Asus Prime B560M-K CPU VRM does not turn on with CPU installed

[Twiesel]

I've got this very strange problem on two Asus boards, a B560M-K and a H410M-A, where the CPU VRM (and all other voltages) are running when the mainboard is powered on without CPU. On the B560, the power draw is about 700mA. But with the CPU installed, the CPU Vcore and another 1V rail (but not the I/O rail with 0.95V though) will not run. The CPU (Celeron G5905) works fine on another board.

The phase controller is an onsemi NCP81243 (Link to datasheet: https://www.onsemi.com/pdf/datasheet/ncp81243-d.pdf). Enable and VR_RDY are high even with the CPU inserted. The PWM signal is also high, but not pulsating.

I guess that the "pre-VID"-voltage works fine, but when the CPU sends its VID, the phase controller shuts off. But I have no idea why this happens. I cant see any fallen off components or scratched lanes on the board.

I uploaded a datasheet of a similar board. Thank you in advance for your ideas!

[noname2024]

PWM B560M-K IS PWM CONTROLLER ASP1400CTBMNTXG//ONSEMI

[DynaxSC]

Check whether DMI bus is OK, bent pins or shorted/open DMI lines. CPU must have a working DMI bus with the PCH, otherwise Bios instructions will not be forwarded correctly you CPU, and the CPU does no even start to think to talk to the VRM controller. You can check the DMI bus with a multimeter in reverse diode mode, each line separately, in the CPU socket, but you must have sharp thin probes, and do it very carefully, best with help of a lens glass or microscope.

Another reason can be missing clocks signals from PCH in the CPU socket. Cut tracks, bent pins or PCH not generating clocks signals. Check on both PCH hot crystals pins if there is a clock present, oscilloscope, probe x10 to reduce probe parasitic capacitance to minimum preventing clocks from oscillating.

Also missing or shorted/decalibrated smd parts in the VRM controller network can be a reason for VRM not to start to work, even if it works without CPU. Especially after some events, like lightnings, total CPU failure, etc.

Check if RAM's have all 4 necessary powers, 2.5V, 2.5V/3.3V, 0.6V, 1.2V.

Backup the BIOS and program a stock version with HW programmer, its possible that the Bios image is partially demaged.

Clean the CPU socket, RAM sockets and CPU/RAM contacts with 99.9% isopropylic alcohol, some soft, small paintbrush and finally with pressurized air.