78L24 outputs high voltage >24v - Harman Kardon AVR 151
The zener voltage ends up being added to the voltage stamped on the part, so 12V+24V = 36V.
Pin 2 of the 78L24 acts as an almost constant current source of about 4.5mA, which flows through Zener diode D929 and therefore 12V will appear across that. The 78L24 provides 24V on pin 1 relative to pin 2, which is thus 12V higher than ground.
As it happens, Izt for the zener is 5mA, close enough to the operating current.