Resolved MP6,1 Power on but no chime and can't boot

[hirakujira]

Hello everyone, I tried to solve this for few days but can't get any clue...

I got a MP6,1 from my friend and when I received it, everything seems to work fine... and then I put it to a box for about a month without touching it.
About 3 days ago, I tried to boot it but it doesn't work. The light of IO ports is on, however the light of the power button is off. And there's no chime and no boot logo.

And these are what I tried:

• Replace a cord: doesn't work
• Reset SMC: doesn't work
• Use an UPS to provide more stable power: doesn't work
• Try to plug it in another room (but the same house): doesn't work
• Reset real time clock (press the RTC button located on the logic board): doesn't work
• Check the diagnostic LEDs of the logic board: no red LED
• If I unplug the cable between GPU and the logic board, the red LED is on (so the diagnostic LED works)
• Check all of the LEDs on IO board: all green LEDs are on

These are what I didn't try because I'm afraid that I would make it worse:

• Replacing the CMOS battery
• Replacing power supply, logic board ... etc

And some times it doesn't boot with these behaviors:

• The power button blinks for about 0.1 secs and then just off. And I heard a sound like something is starting up in the machine, but then nothing.
• The fan starts to spin, but still no chime, no boot.

I also noticed that if I plug a USB stick with I/O light, the USB stick flashes. But If I plug an external USB keyboard, it doesn't work at all, and the caps lock light can't turn on.

Here's a video to describe this issue:

I have no idea what I can do or test, so frustrated...

 

[hirakujira]

Tried to tear it down to replace a CMOS battery and found the issue, it's not the CMOS battery nor logic board failure. It's because my friend forgot to screw one of the screws between the power supply and the CPU riser board... so the power can't be transferred to the CPU...