I recently purchased a non working QSC 1400 amplifier. A few seconds after being powered on the clipping light comes on. I downloaded the schematic and started checking voltages. The +15 and -15 volt power in one channel measured at pins 4 and 8 of the op amp was only +9 and -15 volts. I traced out the power and found +9 at one end and 0 volts at the other end of a 3.9 volt zener diode.
This picture shows the location of the zener in the schematic;
here is the location on the circuit board: