Alternating voltages on vintage calculator circuit board
Transformers work on AC, not DC. AC voltages can be converted to DC onboard if necessary. Just by looking at the PCB traces it can be guessed that there is a diode bridge for rectifying and the big electrolytic capacitor is the reservoir capacitor. Some AC voltages may be used directly such as the heating filament for the vacuum fluorescent display. But ICs need DC voltages, both digital or analog chips.