Calculator blog

Musings and comments about our common interest


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New design in for testing


Up to now, we have created two parts (among others) to repair the back side of the HP41c calculator. One is the brokenback 3D part. It is used to repair the often broken screw head supports in the upper part of the calculator. The other is the battery-ports assembly. It fits on a normal calculator back and also on a brokenback-repaired calculator back. 

However, sometimes I would like to have more adhesive surface to the brokenback part. Sometimes the destroyed part of the screw head support is so big that there is limited spece for the adhesive to bond.

My friend Ignacio Sánchez suggested to make both parts a single one, that would be pasted on a back side that has been flattened and cleaned from debris. This would be glued in place, therefore making it a permanent repair,

Here is the current piece: you can see the holes in the screw area, that mimic those of the original part:



And here you can see the new design. It can, obviously, be improved by closing the hole areas not used by the existing tabs of the back side. This will be the next redesign ater testing.


Now the parts are being 3D-printed at our supplier. Both will coexist - I'd rather prefer to make a repair that I can undo, or re-repair without much hassle - but sometimes the glued parts is what the doctor orders. Also, 100% testing becomes fundamental to make sure that it works in your calculator! Stay tuned for the formal announcement when the tests have finished


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Flex circuit installation for the HP41c calculators

We offer two products to repair the frequent corrossion on the battery contacts. The most frequently sold is the complete assembly - it is ready to be put in the HP41c calculators as it comes. This is the version we recommend to our customers.


However, some of them are more adventurous or more daring, and that decide to go the extra mile and assemble it themselves. for them we have the naked Flex circuit. 

However, I'd like to give some comments from my experience of assembling more than 60 units myself:

  • If you paste the circuit onto the original plastic piece, be aware that sooner or later the contacts will peel off the plastic. There is a solution for that: using our set of 3D printed pieces, the circuit is secured mechanically to the support, with fixing pieces and a total of 8 screws per part.
  • You need first to remove the original circuit. You need to use a X-acto type of knife, and remove the rivets that fix the circuit to the plastic. I usually then cut the circuit by the contact area, and use the freed area to grab it and pull out the rest of the circuit.
  • Clean well the plastic piece from the grease that usually can be found inside the calculator. 
  • Pre-fold the flex circuit:
    • On a table, lay the circuit with the contacts side on top and the while paper down, leaving the funnelled part far from you.
    • image6-2
    • Fold upwards and away from you, and create the two bends that will be the part between the four module contact tabs. These folds should coincide with the short sides of the two circuit opening. Do not fold anything else yet.
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    • Turn the circuit face down. You have now the white part facing up.
    •  Partially peel the paper until the second bend. fold it there.
    • image9
    • Start adhering the circuit to the plastic from the extreme, in the side farther from the battery contacts. Ensure adherence everywhere with a non-sharp tool.
    • image10
    • Fold the circuit by hand on the top of the first tab, and push the bent part into the "valley" between both tabs. With the tool, press the bends you did at the beginning against the "corners" of the valley. The white part still adhered to the circuit should have helped you entering the circuit in the "valley". 
    • image11
    • image12
    • Now you can remove the paper altogether. 
    • image13
    • Bendthe circuit over the second set of tabs. Ensure adherence with your tool, and create the next bend.
    • image14
    • Now paste the battery contact part to the small contacts tabs. 
    • image15
    • Paste the rest of the circuit in between
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    • You have  finished! Now let's ensure it stays put: you can try with memory modules in all four tabs, to make sure the circuit is pressed against the plastic.
    • If you choose all our parts, including the base and the two fixing parts, you can further secure the circuit with a couple of pieces with screws:
    • image17
    • Leave overnight

And you're done!


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Assembled flex and standalone flex circuits back in stock

We have just received a batch of assembled and raw flex circuit.

Therer is a new development created by Ignacio, our chief engineer. All flex circuits can lose adherence in hot or wet situations. This is critical on both extremes of the flex system. Therefore, we have created a several holes in each side and then we screw micro-screws on both sides of the flex, securing it forever in place.

Also we have further modified the base plastic to remove the last bending, the one that makes it more prone to peel off.

In a further blog I will post a picture of the screwed unit, with the flat "escape" bezel.

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New drop-in solution for port and battery connector

We know that many of you are put off by the difficulty of changing the battery connectors, due to the fact that you need to perform several steps:

  • Buy the flex circuit (that's the easy way)
  • Disassemble your calculator (easy too)
  • Peel the corroded circuit off
  • cut the rivet heads and clean the plastic from dirt and rust
  • Pre-fold the flex circuit to suit the plastic shape
  • Carefully adhere the circuit to the plastic shape - usually the most difficult and nerve-breaking part
  • glue the borders with a better, more resistant adhesive
  • Let the glue dry and finally place the assembly on the back side of the case
  • Assemble the calculator and pray for the whole to work.

