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New piece for repairing HP41c broken lower posts
07-19-2021, 10:21 PM (This post was last modified: 07-19-2021 10:27 PM by Divasson.)
Post: #1
New piece for repairing HP41c broken lower posts
Old methods to repair broken post are 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 thread has been destroyed - maybe due to strong tightening of the screws.

Other repair methods have been proposed, including drilling down the screw post and putting in its place a cylinder with screw path, glued in place. We tried this method but there was a problem: the torque used to secure the 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 circuit level plus 3mm, taking care of leaving the hole clear, and avoiding piercing the calculator top!!. We use the drill at minimum speed:


The piece has two claws that clip in the holes we just drilled. We glue all the lower surface of the piece and the clipping claws.


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!

(to be continued)
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07-19-2021, 10:28 PM
Post: #2
RE: New piece for repairing HP41c broken lower posts
I've used this technique to repair one of my 41Cs. It worked well. The toughest spot for me was taking the masking off where the adhesive should not be. The end result is working as good as new. Thanks for the nice solution.
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07-19-2021, 10:30 PM (This post was last modified: 07-19-2021 10:43 PM by Divasson.)
Post: #3
RE: New piece for repairing HP41c broken lower posts
(continued from previous)

The new piece is self-centering: it should enter in the holes and stay placed. (The old piece was only glued to the surface, and needed to be centered on the holes with small screwdrivers or other method. This was critical for a good assembly later)

• 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). Alternatively, you can use a couple of credit cards which will avoid the clothes’ pressing the keys all the time.

• Use E6000 glue. Make sure that there is no solvent or circuit cleaning residue – it will disable the gluing effect of E6000
• Let it cure overnight.
• Remove the tape over the zebra circuit trace: in particular what affect the circuit traces.
• Place the zebra connectors
• Put the processor circuit, Assemble the calculator and test. The screws (more so if they lar of the long variety) press the "claws" of the piece outward and make an even better connection between piece and keyboard circuit.

We recommend repairing both posts at the same time. However, if you are confident on the resistance 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.
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