How to open a HP 28S calculator

  1. Problem
  2. How to open the HP28S
  3. 3D Model (with screws)
  4. 3D-Printer
  5. Cost
  6. Mailing list

Key row ‘Y’ to ‘)’ and ‘EVAL’ to ‘-‘ are not working

How to open the HP28S

Since I could not find any photo instructions on how to open a clamshell calculator HP 28S, 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.

So first set of pictures from today 25.Jul.2017

Picture 1 I started to heat the bottom right corner first (and it worked great)

Picture 2 Heat is set to 2 out of 10, I think a hairdryer on the lowest setting works also
Picture 3 Warming row by row
Picture 4 Some pegs were already broken, so they lifted automatically
Picture 5 cover removed (maybe 5min)
Picture 6 removed cover from the back
Picture 7 Now the top, starting in a corner
Picture 8 So also the top sticker made it.
Picture 9 this took about 2 more minutes
Picture 10 close-up, you see that some pegs broke of (Calculator is 30 years old)

Some close-up pictures

Picture 11 Just one pin that broke off (age of the calculator)
Picture 12 close-up where the pin was in
Picture 13 close-up with the pin added again
Picture 14 pin half way removed
Picture 15 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

Next step: (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 August 5th 2017 I picked up the drill press.

Picture 1 starting to drill the bottom rows first
Picture 2 Not drilling really deep, just about 1mm
Picture 3 Sometimes a pin breaks but mostly the head comes off
Picture 4 Now the top section
Picture 5 About done with drilling
Picture 6 calculator is open, some small pins I did not even drill
Picture 7 the keys are all loose
Picture 8 the cover from the top
Picture 9 the cover from the bottom
Picture 10 the bottom piece
Picture 11 the drilled out heads and some pins that broke
Picture 12 close up back of display
Picture 12 close up bottom piece

3D Model (with screws)

Since you have to “destroy” the case to open the clamshell calculator I am going to have a 3D model designed which I can then print in 3D.
Taking the empty shell and sending it to person for hire which will create a blender 3D model which will incorporate screws to open and close the case for maintenance/repairs.

The metal parts (for example the battery door) should be saved so the plastic part can be reprinted and the metal added so at the end you have a new battery door.

All parts of the calculator should fit into the new case.

3D Printer

Once the prototypes print and are stable enough I will release the 3D file for download and you can print the case yourself or have it printed.


I am not asking for donations but if you would like to support my efforts, you can click on the right “Ad” and it will hopefully generate some revenue.

Mailing list

If you would like to be informed on updates, just sign up and I will inform you on updates. Signup on the righthand side or right below

HP28S open and rebuilding the case