Need power requirements for an HP 67 calculator

Found an old calculator and I want to power it up--the batteries are dead and no charger.

Anybody know what V/A the power input is looking for?

How many cells the battery and what type?

Three batteries in the battery pack. I read somewhere that they are 3.75V, but I don't know if they are in series or parallel.

I just hooked it up to a power supply and gave it 5V. It turned on and seemed to work okay, so I guess 5V is okay. That also must mean that the three batteries are in series. A low voltage light came on if I lowerd the power to 4V.