The symptom you describe sounds like a case of no radar targets. To further diagnose, put radar on the longest range and turn Gain to full, Sea Clutter to Minimum, Rain Clutter to Minimum, and observe the screen. The presence of "grass" or evenly distributed clutter indicates that the receiver and cabling is good and the radar may not be transmitting. No transmit is caused by a bad magnetron or a bad modulator in the antenna, or depending on model there is still an outside chance that the power supply in the display has failed.