The power supply for the Power Amplifier is an unusual double power supply. If you look at the connector which plugs into the Power Amplifier, you will see four pins and a metal shield around them. If you hold the shield with the notch on it at the top, the pins are as shown in the figure below.
To summarize, the bottom two pins are ground, and the top left pin is +12 volts and the top right pin is -12 volts. Note that the two grounds are separate. To test the voltages touch voltmeter leads to both left pins or both right pins while the power supply is plugged in.