HP 35

[AndiGer]

If you want to test before you buy a charger or rebuild the battery pack you can test the calculator from a normal DC converter or another DC power source that gives (3 * NiCad by 1.2 V = ) 3.6 V to 4.5 V. Polarity must be observed: Negative polarity to the upper pole. See this picture: https://www.hpmuseum.org/batts/battcls.jpg
Look for the two pins where the plug goes into. With no plug installed the two outer pins are connected by the brass sheet inside. I.e. use a DMM to check those pins are connected together. Otherwise the calculator will not work as it doesn't with the plug connected but wall plug not connected to the mains.
Good luck
Andi