Musings and comments about our common interest
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!
We have to announce that we're raising a couple of 3D printed pieces prices. This is due to the simultaneous price increases from all 3D printing houses here in Europe. In some cases these prices rises are all over the line, but in others just in the black special treatment that we're getting.
The prices increases are in the following pieces:
Other will have to follow as we place additional orders and we see its prices risen!
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 just received a batch of assembled and raw flex circuit.
Therer is a new development created by Ignacio, our chief engineer. All flex circuits can lose adherence in hot or wet situations. This is critical on both extremes of the flex system. Therefore, we have created a several holes in each side and then we screw micro-screws on both sides of the flex, securing it forever in place.
Also we have further modified the base plastic to remove the last bending, the one that makes it more prone to peel off.
In a further blog I will post a picture of the screwed unit, with the flat "escape" bezel.
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!
It has been a long time since the last time I wrote in this blog. Since then, we have been occupied designing new pieces - some of which are still in the development phase. We will come back to these when we have tested them. But today we would like to introduce you to the Side Cover.
The 3D piece uses a different production method, using the new HP 3D printers that will revolutionize the industry. The surface quality is much better than the common nylon, and although it is blacker than the original model, it looks the part and can be used without problems in the calculators. Also, dimensionally is much better than the other examples we have seen in Shapeways! It took some effort and a lot of measuring to get a cover that fits so well on my HP.
Price-wise, the way we produce it allows us to maintain a very competitive price: 6 EUR each + shipment.
I have now an old HP41c where the upper circuit is pressed down to the zebra connectors via nuts. Now, it is difficult to ensure an even pressure and I am having problems with some of the digits. But if we force the nuts, I may break the post - as it has happened in other cases. These are the situations our new piece (wait until we can put it on sale) is made for.
This piece is intended to be put on the lower posts (where it fits tight to aid in case of cracked posts) and gives the necessary mm. of thickness so that the back side of the original type can pass the pressure on the circuit so that it stands firm on the zebra connectors. This negates the use of nuts, which can be changed only a couple of times, lest they break the thread of the post (or the post directly, as it has happened to me a couple of times). I am testing different thicknesses to find which will be the final product.
Say goodbye to the nuts!
We have now prepared a couple of new products designed to solve more HP41CL repair problems. We will discuss today the first one.
Our bent to do c to CL conversions has led us to specialize on full nut repair, instead of half nuts. But some of the tools and methods are also valuable for half nuts too!
The first of the new items we have created is the adaptor to replace screwed nuts in the original HP41C. You may have seen one of this: the processor circuit is secured to the main board with metal nuts around the lower screw posts. The problem with these: it is very easy to destroy the thread, since he read was formed by the action of the nut on the first attachment, nearly 40 years ago!
In the later units, the processor was secured in place by the pressure of the back side through two cylinders collated to the case. If you have one of the old cases, and you have broken the nut thread, you need to find a solution to push the circuit down against the zebra connectors. This is such solution.
We have created a nylon piece that fits tightly around the lower screw posts, with a small bezel to ease the port entrance, and occasionally making a broken post able again. This piece translates the pressure from the back side of the calculator down to the circuit. Don't need to find the right height, or to adjust the angle: it will work - every time.
The piece has sides! As the back side has an angle compared with the circuit surface, we needed to create this angle to the piece. Therefore it can only be put one way.
As the color here really doesn't matter, we have created the price in the cheapest, clearest color from 3D printing: white.
I hope that is piece helps some of you fixing your beloved calculator!
While waiting for the side cover 3D model with a new texture, we have received the samples of another HP41c repair item that has given us a lot of headaches: the zebra connector between the full-nut circuit and the main body. As in many of this things, it is due to the effort of Ignacio Sánchez to find a proper supplier in China. We have received the first shipment and he will throughly test it with a difficult HP41cv. We will report on the results and put it on sale for you DIYers!
The zebra connector has two different models: the initial one, where both sides were united by a plastic membrane, and had good repairability if it wasn't corroded, and the newer "white-green" gummy-like connector. Once it was detached from the circuit, this latter usually didn't work again. This is the one we're aiming to replace.
Stay tuned for news about the first repairs performed with it! We'll post the results here and in hpmuseum.org.
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!