An HP-35 (Version 2) opened.


The reason I opened this calculator is because it needed a thorough cleaning. A second reason was that I never opened a Classic series calculator before and was curious to see how it was build.

Opening a Classic series calculator it rather easy. The two halves are hold together by 6 screws. Two screws can be found in the battery compartment, two beneath the topside of the back label and 2 under the lower rubber feet. When all 6 screws are removed the two halves can be easily taken apart.

HP35_1 HP35_2

The pictures above show both halves of the housing. The left picture is the most interesting one. On top we see two arrays of four coils, part of the display drivers. In the middle is the plug for the charger. Below the battery holder we find the PCB with the electronics. More information about this chip set can be found here. The logic board is connected to a second print that contains the keyboard and the displays. These connections are made by the single line connector at the bottom.

The backside has some metal shielding. There is also a metal shield on the top around the LED displays to shield the coil arrays.

To reach the keyboard side, another 10 screws have to be removed. The key contacts are covered by horizontal aligned gold plated metal strips. These strip are covered by a thin transparent plastic foil. A transparent plastic grid is on top to keep the key cap hinges in place towards the front of the calculator. See picture on the left. The right picture shows the metal strips after removing the plastic grid and foil.

HP35_3 HP35_4

These pictures already shows the dirt on the contacts. The really dirty parts were the key caps. These have been cleaned with a toothbrush and washing-up liquid.

The next two pictures show both sides of the PCB containing the electronics.


This calculator is build with the following chip set:

1820-0848 Arithmetic and Register Control (ARC)
1820-0849 Control and Timing Circuit (CTC)
1820-0853 Anode driver
1820-0854 Cathode driver
1818-0006 ROM
1818-0017 ROM
1818-0020 ROM
1818-0855 Clock Driver

Finally a zoom in on the LED displays and the drivers. Between the LED displays and the driver chips a rubber strip is inserted. This strip prevents reflection of light by the gold plated pins of the anode and cathode driver chips.

HP35 Display



Web sites about reverse engineering the HP-35

HP-35 repair related links: