Open a HP 28S calculator

HP28S with Leister heatgun
Heat is set to 2 out of 10, I think a hairdryer on the lowest setting works also

How to open a HP 28S calculator

If you never opened a HP28S clam-shell calculator then here is a how to open a HP 28S calculator. This is quite a challenging task in trying to figure it out. I did quite a bit of searching and trying before documenting this post.  Why would you want to open the calculator in the first place? Well because it is not functioning 100%.  My post is divided into 4 sections


  1. Problem
  2. How to open the HP28S (right side)
  3. Cleaning the contacts
  4. How to open the HP28S (left side)

See also the next post for creating a new HP28S case


The key row ‘Y’ to ‘)’ and ‘EVAL’ to ‘-‘ are not working. So either I fix the calculator or I recycle it. I decided to give it a try.

How to open the HP28S (right side)

Since I could not find any photo instructions on how to open the clamshell calculator HP28S, and I would like to fix this problem, I decided to read up on the different text descriptions on how to do it and started to document it with pictures.

Today is the 25th of July 2017. Here is my first set of pictures from taking the clamshel HP28S calculator apart.

In addition, I took some close-up pictures

The next steps I am planing in a few days.
I am buying a small used drill press or maybe just a stand for my drill, so I can remove the top of the melted plastic peg/pins.
Today is the 5th of August, and I was able to buy a used drill press.

Cleaning the contacts

We now have the right side (the one with the display) open and the idea is that if a contact is dirty it could prevent a whole row or column of keys to NOT function. Well lets try cleaning them

What exactly did I clean, are you asking?

How to open the HP28S (left side)

Lets open the left side of the calculator as well.

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As mentioned above, the next step it to put it back together. Well one way is to just glue it or weld the many plastic pegs with a soldering iron. I am trying a different approach with this. See my next post, for creating a new HP28S case.

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