HP-21 - Crazy numbers

[Maximilian Hohmann]

Hello!

To me that sounds typical for a poor battery connection. Some residue of corrosion on the battery contacts maybe. It is able to conduct the "nominal" operating current but whenever the calculator needs more the voltage drops below some value and the calculator operates erratically.

The easiest way to verify that would be to connect a regulated power supply (2.5V 1A) to the battery contacts, using either clamps or little pieces of wire soldered to the PCB traces leading to the battery terminals. I prefer the latter variant.

Regards
Max