HI there very strange black spot appears on my screen at startup snd stays there like a strange black moon. Serial number 4309-2091. It is an older unit but worked great aside from moon we upgraded the electronics on the boat and installed this on our smaller boat. When I turned the unit on the radar antenna now wouldn't spin moon still here. On the big boat we had the fish finder and gps hooked up but all we wanted to use it on the small boat was the radar function. So only the radar antenna is hooked up. Two problems the black spot and how to get the radar antenna spinning again. Thanks Don.