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Calculator blog

Musings and comments about our common interest


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Valentin Albillo's new site


While taking a look to Eric Rechlin's site, I have learned that there is a Valentin Albillo's articles and documents site within too.

Valentin Albillo was one of the original HP calculator users in Spain - together with Fernando del Rey. He wrote a lot of articles and quizzes on all kind of HP Calculators - from the early ones to the HP71. He even collaborated to write Spanish-speaking HP application books (for example, for the HP34c calculator.

If you are interested in something more than the hardware, you're doing yourself a disservice if you don't add Valntin Albillo's site to your favourites!

Some personal highlights:

But there are literally hundreds of them!

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A comprehensive HP literature site!



Eric Rechlin, of fame, has created (with the help of many but mainly based on his own gigantic effort ) what can possibly be the biggest HP calculator related document site.

You can acces it in this URL:

You can filter by language, author, source, related model and free text. Well organized and including many new scans. The list of documents is in continuous expansion and lists more that 1180 pdfs.

I have checked the quality of some of them and it is magnificent! I am far from checking all of them, or even a sizeable part of them - but many of those I have tested have been OCR'd, so that you can look for specific text - what would not happen on manuals taken as image only (many of the manuals that I have been using in the past). The difference is huge - mainly when you are looking for something in particular.

I wish you enjoy your visit to Eric's site - and that you can find what you were looking for!

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Current projects


We are now designing an adaptor to put two AAA batteries in a Spice machine. The Spices were the HP32E, HP33E, HP33c, HP37E and the HP34C - the top of the series.

The Spice machines were designed as low cost compared with the HP65-HP67 series, and as replacement of the Woodstock series. In opinion of many HP experts, too many corners were cut to manufature them - several internal connections rely on pressure exerted by the screws that close the box - while in previous units the box screws did not have circuit integrity functions. Therefore, these machines have higher fatality rate than other models.

One of the issues that plague the HP Spice is the battery bay. The original rechargeable batteries wear down, but the problem for the replacement is that the normal AA size is 1-2 mm too long for the machine - and you break the machine in the medium term. Ignacio Sánchez suggested to design an adaptor that used two AAA batteries - where the size challenge does not exist. 

(continuing with the story: after the Spice series, the lower range was inherited by the Voyager series - for many, the most perfect calculator design ever, with the HP12c and the HP15c as the main examples)


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More about: HP Spice, HP33c
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HP41c upper post repair


We have applied the same solution that we created for the lower posts to the upper posts: a grip that clips on the keyboard circuit, and strengthtens the grip with the introduction of the screws. 

In the case of the upper posts there was a requirement to drill anyway - so it doesn't change the original procedure. But the piece has the same 3-petal grip that we use for the lower post:


The connection strengthtens when the screw is put in, since it makes pressure to the sides of the post and makes sure it doesn't move. We plan to put it in production already!!

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More about: HP41c
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HP41c Repair Spree !


During the last week I have tried to repair (or install CL circuits on calculators) many calculators: 17 of them, of all types and flavours!

Again, repair statistics are quite stable, but there is a jump between 1st and 2nd:

  • Most frequent damage is broken upper screw domes in the back of the calculator. This happened in 13 out of 17 units!!
  • The second more frequent is corroded battery bay connectors. This happened to 11 of the 17 units. The corrosion also spread to the main board in several of the cases, but in all units I was able to remove it with our trusty Caig Deoxit.
  • The third most typical problem is broken lower screw posts. It happens to 7 out of 17 units. I have used exclusively our new piece, which penetrates below the keyboard circuit surface and grips on the sides. The design so so that when the screw enters, it presses harder to the sides, the same way a wall expansion screw works (plastic+screw). I think it doesnt make sense to continue with the old part - but let's see the durability of this new one for a while before making the change and discontinuing the old part.
  • Fourth in line is screen problems. So far 5 out of 17 have this problem, in several flavours:
    • ​a couple of units with a LCD with black areas
    • Not all segments lit, or not well enough.
    • Damaged screen driver. This can be seen when the screen lits only when a key is pressed. 
    • broken screen
  • ​Some other repairs involved changing the zebra connector - but most of the units used the double-zebra original piece, which is much better than the other two original zebra types and can be reused unless corroded.

​​All in all, a good week where we have managed to repair 13 of the units, there are still 3 to go and one completely unserviceable! Some have already been sent to their lucky owners.

