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Calculator blog

Musings and comments about our common interest


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New zebra development!


We have received the samples of a new design for zebras for the HP41 family of calculators. This design will solve the current issues that the elastomeric zebras have. (This part is referred to in the service manual as 1251-5731 )

Let's see it:


Initial tests seem very good - 100% connection on first try! Dimensions are perfect - what we need to find is the optimal fill for the zebra part: how thick and compliant does it need to be, and which material is the best for the application, since it has to be durable and cannot rot, corrode or dissolve. Initial tests show that it works even without filling - the compliance of the film is enough to ensure good connection. However, we will put something in, to ensure durability.

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Interesting video about HP graphical calculators


I have come across this video about HP graphical calculators, in It is worth taking a look at it if you want to enter into these calculators -which were the workhorse of all engineering students in the nineties. It is prepared by Stephen Mendes, and he promises several other videos!!


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Repair your calculator during the lock down!


Now we are locked down due to the Coronavirus global pandemics - and it is likely that we'll have to stay for at least a couple of weeks. Now is the time to start things that we didn't have time in the past to do.

DSC_7337One thing is to fix everything that doesn't work at home - and probably this includes your old calculator.

I have started to repair calculators only recently. The repairs performed were increasingly complicated, but at the end I got Ignacio Sanchez's help and we designed a series of parts that help us in these repairs. But the first you need to do is the diagnostic of the damage.

During the next days we will discuss how to repair yours. However, my goal for this instalment is to make you believe that your calculator can be repaired. In my experience , 90% of HP41c calculators can be repaired - and I'd say that 70% are really easy to repair. And this means that YOU can repair it. 

Even very destroyed calculators are repairable. Curiously, the worst calculators to repair do not show any damage. In many cases users put the wrong battery model and burn the calculator with much higher voltage than needed (for example, put four a23 batteries (12v * 4 = 48v) instead of the 6v that are produced by four 1.5v LR1 batteries. Then the screen is burned and then you need to wait for a donor calculator - something that doesn't happen quite often.

But for the rest of the repairs, there is a solution!

We'll look into it in the following blog issues.

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How to replace a HP41c screen


There are cases (black spot on the screen, screen only works for half the digits, broken screen) where the only solution is to change the screen. These are not produced anymore, so you will need to find a donor HP41c.

To replace the screen: First unsolder the two screens (from the donor and the damaged calculator) Use a small soldering gun and help lifting the pins with the tip of a plier, one by one. Important: unsolder the screen at the calculator side: it is much more resistant!!!

Then solder the donor screen back in place. Locate the screen in its hole first. then start soldering pins, with a very clean soldering gun. There is no need to add more solder. After melting the solder, leave the gun while maintaining pressure on the pin with a flat screwdriver tip, to ensure good, durable contact. Patience until finished. Then reassemble and try again. 

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Testing repaired HP41C

I have received today a fantastic tool for testing the HP41c/cv/cx calculators that I usually repair: clones of the HP service modules. 

HP released two versions of the service modules: one for HP41c and another for HP41cv/cx machines. You can read the details from the service manual:

HP41c Service manual


These modules greatly simplify testing, including memory, screen, keyboard, status, etc. We will discuss these modules and other diagnostic tools in further blog issues.

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Rechargeable batteries for HP41

Some customers ask me why we do not refurbish and repair akku packs for the HP41c family (you know, these packs that were charged through the side hole in the calculator). The reason is that there is a cheaper, more logical solution: the rechargeable N-batteries.

These can be found in Amazon:

Odec rechargeable batteries

You can also find the charger in the same page. 

I have found also other rechargeable n-type batteries in Amazon, but I have not tested them. I have bought the Odec and can attest they work fine with the recharger provided, so this is what I recommend!

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Games for the HP Prime


Games! The HP Prime is so fast (compared with other calculators) and the HP Basic so optimized that you can actually run games!

As always, you can go to to see a lot of them. 

My preferred games:

  • Asteroids: a version of the old arcade game
  • Breakout: who has not played this game? My last time on a blackberry phone
  • Tetris
  • Tetris 3D
  • Space invaders
  • Angry Birds
  • Minesweeper
  • Pong
  • Sudoku


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Battery module for broken calculator backs


If you remember, we had ordered a new time of 3D printed battery modules, specially designed to be used with glue on broken calculators back sides:


This version is intended in case of both supports of screw head broken. For any other case, the standard piece would fit better. The repair is also definitive: if it doesn't work or gets corroded, the repair would be much harder!

Here is the body for the standard piece:


The pieces have arrived and we will do some tests during this week end. We'll come back with the results next week!

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HP54201D mixed signal oscilloscope available again


As I have acquired a new DSO oscilloscope, I have put my trusty HP54201D oscilloscope again for sale.

It tests perfect as can be seen from the pictures. It also has three pods of 8 bits each for a total of 24. It comes with two no-brand probes. Excellent for debugging and testing. Price is 220€+shipping:



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Repairing calculators

This week end I repaired 4 HP41c. The pleasure of seeing again the "MEMORY LOST" label does not wear off, even after many repairs.

I found an alternate way for repairing slightly damaged upper and lower feet. By that I mean the case where it is held together well, but the thread has been broken and the screws do not bite and therefore do not hold pressure.

The method does not work on broken screw posts - only when the thread of the screws is not working anymore. The key is making either the screw thicker or the screw post caliber smaller.

In the past I have increased the thickness of the screw with white nylon tape. It improves a little but not too much.

What I have tried now is to use Loctite® type, applied with a brush, over the interior of the post, as far down as feasible with your brush. Then let it dry for a day. Then try again with the screws - now it bites again!!


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