The symmetrical Power Supply (or +- power supply) has two output voltages, one positive and one negative.
After rectifying the 20-0-20 AC voltage on the bus a LM317 (positive voltage regulator) and a LM337 (negative voltage regulator) are controlled via potentiometers. For the user to deactivate voltage output there is a switch at the front panel.
Since during training we got the layout and just etched the pcb, I currently don’t have a schemativ for it. I will reverse engineer it eventually, but that is not prioritized. More to this you can find at “Power supplies – LPS Linear regulators”
As usual some pictures: