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Calculator blog

Musings and comments about our common interest

 


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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 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.

 

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A couple of audio-related products

As some of you know already, my other hobby is audio. And as a frequent voyager, audio means portable audio. 

We have created a couple of 3D pieces for problems that we have found in our portable devices.

We are lovers of the Stax Lambda electrostatic headphones. However, sometimes the side assembly breaks. We have designed a 3D replacement, that you can find in our audio area or in our webshop in eBay. It just plugs into the hole of the headband and allows some rotation on its axis, since it has been designed with the stem printed within the body - this you can only do with 3D printing!

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This piece helps solving a problem for many users - I hope! I have found yet no other use for the "print inside" capability of the 3D printers.

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HP41CL with Time Module

For some time we have left aside the blog, due to lots of repairs and CL installations. Then comms are postponed and the urgent orders come forward

So we have not informed here that there is now the possibility of integrating a Time Module in the HP41CL. This saves the user a 120-140 € module, and a slot. 

All the rest of characteristics and features of the CL are still there, while the list of included modules is continuing to grow. Also many of the modules designed by Angel Martin have been reviewed, debugged and improved.

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New HP-PRIME G2/B1S (2AP18AA)

HP_prime_front_pictureWe have received the new HP-PRIME G2/B1S (2AP18AA). This calculator is fully compatible with the original, yet twice as fast (while speed was never a problem with the original one). The chipset has been changed, and the new one has several advantages that the previous didn't have. To start with, reportedly you cannot brick the calculator anymore (and this is something that in the past even happened to me, when admittedly I was trying something I shouldn't have done)

In next blogs we will go through this model in mode detail.

The pictures we have are still of the older model. The new model has a more start key coloring scheme, making it easier to distinguish the second and third function of each key.

 

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New batch of springs

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

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Success of new HP41c repair methods

80806dea-8f86-418a-ac37-128e1b53086bWith 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. 

  • Corroded battery terminals
  • Corroded main circuit
  • overdone lower screw posts
  • overdone upper screw posts
  • Broken lower screw posts
  • Broken upper screw posts
  • Corroded or broken zebra connector
  • Broken screw supports on the back side or the case (usually the upper ones)
  • Missing back port
  • Missing side port
  • Corroded spring for battery holder
  • Broken or missing battery holder
  • In full nuts, overdone nuts to keep circuit pressure - an alternative to nuts.
  • Broken plastic window

In next instances, we will go one by one with each of the chapters. Who knows, it may end up as a book!

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Higher 3D printing prices

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:

Side port

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zebra holder

Other will have to follow as we place additional orders and we see its prices risen!

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A new repair method being tested for lower post of HP41c

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!

Material needed:

  • Lower post repair piece
  • Clothes' peg
  • Keyboard support for clothes' pegs

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:

  • Try to avoid the torque from the screw forcing on the glue with the substrate
  • Try to ensure that the front part of the calculator is pulled up when screwing the back side on.
  • Try to be compatible with all known zebra connectors - including our own.

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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)

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When gluing the piece, there are several precautions that need to be taken:

  1. Be sure of covering the zebra circuit part with tape -we don't want the glue to cover it.
  2. Cover also all nearby pin holes of the keuboard. If glue comes into one such hole, the key will not register. In severe cases, the key will be down all the time
  3. When applying pressure, use clothes' pegs to hold the piece in place, preferably on the outer side.
  4. To avoid pushing the keyboard down, and only push down the front of the calculator, use the keyboard cover we have designed to support the pegs. Pushing the keyboard instead will result on separation between both halves of the calculator, since the attachment of the substrate to the pin will be at a lower level (the keyboard is linked to the keyboard circuit, not to the substrate)
  5. Use epoxy glue, type XXX
  6. Let it cure overnight.
  7. Remove the tape over the zebra circuit paths.
  8. Assemble the calculator

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.

 

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Assembled flex and standalone flex circuits back in stock

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.

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Repair of HP41c posts - some ideas

We have already found solutions for some of the main problems in our beloved HP41c:

  • The upper screw supports in the back side of the case, that are usually broken.
  • The battery contacts that are usually corroded
  • The zebra connectors for the full-nut models

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!

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