Calculator blog

Musings and comments about our common interest


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second attempt to card reader side lock

I have received the second prototype of latch for the card reader. The first had not exactly the right dimensions. This second, while OK in the dimension side, does not provide enough spring action to have the latch "lock" in place. We need to angle more the leg (just like the original) so that it provides enough torque so that it latches into the calculator.

On the aspect side, it is good enoug. It will be sold always in pairs, so that no one will be able to see the difference between an original one and ours. Please email to if you are interested on the product!


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Card reader side locks prototypes

I have had the time to test the initial prototypes for a replacemento for the often broken card reader side locks. Still a couple of angles to fix, and also add some texture to the figer pressing areas - and it will work! It does not require metal legs to work, but it uses the relative flexibility of a the P12 nylon stud to act like the metal band. It is integral to the piece so it will not disconnect or fall during assembly - as we are tired of suffering when reassembling card readers! If anyone else is interested, please drop me a line at





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Three new products in the making

Upon request of some customers, we have developed three new products:

  • The first one, we have already discussed about: the tomcat battery module: the one used in the HP92/97 and the HP41c printers. It is bound to be expensive, because the 3D printing process makes the piece expensive. I will show pictures later when I have both pieces and assemble them.
  • A Spice battery holder: there is always a problem with the AA batteries, since they are jusr slightly too long to enter properly. So Ignacio Sánchez had the idea of using AAA batteries, and creating some adaptor with springs that allows it to fit. you can see a picture here. Some samples will arrive soon.
  • Spice_-_4
  • The last one is a couple of side locks for the card reader. These get broken from time to time, and it is a real pain to find others! There is also the added problem of having the metal springs well attached. It is quite difficult to get them in place when reassembly the machine - thus we have made them integral to the part. Let's hope that the flexibility of the part is good enough for the task. In my opinion, this was not the brightest design moment in HP's history! For the sake of manufacturability, we produce 5 pair in the same "tree":
  • sidelocks_card_reader
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Card reader repair



Another typical problem happening with the card readers is the axis' slipping. You have already repaired the gummy wheel and you realize that the electric motor slips.

Once opened the card reader, you take the motor and pull the axis out. You will see that the inner axis attached to the motor is much thinner than the outer jacket of the screw part of the axis. (I have no pictures now but will take them on the next repair). The void between inner axis and jacket is fulled with a plastic compound that also deteriorates over time. What to do?

We have found that securing the jacket on the axis with E6000 glue works too. You need to make sure it remains straight, and that the glue does not touch the body of the motor (lest the motor gets clogged completely!). Allow 48h to dry and assemble it again. What a surprise, uh?

I will have to update the book in that chapter once I have the pictures that go with it. 

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Error in book corrected

I have found an error in the HP41 repair book - in the part referring to the card reader repair.

It is about the order or the cables leading to the electric motor. The black cable is the one furthest from the calculator; the red one is the closed to the calculator. The book said the opposite (or could be understood as the oppositce - it is strange to me since I knew the right order!

Here is a picture illustrating what I say: it is the two cables on the left. The upper part is towards the calculator.


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