Casio fx-61F: Scientific Electric Formula 27 calculator

Solar cells panel able to genetare 3V under normal light conditions.
The hybrid powers supply uses a 1.5V coin battery as a backup power as well.

Processor from Hitachi model HD62021A04 able to work with 1.5V.
The hybrid power supply takes a 3V solar panel and a 1.5V battery as input to a couple of silicon diodes a two resistors and two capacitors.

Hybrid power supply:
Diode MA700 has a Vf=0.5V
Diode MA723 has a Vf=0.3V
Probably both are schottky diodes.