For some time, and taking profit of the experience and steady hand of Ignacio Sánchez, we have offered preassembled kits: we took the plastic piece of a non-working calculator and performed the above steps - so that you did not have to. These parts sold immediately. The problem with it was that we were limited to the number of non-working calculators we could find - and that dried up very soon.

So we set up to create the plastic piece ourselves. We had to get familiar with the new CAD design programs - having used Autocad 25 years ago, there was a lot to be refreshed! Then we carefully measured the piece with a caliper, and once finished with the drawing, we sent it to a 3D printer company. Now we've finished the testing phase and are ready to deliver the first units.

IMG_3850This process has given us the chance to improve on the original (with softer bends and more extended adherence area), but also to create an alternate version (which can be seen on the left). If you have repaired several calculators, you surely have received one with the back side broken: the support for the screws in the back side has cracked. There is no repairing it with any kind of glue, as you know. Well, one of the versions of the battery assembly has a wide flat area, so that it can be glued to the back side and solve the problem with the broken calculator backs! The head of the screw will rest on the flat side of the piece, and the back side of the calculator is held in place due to the force of the glue over a wide area (not just slim borders as would be the case with the original piece)



Each unit will be sold for 62 € - with VAT and shipment included within European Union.

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Repair kits back again!

This is only to signal our customers that we have again some units or flexible circuit for repairing the battery bay of the HP41c family of calculators.


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HP 41c battery bay repair kit


One of the most frequent requirements we're getting is to provide assembled repair kits for HP41C. The problem is that we depend on getting used, destroyed HP41c calculators to peel off the old flexible circuit, clean it and file off the rivet remains, and adhere the new flexible circuit to the plastic base. The issue is the first part - getting the part to perform the work.

Once you have used the part, your calculator cannot work - so you can only get it from damaged calculators that you're going to use as part donors. This is the reason that we don't get more of these assembled repair kits. 















On the other hand, it really saves a lot of time and patience to avoid folding and collating the sticky flexible circuit - I always wonder whether it is going to work properly once finished. The assembled part takes the doubt off the process!

I will post here a message once I get more assembled repair kit parts.

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tips on using the HP41c repair kit

Some hints to properly use the HP41c repair kit:

  • Before all, handle with care. This is a very thin circuit that will not withstand too much handling (as the circuit it substitutes, by the way)

  • You need a thin, straight, acute but not sharp object to perform the bends around.

  • You will also need an object to press the circuit around the above object. it should not be too hard or rough or it will damage the circuit

  • You have bends in both directions (convex and concave) so a good order is required if you don’t want to have impossible bends! Think before doing them. In particular the concave bends between both tabs that form the contacts of the modules need to be done first in my opinion.

  • you really need to punch the circuit in between circuit pistes with a needle before starting to bend. This has the effect of weakening the circuit support, making it easier to be bent in the line defined by the perforations (make sure you use also your sharp, straight object to line up the holes). Use a magnifier glass, since it is very easy to punch in a circuit piste: the pistes are very close one to another.

  • The surface of the “donor” battery pack needs to be perfectly flat and clean, if the adhesive is to work properly. You need first to remove the old circuit. it is fixed by small plastic rivets. While the circuit may be torn away easily without damaging the plastic below, you really need to remove all these rivet heads to the base, with a small, sharp blade. Sand away the rest until the surface is flat. It is the only way to ensure that there will be ample space for the adhesive to work. Also, be sure to remove all debris left by your cutting and sanding.

  • Now, this is the most difficult part. Remove partially the protective paper; start adhering in the valley between the two module connectors. Be sure it is well aligned longitudinally too - a common error. Be sure that as much as possible of the “base” is touching the plastic. The challenge here is to avoid the “sides” adhering before the “floor” is perfect. Press the corners down with a cylindrical object, like the metal pole of a small screwdriver. Above all, don’t use the tip of the screwdriver to push it down, or you will tear some circuit pistes.

  • Now you can adhere the rest.

  • while the adhesive is quite good, enhanced durability can be achieved by pasting the extremes with a stronger glue - the one you’re using will be good enough.

  • Now you can give the “finishing touches”, by bending slightly the battery contacts around its support, so that they mimic close the original battery circuit.

Please write me to if you find any problem with it!!

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