Calculator blog

Musings and comments about our common interest


Published on by

Testing new repair method for HP41c lower posts

I am now testing a new method to repair the lower posts. Do not worry - it does not change substantially over previous methods - it is just a simplification.

I hate gluing. I have always hated it. I always made a mess of it when I was a child and I was assembling aircraft models. A disaster. And I have improved with my skill, but just a little, and I still hate it.

The piece we designed for the lower posts had a grip on the lower part:

Captura_de_pantalla_2024-05-21_a_las_10.05.52This grip has the thickness of the calculator keyboard circuit. If we drill the holes with a little circular motion so that there is a small room below the circuit for the grip to hold, and if we use our long lower screws (this does not work with original screws), and we press down when we screw the calculator together, then we mya not need to glue the piece down!! The piece was designed with a small inward angle - and the grip gets expanded when you introduce a long screw, pressing sideways and securing the part.

When gluing the piece down, you need to be extra careful to cover all small keyboard holes. If these are covered by glue, that key may not work and you can only disassemble the whole keyboard. If we do approve this method, the whole gluing process is not needed and there are some pieces (the keyboard support and the clamps) that are not needed anymore!!!

I also would have to slightly modify the upper post repair part to give it a stronger grip too. Let's see if this works.

You can test with your HP41 during repair - it is a non-destructive testing and you can always glue it if not satisfied!!

Read entire post
Published on by

A new glue for HP repairs


IMG_0583As you know, i have been using E6000 glue for some time. However, the producer has reformulated the glue due to regulatory reasons and there is no more E6000 - there is only E6000+. This is not, by any means, the same product and I cannot recommend it. It does not withstand the same forces or stresses as the original. I have had to re-do several repairs and I was looking forward with anxiety any repair that required gluing.

For some time I have been using epoxy 2-component glue. This requires mixing it properly before applying. You consume a lot, and then when you apply it, the layer is thick - in some cases, far too thick, and it cannot work properly (for example, in lower post repairs). I had to redo several repairs due to the excessive thickness. 

A customer that is also into modeling referred me to a small German company: SCHWANHEIMER INDUSTRIEKLEBER, and their product Industrial Adhesive no.100. They also recommend a primer for difficult plastics - of which our calculators are made of.

This material hardens immediately under pressure: you apply a layer on just one part (it is extremely fluid so be careful) and then press both parts together. If the parts are from our damned calculator bodies, please use also the primer and let it act for at least 10 seconds before applying the glue.

As the material is very fluid, the layer is very thin, and with a good pressure between the parts, you get a very strong bond, and it looks far cleaner than with previous glues. I am considering trying to fix the domes directly - without using the parts we have developed. Let's see how it works over time and we'll be back here with the results.

These glues are not cheap, but they are consumed sparingly (the layers are thin), and the result has been impeccable for the time being!

Read entire post
Published on by

HP41 repairs

I am repairing more and more HP41 calculators lately. I have a backlog of more or less one month - and calculators keep coming. We offer this as a service but do not see it as a business, since it is not "scalable": I have only so many hours, and they cannot be increased - except at the expense of more important things to me. 

My opinion is that most of the repairs could be done by the user himself, and only the very special cases be sent to us. Really 70% of repairs do not require specific skills or unavailable parts. Then 20% may require screen or processors that are not available - and 10% cannot be repaired at all.

We have added a couple of pages to our book "HP41 Repair", which you can find in Amazon - we really think with it you can repair most of the calculators. 

One think that is true is that more and more the plastic of the hp41 calculators is becoming brittle and more difficult to repair. The material does not seem to age well, and lose the plasticity it once had.

Read entire post
More about: HP41 Repair
Published on by

File away these solder peaks!


I have been doing some repairs lately, and I have seen some full-nut modules that did not want to work. In the past, I have always classified them as defective and go with another. 

However, this time it was a CX module. This are far rarer and very expensive to get. What's more, I saw it working on-and-off before the repair. (It was a lower post repair), so it should work - but it was not working in the original repaired calculator. Others were working, thoug, and I also managed to make work the CX module on another not repaired body. What could be the problem?

