HP-21 Hewlett Packard Calculator Interior

The bottom cover of the HP-21 is easily removed by just removing 2 screws. The reveals the logic board
plug onto the keypad PCB.


The two power supply terminal is clearly visible and is silver coated.

This two terminals together with the spring in the battery pack keeps the battery pack in place and form a very sturdy retainer for the battery pack, avoiding the use of flimsy plastic catch.


The bluish tinge on the PCB is common with corrosion caused by NiCd batteries leaking.



Here is the 3 PCBs unplugged.
The keypad, the logic board and the LED display module.

Note the corrosion eated varnish on the resistor on the right of the logic PCB.

From the look of it this HP-21 calculator may have been exposed to the elements besides receiving damage from leaking batteries.