Question Partial Loss USB Port

[Jay_Pee]

I recently replaced the psu on my old pc. Everything appears to be fine with the exception of the top two usb ports in the I/O panel on my Asus Z97 motherboard. Everytime i shut off my pc for an extended period, i get the message at boot up that no keyboard was detected. I plugged my mouse and keyboard to the other ports and they work fine. Now, whenever I go to the bios and load optimal settings and reboot, the top two USB ports start functioning again...until I turn off my pc for an hour or longer.

 

[Mr Evil]

A couple of the USB ports on my ASRock X370 Taichi have partially failed. Measuring the voltage being output by them under load shows that they have high internal resistance, so they work when the connected device is drawing little power, and then fail if it draws more, with some devices being more tolerant of low voltage than others.

I haven't poked around inside to find out for certain, but my guess is that there are polyswitches (fuses that reset themselves) for short-circuit protection that have failed, leaving them with a high resistance. Your symptoms could be explained by a similar fault.