-15 V 1-A Regulated Power Supply

-15 V 1-A Regulated Power Supply
The actual supply should get -20 V coming from rectifier/filter that is given to the collector of Darlington PNP-type pass transistor, a TIP 105. The base drive for the TIP105 is supplied  via resistor R5. The base of the TIP is driven from Vz terminal at pin 9, this is anode of 6.2 V zener diode which links with the emitter of uA 723 output control transistor. The way of providing the positive feedback needed for feedback action is