Posts on topic: HP41c repair

Calculator blog

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HP41c repairs!


We've just received three calculators for repair: a CL that worked on assembly but not on delivery, a C and a CV. The C is easy to diagnose: corroded battery terminals. Let's see if it is the only thing needed when we open it. The CV has a beautiful battery terminal and looks solid, without any post broken - but it doesn't work!

It will be an interesting week end...

I'll try to take some pictures to illustrate further bolg issues - also with oscilloscope output views, to compare a new CL with older models, when it comes to wave frequency stability.

Please contact us at if you need your HP41c serviced!


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Back to the office!


We have returned from holidays with a new 3D design, although unrelated to calculators - a battery door for the ubiquitous Samsung TV remote!

We will have it in different colours - as a way to distinguish different remotes. It is still not created as a product, but it is on its way.

We have several products created as 3D: here is the list so far (not counting repair parts):

- Side port HP41c

- Back port HP41c

- Battery holder HP41c

- Battery holder Woodstock series (HP21, HP22, HP25, HP27 and HP29 calculators)- Shanling M0 DAP cover

- Samsung TV remote battery door (coming)

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Development priorities


We are preparing the development plans for 2020. Your requests would be listened to, but be aware that someone insisted heavily on creating the Woodstock battery bay and we didn't get an order from her finally! So we would like to have some quorum for the demands.

My list so far:

- USB connected HP41c adaptor (not battery - putting any kind of electronics inside would require huge certification effort (or alternatively huge risks for our very small company). It will be a 3D printed part with a connector. The jury is out as to whether to include a capacitor to hold the charge for longer.

- A permanently glued battery-ports connector, to hold better together with the back case.

- Many other small items for HP calculators: screws, spacers, screen plastic covers, etc.

- Finish a HP41c repair manual (we're already half way through)

Please also submit your requests to too!

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Testing HP41C with oscilloscope

When repairing a HP41 machine, it is always interesting to know what it is happening inside - regardless of the screen being blank or not. Using an old memory module, we have connected the pins with a ribbon connector, and then this connected to the oscilloscope we can see several things: is there the positive voltage? when you press a key, does it appear on the interface?

The system is designed so that when a key is pressed, there are two lines ( ø1 and ø2) that output a synchronizing wave - delayed some time the one versus the second. This is a tell-tale that the circuit is working - regardless of what happens to the screen.

Here you can see in the case of a CL just installed (thanks Mr. Lerich). The circuit is outputting 360 Khz, much more stable than the usual 343-365 that you see on normal calculators. The ringing at the corners may be due to the fact that the cables were not terminated properly and therefore there were reflections. Also it can be seen the induction effect on the rises of one curve on the other, due to the parallel 150mm of the ribbon cable.


If we see it closer, we see the rise-time of the curve is equivalent to 25 MHZ - quite a high bandwidth, even despite the mentioned termination issues. Also the stability of the voltages seem very good. Both are testimony to Monte Dalrymple's good engineering!!


We need to get deeper in this method to be able to better debug and repair our calculators. Now we need to get a service module - or an image burned into an eprom of such.


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Feet arrived!

We have received the sheets of feet for the HP41c. Tested them and work much better than expected.

We will send the 100 unit sheet for 50€ plus shipping - half the price than the 4-unit set. Ideal for collectors and repairers. 

Please contact us at


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Four new products


We have ordered production of four new products for HP41c repair - which will help many an amateur in our beloved calculators:

Lower screw - slightly longer (⅜ in) to "grab" on the lower part of the screw post (when the thread is broken with the normal size screws). Here you can see a pair compared with an original lower post screw.


Black screw: same, in A4 steel in black. Much more expensive but looks like the original.


Full set of screws!


And a spacer set to substitute the nut often found in early HP41c full-nuts. This nut can be used 2 or 3 times before it breaks the post or the thread is unuseful. The you can put a pair of spacers. As the height would depend on your zebras and your circuit, we supply 8 different heights in 0,2mm steps: from 1,2mm to 2,6mm



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Feet for Coconuts and Voyagers


I have received a lot of feet for the HP41c family and for the HP12c family. Now all repairers will be able to get them for their own calculators. They adhere to the ground much better and look also much nicer. 

Stay tuned for tomorrrow.

There are other products coming - we'll be introducing them as they arrive!


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Repair service for HP41C

4e4da4ed-87a5-4e0b-a8c1-0e53b7642c46We have developed a number of techniques to repair HP41c calculators. These include the corroded battery bay, corrosion on the zebras in the full-nut calculators, broken lower posts, broken upper posts and broken back case.

However, we understand that to open a calculator is quite a daring enterprise for most of us - even if we're technically minded, it doesn't mean that we're DIY-inclined.

So we offer a repair service for HP41C, CV and CX calculators. The conditions are as follows:

  • Initial diagnosis is for free - but we may decide that it is non-repairable.
  • If repair is possible, the price is 40€ inclusive of all repair parts, except if a battery bay replacement module is needed, where the total price is 75€.
  • Replacement parts (back port covers, battery bay, side port cover) come in addition at 20% off the list price.
  • Owner pays back and forth shipping, and all import/export taxes.

If you're interested, you can contact us at , and we'll arrange shipping.


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Battery bay repair piece

sketch_moduleThe latest battery piece is used to repair the typical corroded contacts in HP41c calculators. By far this is the most typical repair need in these calculators.

Some years ago, Diego Diaz created a foldable, adhesive circuit that could be used instead of the original cicuit. Initially, you would peel off the original circuit, remove the plastic rivets with a X-acto knife, and paste the new circuit once pre-folded. This was a fantastic solution!

This gave fantastic results - for a while. The adhesive, while strong, would give after several months of use - sometimes up to a couple of years. No matter how it pasted and pre-folded it, after a year of use the adhesive would fail and the outer part of the circuits would peel off. Good for your own calculator, but not good for somebody else's file. 

So we set to find a permanent solution. 

After some thinking, we addressed these failures. We decided to fix both sides with screws, with a total of 8 of them. We use also two 3D printed pieces to fix these circuit ends:

On the right you can see an exploded picture of the part before adding the circuit. This makes sure that it will never peel off! It has also tighter measures than the original, and gets secured by its shape too. We still recommend to put the back ports when closing the calculator, since it pushes the module in place.



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Zebras for the HP41c

IMG_2838The HP41C full nut requires two zebra contacts betwen the keyboard circuit and the main processor. The original zebras seldon get destroyed, but in severe corrosion cases, the zebra connectors are also affected. Then you need to change them.

In there were several threads about alternatives for the zebras. We have done something else: we have custom-ordered a couple of zebra connectors that work for our application. The length, width and height are based on the originals. 

Now, the original is two round copper cylingers joined by a plastic membrane. As we don't have this membrane, we have designed a "zebra-holder" that keeps the zebra pair in place. It is made in 3D printed nylon (with a HP printer, of all brands!). It replaces the old structure, so that it can be round the zebras to keep them firmly in place. The old structure can be removed with pliers and a little force. Be careful to avoid breaking the screw posts.

If the screw posts are damaged, broken, or the screw rotates freely in them, the required piece is a different one, the lower post repair, which replaces both the zebra holder and the screw post, and that is glued in place.


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