I'm trying to repair my old audio CD player, and I've found a fault in or around this voltage regulator circuit (it's sourcing VCC for the DAC chip):
I understand how the Zener + opamp work to produce Zener-limited voltage with higher current load capacity than the Zener alone can withstand. What I don't understand at all is the role of the FET. What's going on there? Why are they feeding -13 V into the circuit that generates +5 V, and how does this work? I don't know which part(s) to check for faults without understanding how this works.
What I'm observing is -12.6 V at the source of the FET, which is the actual voltage of the "-13 V" line throughout the board, and -5.6 V at the drain (and the pin 2 of the opamp) instead of 0 V. The opamp's output is -0.6 V (the forward voltage drop of the Zener?), and I've checked +/-13 V power supply of the opamp as well as GND. Also replaced the Zener (5.1 V, 5 mA HZS5.1NB), but it was fine.
P. S. I have looked at the datasheet of the 2SK246 FET, and I've read about common-source application of FETs, but I don't quite understand what's special about this particular FET model (why is it specifically "for impedance converter circuits") and why the whole circuit is designed like this - I would expect the non-inverting input to be connected to +VCC through a resistor, not to -VCC through a FET.