This is a fault in the NEC screen
unit. I had a Northstar M84, the same unit in a different box. It didn't seem to affect the operation of the plotter. However one day when the unit was about 3 years old the screen
went black. I traced it to the failure of two very thin fluorescent tubes that light the LCD screen from the bottom edge. The electronics
still functioned but could not be seen as there was no light in the unit. I tried to replace the tubes with a LED replacement but only managed to "fry" the LED also. It was operational in pieces on the bench but when I assembled it I must have shorted something and smoke billowed out! The screens and fluorescent tubes are replaceable if you don't mind opening the unit. The displays are available on Ebay for around AU$700.