The AC power 220V is entered to through input of transformer-T1 to reduce voltage as 9VAC, then wire connected to four diode D1-D4 as bridge rectifier became to 11VDC, then the current is filtered to dc voltage that low ripple output and then the voltage is regulated to 6VDC constant voltage by IC-KA7805( IC-7805 type) In normal it is used for 5 volt only but now we add two resistor to determined output voltage is 6 volt and passed through diode 1N4002-D6 to output is 6 volt.
Read more source: http://www.eleccircuit.com/6v-backup-battery-power-supply-regulator-with-ic-7805/