I have a motherboard which seems to have the power signal shorted, because the psu y cpu fan are always spinning whenever the power cable is connected to the PSU. Otherwise than that the computer seems to work normally.

I have tried different PSUs with the same result.

Disconnected everthying from the motherboard except the power supply cable that feeds the atx connector, and the processor and ram.

Can it be a bad capacitor ? it seems to me that there is small of capacitor in the lower part of the motherboard with a little bump on top of it. Should replacing it fix the motherboard ?

Is the +5VSB the signal that manages the power on and off of the motherboard ?

Thank you

If a capacitor has a bulge on it, that's bad news for the MB, because other caps are likely about to fail as well.
However, for your problem, I'd take a look at the header pins on the MB for the front panel switches. Perhaps there's a jumper installed there or something else shorting the two power on/off switch pins together.

The +5VSB (SB for standby) just provides a small amount of +5V to the MB while it is "off", to power the circuitry needed for managing the on/off function, wake-on-xxxx functions, etc.