[SOLVED] Computer incorrectly starts on "AC power on"

Hello

Preamble
This may be a rare problem; it is certainly hard to find solutions for it by netsearching. My instance of the problem was triggered by a specific device but many devices could trigger the same generic problem.

Setup and symptom
With the BIOS configured

• not to "power on" on "AC power on"
• to "power on" on any keyboard key press,

when AC power was turned on the computer started.

Problem analysis
The keyboard was doing something on "AC power on" that the BIOS interpreted as a key press so the BIOS turned "power on".

Solution
Changed the keyboard.

Problem analysis method

1. BIOS was re-configured with all "power on", "wake up", and "resume" events disabled.
2. Test: AC power was connected. Result: the computer did not start.
3. The previously enabled "power on", "wake up", and "resume" events were re-enabled and AC power connected after each change. This showed that "power on" on any keyboard key press casued the problem.
4. Test: the PS2 keyboard was disconnected and AC power connected. Result: the computer did not start.
5. Test: a USB keyboard was connected and AC power connected. Result: the computer did not start.

No other PS2 keyboard was available for testing to determine if the problem was caused by the specific keyboard (a keyboard problem) or by having a PS2 keyboard connected (a motherboard problem).

Specifics

• Motherboard: GA-MA770-DS3 Rev 1.0
• BIOS: Award Software International, Inc., Version: F3, Release Date: 12/24/2007
• BIOS settings: AC Back Function: Soft-Off, Power On By Keyboard: Any KEY
• PS2 keyboard: Logitech "Deluxe Keyboard", model name: V-AU61, P/N: 867462-0403

Related thread
https://www.motherboardpoint.com/t25714-power-supply-always-on-knxp.html