The DC output is fed to these transistors. T1 acts as a series pass driver or a current regulator. Base bias for transistor T1 is achieved from the supply through resistor R3 of 680 ohms as resistor R2 of 10k is a base bleeder and capacitor C2 1 μF filters base potential. When the test probe is fully open with no zener connected, the base potential of transistor T1 is around 32V that is across resistor R4 or capacitor C2.
New Zener Diode Circuit Diagram