Had a chance to fix friends Xbox 360. It was not powering on. Repair center said it was not fixable.
Symptoms were: psu was glowing yellow, but Xbox was not turning on, no reaction, no sounds.
After disassembly found that only +5V were present on the motherboard. No 3.3V, no 1.2 or 1.8V.
Later identified that 3.3V line was shorted, and traced short to C5 capacitor on the RF board.
Found that u5a1 u5a2 on the main board are responsible for generating 3.3v and 1.8v. Removed those as they seemed faulty. Also removed coils near them on corresponding lines.
Tested with external power suply providing 1.8 and 3.3 volts directly to corresponding lines near u5a1 u5a2. Xbox powered on and booted ok.
So I took LM1117IMP-3.3 and LM1117MP-1.8 and wired up ingenious quick and dirty pcb to generate 1.8v and 3.3 volts for the pcb.