I'm troubleshooting a nearly new system built around an ASUS TUF Gaming B450-PLUS II and a be quiet! System Power 10 650W. The Ryzen 3 2200G, stock cooler, and XPG DDR4 memory are known-good, and this minimal setup initially reached BIOS and booted Windows using the integrated graphics.
After that, the system became intermittent. On three occasions, unplugging and reseating the 24-pin ATX connector immediately brought it back to life, although one reseat did not help. While the computer was running in BIOS, I pressed near the motherboard's mounting and 24-pin area and it instantly shut off. I then discovered that the board was not properly aligned with the case standoffs and visibly flexed when the 24-pin connector was installed.
The board is now removed from the case and sitting on its cardboard box with only the CPU, cooler, one DIMM in A2, 24-pin ATX cable, 8-pin CPU power cable, and integrated graphics connected. The power switch has been tested both through the case and by directly shorting the PWR_SW pins. The board has standby power and lighting, indicating that 5VSB is present, but it will not start: there is no fan movement, PSU startup, or operating power to SATA devices.
The CPU has been tested in another working AM4 system, and the memory and CPU have been reseated. The main possibilities seem to be a damaged motherboard, cracked solder joint, faulty ATX socket or terminal, damaged PSU cable, or a problem with the PS_ON control circuit. What electrical test would best distinguish a motherboard that is failing to assert PS_ON from a problem with the PSU or 24-pin power path?
3 Answers
The combination of the 24-pin reseats temporarily restoring operation and the board shutting off immediately when flexed strongly suggests a physical fault around the motherboard’s ATX connector or nearby solder joints. A cracked joint, damaged connector, or poor terminal contact could make and break the main power connection as the PCB moves. Since the board behaves the same outside the case, the original standoff problem may have caused or exposed damage, but it is no longer the only likely cause. Inspect the connector and surrounding traces closely, and an exchange or warranty replacement for the motherboard is reasonable.
To separate the PSU from the motherboard, disconnect the 24-pin connector from the board and briefly bridge the PSU’s green PS_ON wire to any black ground wire on the 24-pin plug. Use care and make sure the bridge is secure before switching on the PSU. If the PSU starts and its fan or connected load responds, the PSU can at least leave standby and the motherboard is likely failing to pull PS_ON low. If it does not start, try another compatible cable if the PSU is modular and verify the PSU independently; never mix modular cables from different power supplies.
Also inspect the CPU socket and the underside of the processor with magnification, checking for bent contacts, debris, or damage. Reseat the CPU and memory once more, but the known-good processor and the direct connection between PCB pressure and shutdown make the ATX socket or a cracked motherboard connection more suspicious than the CPU itself.
The PSU is only about two weeks old, so I’m leaning toward returning the motherboard first, although the failure is still very strange.