I've always had trouble understanding potentials and current flow between secondary transformer windings and this example is fairly typical.
The schematic shown is a straightforward linear regulated power supply circuit from Heathkit (IP-28) with two secondary windings, one of which creates a reference voltage for the control element (Q5). Looking at the schematic, I'm struggling to see why current would flow through D6 and into Q5 at all. What would entice current to flow between two otherwise isolated secondary windings?
Original schematic here.
Assuming the bottom of the secondary winding was ground (it isn't in the schematic as far as I can tell), you could say that the negative rail was 4 (or two) diode drops below the reference voltage. But, that assumes you know that the negative rail sits below the reference voltage; what establishes these potentials?
It could be diodes D9 and D10, but again how are the potentials established. I'm guessing no current flows through D9 and D10, so this just adds to the confusion for me.