Open a HP 28S calculator

HP28S with Leister heatgun
Heat is set to 2 of 10, I think a hairdryer on the lowest setting would work as well

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

1. Problem

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.

2. How to open the HP28S (right side)

Since I could not find any photo instructions on How to open a HP 28S calculator, 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 clam-shell HP28S calculator apart.

  • I started to heat the bottom right corner first (and it worked great)
  • Heat is set to 2 of 10, I think a hairdryer on the lowest setting would work as well
  • Warming row by row
  • Some pegs were already broken, so they lifted automatically
  • cover removed (maybe 5min)
  • removed cover from the back
  • Now the top, starting in a corner
  • So also the top sticker made it
  • this took about 2 more minutes
  • close-up, you see that some pegs broke of (Calculator is 30 years old)

In addition, I took some close-up pictures

  • Just one pin that broke off (age of the calculator)
  • close-up where the pin was in
  • close-up with the pin added again
  • pin half way removed
  • just a measurement so I have an idea for the size of the drillbit, looks like the hole is on top 3.2mm in diameter, so a good size to start with would be 3mm

Drill Press

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.

  • starting to drill the bottom rows first
  • Not drilling really deep, just about 1mm
  • Sometimes a pin breaks but mostly the head comes off
  • Now the top section
  • About done with drilling
  • calculator is open, some small pins I did not even drill
  • the keys are all loose
  • the cover from the top
  • the cover from the bottom
  • the bottom piece
  • the drilled out heads and some pins that broke
  • close up back of display
  • close up bottom piece

3. 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?

  • Use a Q-tip with alcohol and clean it. I also like using a soft eraser on the gold contacts.
  • carefully clean the foil with the q-tip
  • Lets see if that worked and put it back together based on the picture
  • Who would have thought. Keys in that row are working again
  • Just assembled it quickly to test
  • This shows which contact has the problem. The initial problem showed an entire row not working so based on this picture this would be “12” needs extra cleaning

4. How to open the HP28S (left side)

Lets open the left side of the calculator as well.

  • starting again in the right bottom corner
  • slowly heat row by row and pull
  • overlay removed, no issue
  • lets start drilling again
  • sometimes the peg is broken on the bottom and comes right out
  • all are drilled, ready to open
  • the stuff that was removed during drilling
  • carefully pull on a corner
  • its removed, do not worry about the keys
  • now open the hinge, use a knife and screw drivers
  • left side is now also open
  • final picture

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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1 comment

    • Arjunaidi Jamaludin on 15/01/2020 at 17:17
    • Reply

    I enjoyed your tear down of the HP 28S calculator. Thanks for sharing

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