What I found is pin 10 (ground) of all 3 U301~3 has 0.03V. But pin 17 of U303 has perfect zero volt. I think 0.03V within same chip is a lot leaks or current. I have 3 same type of chips which gave same result of 0.032~0.035V. Without chip, the socket's pin 20 goes to +7.35 and pin 10 has 0.0215V. My scope shows the pin 10 without chip 22~26mVpp ripples. All 3 chip's pin 10 are very noisy. Maybe it is because pin 10 (7) and pin 17 (2) are on different ground plane? So +7.5V current flows into the ground changing +7.5V to +7.35. But why it goes to 4.5V when U303 is inserted?

 

What I found is pin 10 (ground) of all 3 U301~3 has 0.03V. But pin 17 of U303 has perfect zero volt. I think 0.03V within same chip is a lot leaks or current. I have 3 same type of chips which gave same result of 0.032~0.035V. Without chip, the socket's pin 20 goes to +7.35 and pin 10 has 0.0215V. My scope shows the pin 10 without chip 22~26mVpp ripples. All 3 chip's pin 10 are very noisy. Maybe it is because pin 10 (7) and pin 17 (2) are on different ground plane? So +7.5V current flows into the ground changing +7.5V to +7.35. But why it goes to 4.5V when U303 is inserted?

 

I solved the problem now. +7.31V stays on whether U303 is inserted to socket or not.

I didn't suspect U303 is bad because I pulled two more chips out of other MBs. Exactly same number. Any how I pulled 4 more same type of chip from other MB untill it worked. Amazing thing is these ICs (3 U303 from 3 MB) are all bad.

Thank you guys!