Respuesta a "On start up I get the tone then immediate shut off"My iMac 2007 did this after plugging in to a new outlet. What worked for me finally was disconnecting the keyboard, etc. turned off the machine, disconnected the power cable. Held down the power button for 10 seconds. Plugged power cable back in. On start up I held the start button down until it beeped a long horn-like sound and then started as usual. Thanks Madmac252.