Power Supply (Amiga 500)
The first check is to make sure your power supply has the required voltages when not connected.
The Amiga 500 has -12V (but usually shows around -9V) on the center pin. Some specifications mention this will be as low as -5V.
Although the physical connector for the Amiga is identical to the Commadore 128, they are NOT the same pinout and will damange the consoles if used in the wrong console.
Remove the cover to the computer so you can access the pins of the power socket, then connect the power supply to the computer and turn on.
Measure the pins again by probing the back of the power connector, to ensure the voltages are still ok while under load.