Check the capacitors on the motherboard, if they’re good (no leaks, no bulges, at least for electrolytic capacitors), it’s almost certainly PSU. That said, PSU failure is much more common than ...
When you disconnect your computer’s power supply from the wall, it jumps in to save your BIOS or UEFI settings in your motherboard’s CMOS-RAM (Complementary Metal-Oxide-Semiconductor). But even the ...