Radar isn't consumer friendly since repairs that involve opening the cabinet of the unit or the scanner -- the rotating antenna -- or any change that can't be controlled by the knobs or switches on the front of the unit must be made by a federally licensed radar repair technician. In spite of that constraint, though, most radar problems are related to blown fuses, open circuit breakers or even a power switch in the wrong position.
Turn the radar unit on. If the unit fails to light up, check the fuses -- the location is marked in the radar owner/operator's guide; if these are in order, take the unit to a licensed radar technician. If the unit lights up, but fails to display an image, look at the scanner to ensure it's rotating.
Check the scanner for a blown fuse if the radar isn't rotating, then follow the wire from the scanner all the way to the radar -- if the scanner draws power from the radar -- to the circuit breaker. If the scanner doesn't draw power from the radar, check the scanner power switch to ensure it's in the "On" position. Set your multimeter to test for continuity. Place one probe of a multimeter on one post of the switch and the remaining probe on the other post of the switch and ensure the switch is viable; if so, call the tech. The problem is within the scanner antenna.
Turn the gain knob all the way to the right, clockwise. If the display doesn't show an image, take the unit to a licensed radar technician. If the display comes on but shows a "white screen" -- the screen is covered with interference -- turn the gain knob counterclockwise until images are suitable and little, if any, interference remains on the screen, and adjust the range.
Find a "target" that's visible visually and make sure it's visible on the radar. If not, turn the "Gain" knob or press the "Gain" control and the "+" side of the common turning bar until the object appears as a target on the radar.
Reduce the range (how) to 1/4 mile or less. Turn the "STC" control until there's a very small, fine scatter of "hash" at the centre of the screen
Raise the range (how) to 6 miles and turn the "FTC" knob until there's a barely discernible amount of interference on the screen.
Modern radar units run a diagnostic protocol when activated. The gain is set automatically. If you have a problem where you have to adjust the gain yourself, either the knob has been turned (on purpose or accidentally) or there's a problem with the unit. After adjusting the gain, turn the unit off and back on. If the gain isn't just as you left it, take the unit to a technician.
If you are making way in conditions of reduced visibility when your radar goes out, reduce your speed to the minimum required to maintain control of your boat -- called "bare steerageway" -- and post a lookout in the bow of the vessel, if one is available. While the nautical "Rules of the Road" recognise radar as an "aid to navigation," the rules require you to rely on your eyes. If conditions make that impossible or uncomfortable, the prudent mariner will suspend his voyage until visibility improves. Don't tinker. Depending on the radar and the power source for your boat, some circuits may be high-voltage, high-amperage circuits that can kill.