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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 ‘Yto ‘)’ and ‘EVALto ‘-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



Source: Marcel in 2014

Personal Notes:
If you bought some plums but still have about 2 pounds left over then here is a recipe that will make them disappear.

plum cake, Zwetschgenkuchen

yeast dough for a whole tray:

500 gr. flour 100 gr. butter
20 gr. fresh yeast or 1 pack dry yeast 1 egg
250 ml milk 1 pinch salt
75 gr. sugar some breadcrums

Mix the dough and let it rise for about 1 hour.
In the meantime take about 2 to 3 pounds of plums, wash them, then remove the pits. After the dough has risen, put it on the tray and form a boarder. Now put a little bit of breadcrumbs down. Now add the plums evenly.

For about 25 to 35min bake by 200 °C.

Option 1 (Vanilla pudding filling):
while baking the cake, make a fresh vanilla pudding. After about 20min, take the cake out and fill all the gaps in the cake with the pudding. Now bake for another 5 to 10min.

Option 2 (crumble topping):
Instead of the vanilla pudding, you can add crumble topping. Prepare the crumbles like below and put on top of the plums.


300 gr. flour 200 gr. butter
125 gr. sugar some cinnamon or vanilla sugar

Directions crumble topping:
Mix the flour with the cold butter and sugar by hand for about 2min.

Nonni’s Italian Pizza

Nonni’s Italian Pizza

Source: Mama Joanne about 1990’s

Personal Notes:
We visited my mom for a few weeks during the fall. The central point to our visit was the food! She cooked up a storm and I was there to shoot the pictures, and sample of course! Who could complain? This pizza was very different to mine and all with a few variations of toppings. Normally, my pizza is simple; mozzarella, tomatoes and basil. I learned it was fun to change things around on the pizza toppings.

I hope you enjoy the variations too.

click here for the pizza dough
Nonnis pizza


Italian Sausage Manchenco Cheese
Mushrooms White Cheddar
Mozzarella cheese

Prepare the dough, add a bit of olive oil on the dough once it’s been spread out on the pizza tray. I prepared the pizza’s @ my mom’s house by spreading out the dough with my fingers (no rolling pin), adding a bit of olive oil, then the toppings. My mother said the pizza crust came out much nicer than hers normally does. More crunchy, less soggy.

Once the dough is all spread on the tray and oiled, layer your toppings. Sauce first, then cheese, then meat, then oven.


Rhubarb “Pie” Crumble

Rhubarb “Pie” Crumble

Source: Myself in 2013

Personal Notes:
I used to work at Marie Calendar’s pie shop in California when I was 16 years old. From the name, you can guess, there were over 40 pies and I tasted every one! I remember only having rhubarb once in my life, I guess it wasn’t my favorite, and I never tasted it since. Now, a good uh-hum 20 years later, I see we have rhubarb growing in the back yard here. Simultaneously, I picked up a gardening magazine this week and read a nice recipe for a rhubarb tart. I decided today was the day for trying rhubarb! I reviewed the magazine recipe, and didn’t have cream, nor eggs as it called for. I searched around on the internet for a good looking recipe, and found quite a few which didn’t use eggs nor cream. I didn’t like the looks of what I was finding though. I searched for a Rhubarb Crumble, and all of the recipes called for oats, and cinnamon. I liked the recipe from the magazine I found, which called for Hazelnut “mehl” or flour. It’s quite popular here in Europe to use nut meal in baking. Trader Joes was the first to carry nut meal in the U.S., I remembered it was such a big deal when the carried it to all my German friends! I see why.

So, the long of the short here is…I took a little of this and a little of that from several recipes and created my own. I didn’t measure much, just mixed and eyed the consistency I liked. I’d say give it a go. It turned out to be an acceptable dessert with my family. My kids thought it was apple, and my husband knew the taste but liked it quite well too, I hope you find the same.


3 C. thinly sliced rhubarb Hazelnut flour
1 C. Sugar Cream

Slice your rhubarb and put in a metal mixing bowl along with your sugar. Mix around. You will notice the rhubarb produces a lot of moisture, so I added a bit of ground hazelnut to absorb the moisture-maybe ¾ cup You decide. Then, I had a bit of cream which I added just for texture.

I mixed it all up, tasted it (a bit sweet/tart) then poured into the pie shell which I made (see pie dough recipe)

I topped with a crumble of butter, sugar and flour. Again, mix about a cup of sugar, ½ cup of flour and about 1/3 C. butter and blend together with your fingers to form a ‘crumble’. Pour on top of the Rhubarb mixture, and put in oven.

Bake @ 350 degrees for about 30 min.

Some sources recommend to serve with ice cream or cream. We ate it plane, and was super!

Obwalden anis birds

Obwalden anis birds (bread)

Source: Swiss Farmers Cookbook (circ. 1800’s)

Personal Notes:
As they say, the Swiss farm woman plays a great role in cuisine. Being that the men, and children traditionally come home for lunch promptly @ 12:00, the woman must make her meal complete, full of flavors and to be adored by her hungry group. Upon searching around for some traditional Swiss recipes, I found this anise bread in the shape of a bird (if you looked hard enough). Now, you might ask yourself why in the world would you ever attempt to bake at home when there are so many bakeries that do it for you and most of the time better than you. My answer is, I’m always looking for a culinary challenge. When I saw these bread birds in the book, it inspired me. I challenge you to give it a try as well. I’m sure you will not only please yourself, but those adorning and eating them! They are wonderful for breakfast, lunch, or dinner.

anis bird from Obwalden (OW)


500 gr. white flour 1 dl. heavy cream
¼ Tbs. salt ¾ Tbs. anise
1 Tbs. sugar 2 Tbs. raisins
20 gr. yeast 1 egg yolk (for brushing)
2 dl milk

Mix together the flour, salt and sugar. Dissolve the yeast in a bit of warm milk, then add the dissolved yeast to the remaining milk mixture. Add the cream, anise and raisins and kneed it together. Let the dough rise in a warm place until it’s twice the size.

Divide the dough in 8-10 equal balls. Roll out each ball to about 25 cm. long. Make a simple knot in each roll. If you’d like, form a beak and add wings if you’d like to enhance the bird look. Brush on the beaten egg yolk and let stand for 20 min. Bake birds for about 20 min., at 200°C Celsius.