HP-25 not working

[teenix]

Apart from checking for visual problems like corrosion or cracks on the circuit board, I'm not sure there is much more you can do. Unplug the keyboard and display from the CPU board and then reconnect them. There may be a connection problem. Clean the battery terminals.

If the calculator has been sitting unused for 40 years, then maybe some capacitors are on their way out.

The best option would be if you have a multimeter and/or oscilloscope, or know someone who has, then you can check the power supply voltages and signals on the data buses.

cheers

Tony