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Broken lower screw posts

Broken Lower Screw Post Repair

Material needed

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

Open the calculator

First remove the rubber feet. To do so, use a small, flat screwdriver and introduce it beneath the rubber, taking care of pulling also the adhesive sheet that glues the rubber to the calculator. Try to make here a good job because usually you will have to reuse the rubber feet when you're done. Put the rubber feet aside so that they don't hinder the work (they have an uncanny ability to adhere to shirts, tools, calculators and eventually get lost.

Unscrew carefully and slowly the four screws beneath the rubber feet. Turn gently the screws: they operate on plastic that, after 30 years, is very brittle and fragile. It is very easy to break a screw post or "overdo" the thread.

Remove the back side of the calculator and put it aside. For comfort, remove also the intermediate piece between backside and main body. Later we will take care of putting it the right way, since it has two ways.

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 the pressure between both halves 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:

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

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)

ZPN_foto_v2

 

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

  • Be sure of covering the zebra circuit part with tape -we don't want the glue to cover it.
  • Cover also all nearby pin holes of the keyboard. If glue comes into one such hole, the key will not register. In severe cases, the key will be down all the time, glued in place!
  • When you position the piece, make sure of using some pins through the holes to make sure the piece is well centered and the screws can go through.
  • When applying pressure, use clothes' pegs to hold the piece in place, preferably on the outer side.
  • 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)
  • Use epoxy glue, type Araldit. 
  • Let it cure overnight.
  • Remove the tape over the zebra circuit path
  • Place the zebra connectors
  • Assemble the calculator and test.

We recommend to repair both posts at the same time. However, if you are confident on the resistence of the other post, you can cut the piece in two. 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.

 

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