|Cause: Incompatible mains adaptor being used to power the unit, where the voltage and/or current rating is too low. This allows mains-ripple (at 50Hz or 60Hz) to become evident in the video signal you see.|
Solution: Use a correctly-specified mains adaptor, preferably regulated, with a current rating of at least that shown on the label on the underside of the unit. Do not use mains adaptors with a lower or higher voltage than is required. A higher current rating will not usually cause a problem, since the unit being powered will just draw less current than the mains adaptor can supply.