculator blog

Musings and comments about our common interest


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Possibility of a new product - query to customers


We have been asked to produce new HP IL cables, and we weould like to know if you are interested.

From a technical point of view is not too difficult - the extreme low speed of the nework make it very simple to make it work - there are no complicated microwave effects to solve. (Hey, were talking of less than 50kHz frequency!!). The issue is to get the connectors, both housings and metal. In particular the metal parts are quite expensive: close to two euros each metal contact, of which each cable pair has 4! And then there is some soldering and finishing required.

The question, of course if you as customers may be interested in it. Please respond to this thread, or send a mail to with your opinion.

By the way: we do have an HP-IL printer if needed. Please remember that if you have the HP-IL module, it doesn't consume an additional port, but instead can be dasiy-chained tothe other items in the HP-IL loop!

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Changes in the battery assembly

We have designed some changes to the battery assembly. The main of them is to create a small dip in the main body so that the band that maintains the film in place but that doesn't interfere with any kind of battery holders (we have seen some cases where there was minor interference.

Here are some pictures:



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New upper and lower post pieces



We have designed new upper and lower post repair pieces, to replace the ones we were using. 

The original ones were flat (in the case of the lower post) or with a cylinder projecting down 3 mm (in the case of the upper post):


The lowe side was flat, with a hole so that the screws passed. All resistence to opening was provided by all the adhesive area. You also had to be careful with aligning piece with keyboard circuit holes.

The new piece requires the user to drill with a dremel tool for 3-4 mm more, and then you get a piece that "clips" to the keyboard circuit. The lenght is designed to do so:


The section is also slighthly angled too, so that when the screw comes in, it presses on the sides, securing even more the part. We have not seen any unit so far that fails. We still use E6000 adhesive, but curing is not as critical as with the previous model.

We are still under test - please specify if you want the old- proven piece or the new one. This also impacts the upper post repair piece!

Some more examples:


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Clean your HP41c case!!!


fundaWe have received many units for repair. They suffer from all possible damages so we have a quite good overview of what the problems are. But we have identified a damage mode that will surprise you - and it is due to the dust that is generated by old HP41c case lining disintegration.

If you see black dust on your calculator when you take it off the case, then you suffer from that problem. This dust is incredibly thin and it is a pain to use the calculator with it. As the grain is very small, it can enter the keys and also into the screen. It can be improved by blowing on the calculator with a hair dryer - but this alleviates the problem only. And in already many cases, it can damage your calculator!

The mechanism is always the same: when I open the calculator, I see dust all around, but it concentrates in particular on the +5v contact between the board and the battery contacts. There it increases the resistance and it is eventually burn, thereon stopping the passing of current. 

The solution is not too difficult and in most cases you just need to disassemble the calculator and clean both contacts thoroughly with Caig Deoxit until they shine again (and by the way the rest of the innards too). But the problem with the case remains!!

As soon as you detect dust on a case, just wash it with your hands and liquid hand soap. Use your hand vigorously in the inside until there is a lot of foam. Rinse thoroughly and marvel at what is coming out to the sink! Do the process a couple of times until there is no more dust coming out. Let it dry and you're ready to go. The case will still look good in the inside.

Wash also the sponge in the bottom of the case separately. You will see that it is actually yellow, not dark green!!

Now your calculator will be clean always. Now it is a pleasure to use it!


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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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