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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):

enter image description here

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.

The opamp is NJM4565 and the FET is 2sk246.

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.

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The JFET is a constant current sink so the current through D13 is stable. It's not very well controlled, but it is stable. This is the same idea as the (relatively uncommon) current regulator diodes.

The JFET has a relatively high dynamic impedance compared to a simple resistor so less of the ripple and noise on the -13V rail appears across the zener diode.

enter image description here

Undoubtedly they would have used only one of the binned Idss ranges for Q7, and it should be marked as such.

-13V is because the JFET needs some voltage across it to maintain a constant current, and because they had that rail available. The anode of D13 is at 0V with the op-amp balanced so there is 13V across the JFET.

P.S. From the symptoms it looks like the op-amp is scuppered. It is easier to find from authorized sources than the JFET.

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  • \$\begingroup\$ Thank you very much! You're right, the service manual specifies that Y or GR bin is to be used. And I couldn't find NJM4565 locally anywhere, so I ordered NJM4580 which is said to be a suitable replacement (I hope) with better specs. \$\endgroup\$
    – Violet Giraffe
    Oct 26, 2021 at 11:03
  • \$\begingroup\$ Is there a quick explanation you could give me of how you concluded that the opamp fault is the more likely explanation? Is it because of the voltage difference between source and drain, indicating that some current is flowing through the FET but the opamp is not trying to match its inverting input to the + input as it should do in this application? \$\endgroup\$
    – Violet Giraffe
    Oct 26, 2021 at 11:05
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    \$\begingroup\$ The output is negative despite the (+) input being 0V and the (-) input being negative. So unless Q25 is somehow forcing that.. \$\endgroup\$
    – Spehro Pefhany
    Oct 26, 2021 at 11:24

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