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corroded connections


caig_deoxit_100Now I have a pile of calculators to fix. Many of them have corrosion points in the circuits and -above all- in both sides of zebra connectors. 

No matter how thick the corrosion layer is, our recommendation is to put some drops of Caig DeOxit 100% (although other formulations (5%) may work too) and let it act for a little while. Then you can remove the crust of corrosion with a flat small screwdriver. You will be surprised because most of the crust will go and you will see the yellow colour of the copper below!

There are some areas where the corrosion is more difficult to remove: the parts where the main current happens. The additional resistance that builds up with the initial corrosition creates some heat that burns the debris in the metal path and makes it much more difficult to remove. One of the main causes is the "dust" that comes from the disintegration of the fabric inside the vinyl cases. 

(By the way: it is a good practice to wash the vinyl cases to remove the dust. Use normal soap and doit by hand)

This product barely leaves any residue, and although it is quite expensive lasts a lot (if you use it drop by drop). 

This product is typically used in audiophile circles to improve the conductivity of the Hi-Fi connections. I discovered there but it can be used in any case the connection is not good. It is one of the products you feel proud to recommend, like in "you will thank me to bring this to your attention.

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HP82104A Card reader repair


We have detailed with pictures in our book how to repair the most typical card reader repair: the "gummy wheel" problem. This appears when the original material that was used to move the card disintegrates into a gooey mess. This is covered both in the book but also on a number of other websites.

libro_cardreader_-_29We have found also another type of damage: a similar disintegration in the material between the engine axis and the endless screw that transfers rotation to the wheel. It shows in the noise of the transprt (which is the endless screw skipping over the engine axis). This will appear in the next version of our book. It basically consists on:

  • pulling the endless screw away from the engine.
  • Clean with a needle the inner part of the endless screw. There is the material that has dried up and is now skipping.
  • Fill completely the cavity with E6000 glue. Let dry for a very short while (15 seconds)
  • Join again the endless screw with the engine axis. Make sure that the glue does not reach the engine body! Remove excess glue.
  • Make sure the ensemble is straight and let it dry fro 48 hours before installing it again.
  • Ready!  Now it'll sound much better, and it won't skip.

I will include pictures once I have done another fix.

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Interesting units arriving!!!

Some interesting units arriving! Some will need repair, but altogether very interesting details! a family picture:


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New low post part

We have been using for already a couple of years our lower post repair piece. It is designed to replace both lower posts of the HP41c, since these are often broken in the full-nut HP41c clculator, and which are key for the calculator work since they hold in place and under pressure the nut processor circuit. The idea for the part was to have a single piece for both posts - the attachment on one side provides torque resistence for the other side and vice-versa.

This has been quite a success but it requires good alignment of the piece with the original holes, and it depends heavily on the glue quality. If the pulling force was too strong, we risked to have the piece off the keyboard circuit. This is despite having given a wide gluing area. 

We have created a new low post repair part that requires some drilling of the rest of the original post, and introducing "claws" inside the hole. The piece is designed so that when the screw enters in, it pushes the 3 "petals" of the claw to clip below the keyboard circuit - making it much less dependent on the glue!. It is really strong - the longer the screw the stronger it is. Definitely an advancement over the previous method. What you lose having to drill deeper the original posts, you gain ensuring a perfect alignment, and a much better resistance to pulling forces.

Here are some pictures of the piece (sorry for the bad lightning conditions!) and 3D-renderings too:


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Several HP41cv and cx - Half nut!!

HP41 half nut units: CV and CX 

We have received some calculators: a couple of CV units and a CX unit - all of them half-nut. 

What is a half-nut unit? The original HP41 calculators were build with two circuits: one for the keyboard and screen and another for the processor. All parts were soldered through holes, the "old way".

Around 1985-1986 HP had developed far better circuit-making, and they developed a single circuit, surface mounted with just one IC. This model (which was done as C, CV and CX too) was called "half-nut", and the previous model was the renamed "full-nut". Performance of both is identical, but half-nuts have several more "failure modes" than the full-nuts!

We test all units with the test pack of the HP41CV Service module.

It is an opportunity to get a dependable, tested and revised HP41CV  or HP41CX with the full set of memory, 

You can add some of our modules that you can find in the HP41C corner.
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More about: HP41cv, HP41cx
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