[Mondial]

Try connecting the green ground wire to the chassis. Although not shown on the schematic, the manual mentions that the chassis and case of the scope is grounded with a 3 wire cord. See page 2, paragraph 7 of the service manual "test instrument safety"

Some previous owner probably " lifted " the ground by disconnecting the green wire, so they could work on "hot chassis" sets without an isolation transformer. This is a very dangerous practice and should never be done.