Musings and comments about our common interest
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!
We 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:
If you're interested, you can contact us at email@example.com , and we'll arrange shipping.
The 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.
The 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 HPmuseum.org 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.
We have received a new batch of springs. Due to the quantity, we've been able to reduce the price from 4 to 3 € apiece. You can find them here
With the help of Ingancio Sánchez, we have begun analyzing the failure modes of the HP41c, and we have been developing repair techniques for most of them. In the next articles, we will present the failure modes and the possible techniques in each case.
In next instances, we will go one by one with each of the chapters. Who knows, it may end up as a book!
Lower Post repair
We have gone through many interations, both to the pieces used and the process. We think we have found a process that is durable, easy to apply and works all the time. You don't need to have steady hands to succeed!
Let's assume that both posts are broken or cracked. The effect of this cracking is that the screws do not do their job. Usually, using longer screws may help, but this typically is short lived since the cracks will continue downwards. Once this pressure is softened, the calculator doesn't work anymore. The solution is to repair the broken posts.
Old methods to repair broken post is to glue them (but this doesn't hold together too long) and to tie them down with very small gauge copper cable, applying pressure so that it holds together. This may solve some cases, but not when the screw path has been destroyed - maybe due to strong tightening of the screws.
Other repair methods have been proposed, including drilling down the screw and placing in its place a cylinder with screw path, glued in place. We tried this method but there was a problem: the torque used to screw resulted in cracked glue and the cylinder separating from the substrate - every time.
Our method was devised with three goals in mind:
The piece that we have designed requires drilling the old posts to the keyboard ciruit level, taking care of leaving the hole clear. The design on the piece is such that the pin will be glued to the inner part of the rest of the screw post; and the surface of the piece can be glued to the keyboard circuit, so that the torque on the substrate is avoided (it also helps that torque is also resisted by the other post, when using the full piece)
When gluing the piece, there are several precautions that need to be taken:
We recommend to repair both posts at the same time. However, if you are confident on the resistence of the other post, you can use our 1-side piece. As it is symmetrical, it can be used for both posts, just twisting it 180º. Using the full piece, the other post side does not need to be glued - the torque resistence is done by the post itself over a much longer distance.
Alternatively, you can cut the other side of the 2 post piece with a tweezer and glue in place the rest of the piece.
Upper post repair
Same procedure. You need to use our upper post repair piece. Same precautions apply - be careful of covering with tape the zebra part; and also the pin holes of the keys, just in case.
We have already found solutions for some of the main problems in our beloved HP41c:
But there was an area that we have not been able to find a solution for: the broken posts, both upper and lower. Until now.
So far we have tried withe metal or nylon posts, but the repairs never lasted too long: the torque applied by the screw on the limited adhesive area was too much when tightening the calculator case. But stay tuned for a slution that may work finally!
Most of the HP41c that we receive for conversion are missing the side cover. You can find in ebay side covers but as they are from overseas, they are quite expensive for us Europeans. And you can find some in 3D printing sites, but neither the material quality nor the design makes for a good replacement. So based in our experience with the battery-port assembly 3D printing and with a couple of new materials, we think we may get it right!
Here you can see a rendering of the peice we have designed. We have tried slightly thicker arms, since this is the breaking point, but not too much. We have ordered the first set of samples and we are eager to show you how it looks like in reality!
Some of you guys have several calculators to repair and have asked for a better price if you buy more units. Also we wanted a more convenient way of selling and producing these parts. So we decided to join them and sell them together. You can then cut them by the crosses and use them as individual pieces. Exact length of the cross is not critical.