Hi orange,
From what you have told us, your power supply is bad. The -12 volts is too low. Get yourself an PC AT power supply out of an old PC. Then cut the cable with the square connector off of the Amiga 500 power supply and strip the wires and solder to the power cable coming from the PC AT power supply. Use the connector with the orange wire. That is the power good wire. It needs to be soldered to one of the black wires. The following wires from the PC AT power supply is what you will be soldering to the Amiga 500 power connector. Red = 5 volts, yellow = 12 volts, blue = -12 volts, orange = power good, and two black wires which are ground. These 6 wires are together on one connector. Cut that connector off and strip the wires. After you are done you will have a fan-cooled "Big Foot" power supply that will work with the Amiga 500, 600, and 1200. :-D