Here is a picture of the lower post repair piece:


I have designed a couple of pads on the left and right of the screw posts to allow for more surface to "glue" onto the keyboard circuit. In principle this surface does not collide with any part of the circuit. In practice, it does: there are some solder points that are a little bit too high - and these press onto the pads and avoid a good contact with the zebra. I have discovered that last week and filing them away has allowed me to "save" this CX circuit - and probably many others goign forward.

The ones to file are the ones at the right and left of the connectors, and sometimes also the ones between both connector lines.


This may also happen on some cases with non-repaired units when using the zebras over the old support: I see the marks of the solder points on the surface of the film connecting the two parts of the zebra. 

I am updating also the book with these comments.

Read entire post
Published on by

Glue for HP41 repairs

In our book we insist on using E6000 glue. This product has been stopped and replaced by the E6000+. The reason for the change is a legal (safety-based) one: there are components and odours that may be hazardous to our health in the old mix. (You can find some in Ebay still - go grab it while it lasts.)

Unfortunately, the E6000+ does not work for our application. I have used it for a low post repair and for a glued battery assembly and it does not hold for too long. In the low post repair, it is enough to impede proper working. The glued batttery assembly did not allow for very tight screwing. The material is too flexible and not adhesive enough for the material we are using. Even if I am too repetitive: E6000+ DOES NOT WORK WELL FOR OUR APPLICATIONS!!

We have heard a lot of good things about the epoxy 2-component glues, and we re testing, but the challenge with these is that you need to do a previous preparation of the glue, by mixing both components and applying the paste onto the parts. I have run some tests this holidays and I am waiting for the glue to cure to make some strenght tests. I will report once I have tested the samples thoroughy. Again, it is a pain compared with the plain E6000 !


Read entire post
Published on by

HP vintage calculators prices in Europe


I wanted to discuss briefly the current price situation for old calculators in Europe. I don't know if this situation is temporary or it will just increase over time. 

We all know that calculator prices have been increasing during the last 2-3 years. I just did not realized how much. 

We do repairs for our customers, and for the HP41 family 90% of the repairs are quite easy and straightforward with our tools and parts. However, something that still requires an original part is when the screen is defective. It may be the LCD, or -more frequently- the display drivers. In these cases, the screen does not show anything or it shows the screen only when pressing a key. According to the service manuals, you need to change the screen then. Your otherwise perfect calculation cannot work unless you find another screen from a donor calculator. (apart from the difference between half-nut and full-nut calculators, the full-nut displays are largelly compatible with each other, save for a little capacitor if you're to use one of the very ealry screen with a later calculator processor)

So, we are always looking for calculators to act as donors. And for that purpose, it is better to find a defective calculator (if it were working well, why use it as donor? you just use it!!)

When I found a lot of 3 defective calculators in eBay Germany, I said "Perfect! I have most likely a couple of screens to repair and maybe other spares too). Be aware that these calculators had already been stripped of all port covers (which are the easiest and nices spare you can get from a defective calculator), so I said "OK - I can offer 150€ tops: two of them are C (for which I have already many processors) and there is only just one CV - for which the processor has a little market value. Then the interesting part are the screens! Let's assume (since these are sold as defective, never forget), that 2 of them are fine, the other really defective. I would be paying 150€ for 2 screens and a CV processor. One of the bodies is tall keys, also - that may have a slighllty higher value. I have plente of bodies without screen, so the body in itself has no value to me (unless it is tall keys, of course). So I bid 150€.

You have here below the results of the bid: 330€! This is crazy! This makes buying an old calculator in Europe (seems that the US is somewhat different and cheaper) totally crazy and you really need to repair your own calculator. The secopnd thing is that we need to say goodbye to calculators that have defective screens - it is just not economical to buy defective calcs for spares, at the price of roughly 100€ per screen (not counting the work of unsoldering and soldering back in the calculation that is being repaired). A pity!


Read entire post
Published on by

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.
Read entire post
More about: HP41 Repair