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Musings and comments about our common interest


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Financial calculators


Many of us are working in financial positions. I have seen in many offices financial personnel using what in calculator business are called "four banger": just the basic four and typically percentages. In some cases they also have a small printer. This is more than enough in most cases. And in the cases this is not required, you use the Excel spreadsheet - which can also show in a presentable way the results that you are looking for.

The financial calculators are now used in more informal situations: meetings, discussions, workgroups, etc. In that setting, there is still a need for a financial calculator.

We currently sell the follwing financial calculators:

  • HP10bII+
  • HP12C
  • HP12CP
  • HP17BII+

In addition, we typically have the oder HP17BII units from the Pioneer series: a different feeling and a still very valid calculator.

Tomorrow we will describe the different units

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New Product: 4 x back case repair piece!

Some of you guys have several calculators to repair and have asked for a better price if you buy more units. Also we wanted a more convenient way of selling and producing these parts. So we decided to join them and sell them together. You can then cut them by the crosses and use them as individual pieces. Exact length of the cross is not critical.

repair piece

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You can now order the Battery-ports assembly!

Just wanted to tell you guys that I have fixed the article Battery-port assembly. Apparently it was configured with options, but no option was activated and it couldn't be bought!

You can find it here :Módulo conector de baterías y puertos de expansión de las HP-41c/cv/cx

Sorry for the inconveniences of those who wanted to buy it!!

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The beauty of RPN

A short one (very far from the long posts involving repairing manuals):

One of my sons is learning to do divisions, and he has to practice. So I took the HP41CL to generate random divisions. I always have among the modules virtually plugged in the system Sandmath 4x4, which has the function RND (I think it is there - the only thing for sure is that having only the advantage pack and Sandmath, it DOES work), so I made this short program in all of 1 minute, plus other 15 seconds to assign it to a key:



3 1E2

4 *


6 1E6

7 *


It stores a 5 digit number in X and a 2 digit in Y. My son can execute it and press x<>y in order to get the divisor too.

There is no way I could get a faster solution with a computer or any other more modern calculator!!


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Manual for Battery-port assembly


Disclaimer: this is a manual for the exchange of the battery-port assembly. It doesn’t pretend to be a full repair guide. There may be many failure modes in a HP41, and to diagnose even the most frequent of them is beyond the scope of this manual.

We assume that the machine doesn’t work because of the rust in the battery assembly, but it may be due to many other things:

- broken screw posts
- broken upper screw supports.
- rust in the main circuit

1. Do you have a) a readymade assembly or b) the piece and the flex circuit, and you need to put it together? If a), keep on reading; if b), go first to the manual for the flex-pcb.

2. Remove the battery pack and disassemble the calculator. With a flat screwdriver, pry in-between the body and the rubber supports. Be careful because below the rubber there is an adhesive band, and we want it to come up together with the rubber, to be able to use it thereafter. Also, we want to avoid damage to it. Put them away on top of a paper. If they are around your work area, they will be a pain since they adhere to everything!

3. With a Philips screwdriver, unscrew the 4 screws. Do not exert too much force - the holes are not screwed internally, and pushing it too hard will break the inner surface, rendering it “unscrewable” henceforth.

4. Pull the back side of the calculator upwards. and leave it upwards beside the calculator. Look inside: in the interior is green, with an integrated circuit in the middle, you have a “half-nut” calculator; if you see a whitish keyboard circuit and another circuit on top of it, you have a “full-nut”. The full nut upper circuit may be screwed to the screw posts with a nut, or held in place by means of pressure of the case (in this case (no pun intended) there are two cylinders protruding down from the case with the task of applying that pressure. 

Take out the black plastic that covers the circuit.

Now, take the time to observe:

- Are the screw posts broken? look into the upper and lower screw posts. It may happen that they aren to completely broken, but there are cases in which there are close to invisible cracks, that may lead to the system not working.
- Is there green rust in the circuit? if so, you will have to clean it. In our experience, there is nothing better than Caig De-Oxit D5. You have to apply it and rub it with a cotton stick. You will be surprised about how well it removes the green part. If you have a full nut, you will have to lift the upper circuit, in order to clean below it as well. Do it until all green has been completely removed. In some cases, it was so corroded that a circuit piste disappears. There are silver conductive pens that will allow you to “draw” the piste again.
- In the back case, are the upper screw head supports broken or cracked? If so, the best solution we have found is to remove the support altogether and use our brokenback repair piece.

5. Pull up the original battery-port assembly. It is removed by just pulling it up, vertically (this is important). It is not glued in place, and it should come up. Increase force carefully, and do not move it horizontally, since you will break the vertical table that hold it in place.

6. Clean everything below it. Usually it ends up full of grease - I don’t know where it comes from!. Pass Caig Deoxit through all contacts in view in the main circuit.

7. Place the new assembly on the back case, making sure the tabs in the case enter in the holes. It fits tight but comfortably.

8. Replace the black plastic. In a full-nut, you will have to pull up the processor circuit.

9. Put the back case in place. Be sure of placing the intermediate plastic piece (the one that covers 3 sides of the calculator) the right way: with the calculator upside-down, the angle of the piece seen from the port side should be like this: \  /.

10. Screw back the screws. I usually start with the upper screws, but I don’t think it is critical. What is important is to do it tightly, but not forcing it too much since you can break the 30-year old posts. And you are now done!

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Broken HP41c back repair manual with 3D piece


Instructions for use of back side repair piece.

1. Be aware, before everything else, that the piece has a right side and a left side. This is due to the space for the tabs in the calculator that keep the battery-port assembly in place. You need to be aware of this at all times - more so if you want to cut it as explained below.

