If you are connecting to a laptop, try running it off batteries (unplug the A/C cable from the wall outlet. If you are using a desk PC, make sure that the computer and SQ-80 are plugged into the same wall outlet. If neither of these works, get one of these:

Ebtech HumX

I keep one of these and also the Ebetch Hum Eliminator in my gig bag and they have both saved the day (or night as the case may be) many, many times.