Alex had the official Lenovo schematic (PDF page 43, sheet 6 of 11). Everything looked standard: the 3V/5V regulator, the PCH power sequence, the Vcore controller. But when he traced to the embedded controller (IT8226E), he saw something unexpected — a 1kΩ resistor marked R1401 that wasn’t populated on half the boards he’d seen.
Why would Lenovo add an optional resistor in the PS_ON wake path? 17ips72p schematic
Most technicians ignored that. But Alex remembered a bricked Y720 that wouldn’t power past 0.2A. He measured resistance between and ground — shorted. The short led to a capacitor near the GPU, but that capacitor was fine. The real culprit? A leftover solder bridge on R1401 ’s pads, permanently disabling the JTAG isolation and holding the PCH in a debug state. Alex had the official Lenovo schematic (PDF page