Thank you for your interest for help.
I want to know what the problem is so that I can replace the part or give up at least.
SYMPTOMS are :
1. No green light on the front panel board (near the power switch)
2. No "Bong" sound
3. Red steady light on the motherboard
4. Power fan, cpu cooling fan, and vga fan all rotates.
5. DVD drive door doesn't open if the cable is attached to the motherboard.
However it DOES open if the cable is disconnected.
6. Hard drive doesn't rotate if the cable is attached to the motherboard.
However it DOES spin up if the cable is disconnected.
WHAT I DID RIGHT BEFORE HAVING THIS HEADACHE :
1. I pulled out MODEM daughter card and gave it to my friend who needs modem.
I asked him to give me back just to test whether that's causing problem or not but
it turned out it's not.
2. I don't know why I did it but I was very curious about macintosh CPU so I removed the
CPU board and enjoyed(?) looking the CPU... it was smaller than I expected
THINGS I TRIED SO FAR :
1. After doing extensive google search, I figured out in most case pressing PMU button fixed the problem. I also read a warning not to press it in a row. I tried more than hundered time so far no hope.
2. Measured the voltage of on board battery. It's 3.9 Volt. Just in case I bought a new one. Still it's holding good charge.
3. Tried to replace a ram also. Pulled one stick from working powermac g4 so I'm sure ram is okay.
4. VGA card is tested also. No problem.
My guess is either main board is gone or cpu board is gone. I want to know exactly which parts is causing a problem. Any idea ?