How does this voltage adjustment circuit work?
R15 and R15 provide enough current to bias D3 and D4 "on" so that each has roughly 0.7V across it.  So the 10K pot has about 1.4V across it and 140uA of current through it.

DR14 will have about (15V - 0.7V)/1.5K or about 9.5mA through it.  That current, minus the 140uA in the pot flows through the diode D3.  Similar reasoning for the negative branch of the circuit.

So the pot can set a voltage of anywhere from ~ +0.7V to -0.7V depending on the wiper position.