2. Decide whether you will replace both sides or just one. The decision is easy when both are broken, but if one is not, you'll have to check whether the old standing support will withstand the pressure for long. Our recommendation is to replace both sides. If later one of the sides breaks down, the repair will be much more difficult and probably less solid. And you have already paid for both sides!!

3. If you are going to replace just one side, cut with a hobbyist scissors just the side you don't need, according to picture enclosed, again, be careful of cutting the side you dont need, since the piece has a single correct orientation!. The rest of the piece will serve to increment the glued area, better align the piece in place and also to better support the battery-port assembly.



2. With a Dremel machine, file and level the area that had the support. You don't need to remove the tabs in between the two screw post, but you need to remove them if they are twisted or half-broken.


5. Put the piece in place. There is only one "right" way. The cross part is tight because it is needed to properly locate both holes at the extremes. Do not force it. Please make sure it is not bent downwards. Make sure it makes good contact. Remove the piece.

6. Put some glue in the bottom of the piece. Do not put too much or it will go out when pressed. We have used E6000 black glue with good results. Do not use cyanoacrylate glues. Put the piece back in place. Press and remove with a stick or a blade the excess glue, if any. If the screw holes are partially obstructed by glue, pass a screw in the opposite direction (to avoid separating the piece from the back side).

7. Secure the piece in place with a pair of clamps - lighter pressure like clothes' pegs will not be enough. Be careful that there is no excess glue from the other side of the case and the clamps are adhered to the piece! Leave for 24h for the glue to cure. And you're done! You have a repaired back that will withstand a lot of abuse again.


8. The piece is designed to accept both the original and the new battery-ports assembly - so you need to press it down to have it in place. The screw holes should fit a little bit tighter than in the original calculator, but should be perfectly aligned with the assemblies and the screw posts below.


Also, if you're buying from the US, it is maybe better to buy it directly from Shapeways there (just click on the link)



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Questions about the new pieces for repair

1. The battery-ports assembly includes the base piece and a flex circuit expertly adhered by our expert. According to him, the adherence over the nylon material used in thos piece seems to be better than on the original's material!

2. You can order the nylon piece alone at the price of 26 € each plus postage (while the finished piece includes shipment too within Europe), but then you'll have to paste yourself. My experience tells me that it is worth to leave it to the experts!

3. The battery-ports assembly fits tighter than the original piece. This is made on purpose: we have seen some instability on original pieces, so much so that it is predicated to assemble the calculator with the port covers in to make sure that the assembly doesn't move. This is not the case with the new piece.

4. The small piece used to repair the upper screw supports fits perfectly under the repair piece - it is DESIGNED to be like that. The gaps and the slots it has fit perfectly into both the original and the new assemblies. In fact, it was designed to improve it support in case the original calculator small stands are damaged

More questions? please ask to


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Further pictures of the backside repair pieces

Here are further pictures of the repair pieces once collated to the back side:


It also serves as guide for the port-battery assembly. It will also help in case the vertical supports are broken (another typical failure, although not as damaging)

Look ar how it looks when assembled:


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A repair solution for the back side of the HP41c family

If you have bought a HP41C, chances are that you have found a machine that has a broken back half. Typically, this happens when the top screw supports break under the tension required to bring together the two halves, putting presure on the foam under the flex circuit. A strong pressure is required for the calculator to work properly, as all of us that have assembled calculators remember.


The problem when repairing the broken supports is that it is very difficult to paste again the broken pieces - and when you do it, they tend to break easily. When you see a completely broken post, it is easy to see that the adherence area is very small. So we decided to find a solution. We designed a piece that can be adhered over a wider surface, that can be used for both sides or one side, and that can integrate woth the original battery-port assembly or with our new 3D-printed model.


This is how a partial repair looks like: first we cut the part we need:


Then, we need to remove all rests of the original screw support with a Dremel machine or similar. Then, we put the part in place. The way it is designed, it can only be put on way and it also serves to keep it in place. Then, we paste it and keep together with a clothes peg or similar overnight. And this how it looks with the repaired piece:


The part will fit under the battery connector module. Also, the way it is designed, it helps to keep a strong connection between the module and the circuit, even if for some reason the glue doesn't hold.

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A brief description of the HP41CL

We have now 3 more circuits to be built on HP41CV bodies. These were just uploaded with the very latest versions of all modules.

The news is that we have a Time Module available to create the ultimate classic calculator.

The first order will be shipped with an original HP41C oversized vinyl case. Next units will go with leather-like new cases.


For those that are not aware of the capabilities of this machine, here are some of its features:

  • All features of an HP-41CX except for the Time Module. CX Time functions (the soft- ware) are included, but a Time module plugged into a Port is required for full timer functionality. 
  • Full 600-register Extended Memory is built in.
  • Over 295 plug-in module images are built in. Functions are included to allow these images to be virtually plugged into a calculator Port and unplugged from a calculator Port.
  • Turbo mode, which allows the calculator to run at up to 50X normal speed. Actual values available are 2X, 5X, 10X, 20X and 50X.
  • Several empty pages (4K in length) of Flash memory are available for non-volatile storage.
  • 122 pages (4K in length) of RAM are available. All RAM is continuously powered.
  • A sophisticated Memory Management Unit (MMU) allows full access to the large physical memory.
  • Full bus compatibility for the Ports, allowing the use of any peripheral designed for the HP-41 system.
  • A full-duplex serial port is available when the optional serial connector is used. This optional connector uses a 2.5mm stereo jack mounted in a blank port cover.

Please see all options in the HP41CL page. Prices start at 625 EUR.

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