The schematic is listed as below.
When powered on, Vout=12V just as designed.
However, after powered on for a while, Vout will drop to 6V.
We just gauge the voltage at the junction of R425 and R426 and it's 1.25V.
We change a new R425 and Vout go back to 12V again.
However, after powered on for a while, Vout droped to 6V again.
It's very strange. We just want to know the reason.