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Calculator blog

Musings and comments about our common interest


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Classic charger substitution

Some customers have asked for a solution for the Classic battery. What has been suggested is something similar to the Woodstock battery holder: a 3D piece that can handle 3 normal batteries (still to be decided if we can fit AA or it'll have to be AAA) and that integrates the contacts with the calculator. All in the HP original black and with the usual HP classic fixings. 

Anyone interested? Just to know whether to devote design time to it or not!

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zebra utilization

The zebra comes with a holding harness to avoid damage. In addition, it comes with a couple of staples to hold it together too. But in order to put it in the calculator, you need to remove both the staples and the holder.

The zebra can be used on the previous support in the calculator, or even without support. In case of damaged hardware, it can be removed (provided that the screw posts remain), and the zebra be put on the screw posts - it will work all the same! The plastic of the PCB circuit is far more resistant than the original, and the connectors can handle much better the current than any elastomeric zebra (included the white ones found in some HP41c).  


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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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Broken back piece vs integrated "flat" assembly battery part



As you know, we have two methods to repair the ubiquitous upper screw broken head support: the broken-back piece and the integral assembled "flat" piece.


If you have an original, working battery module, the only possible solution is to use the brokenback. Be sure of 1. putting a good deal of glue, and let it cure for at least 24h - 48 is better. And then, you will be able to assemble a number of times - but not many until it peels off again.

Now, if you can use the flat piece:


Then there is a much wider surface to adhere! it usually is a better solutiion. I'd first test it with the module in place but not glued, and pressing both calculator halves together with the batteries just to chekc it does work! Then you can adhere with E6000 glue, and wait 24 to 48 hours to cure.

Usually this is a better solution, since the piece holds better. I prefer it, but then I have more back in case something goes wrong. So far it has never happened.

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Zebra may go alone


We have received some inquiries on whether the zebra PCB assembled requires the zebra-holder. The zebra holder piece was designed to hold the separate zebras you can find in older calculators: the golde ones, not the elastomeric ones. If you find these, it is very likely your calculator won't work when reassembling - you'll have to replace it.

When you are using our zebra, it can very well go alone, without any support. It is strong enough to keep shape no matter what:


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Just learned that Systemyde will start another lot of HP41CL circuits. We already have two excellent calculators for conversion - and we will get others in time for end of January, which is the expected arrival time of the new circuits!


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new zebra PCB


When you are repairing full nut units, you may find different zebras, that may or many not be corroded. Here is a picture of the three different models of zebras, plus our own:


Today's message refers to the first set of zebras. On one side, they are the least common; on another, they have also the highest resistence, which is good enough for a calculator to work, but makes very difficult to operate peripherals - and in some cases virtually impossible - in particular the card reader - that power hog.

If your card reader seems anemic, this may be the reason. The second pair in principle should work - but I had unconsistent results with it. The third is the best of the originals, and works every time unless it is corroded or broken.

We have found that the replacement we have designed works every time. The connectors are exactly aligned with the upper and lower contacts, and there is no way it cant work unless corroded: no mobile parts, very solid and thick circuit paths, and there is no topological way for it to unfold after being put in the machine.

You can find it here:

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An early HP41C converted to CL

I have prepared an early HP41C (serial number 1944A...). I write it here because initially I put it on sale with not good enough pictures, but now it comes in all its beauty.

It is so early a model that it has tall keys and the original, early metal plate keyboard surface (not textured plastic)
















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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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HP Repair book


Finally it is here!! 133 pages of our repair experiences and methods. Thanks also to the help and advices of Ignacio Sánchez, who suggested many of the repair methods and helped testing them.

You can click on the link below:

HP Repair book in amazon

It is also available on all amazon country sites.

Here is the chapter list:

1. The HP41C family of calculators
Some History
Special features
Serial number and places of production
The HP-IL loop
2. Identify your HP41
C, CV and CX
Full nut and half nut
Tall keys and normal keys
Screen revisions
Special models
3. Tools and Parts
Tools - Required
Tools – nice to have but not required
Contact enhancers
Conductive inks
Other tools
Spare Parts – replacement parts
4. Modes of Failure (or typical problems in HP41c Calculators)
Types of failures
5. Opening and Closing the calculator
6. Battery and ports module replacement
7. Zebra replacement
Installation instructions
8. Broken lower post repair
Material needed
Use longer screws.
Cyanoacrylate over the inner part of the post.
Broken post repair
9. Upper Post repair
10. Broken screw head supports
Instructions: first method: back repair piece.
Alternative Method: integral piece.
10. Corroded Circuit Traces
11. Screen Problems
Dirty screen
Screen repair
Damaged Screen Protection
12. Damaged Keyboard
13. Calculator Testing
Test with a Service Module (C or CV/CX)
Test without Service module
14. New: CL module installation
15. Peripheral repairs and other items.
Hp 82104A Card Reader
Vinyl Case
Missing port covers
Battery holder
16. Conclusion
Appendix 1: The Calculator Store Spare Parts
Appendix 2:HP41C List of parts
Appendix 3: Full-nut circuits
Appendix 4: list of software modules
References and interesting websites.
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