It could be a few things,
The 1621 was in production around the early 90s and if no service has been done to the radar since it was originally installed than it could be a component that is starting to fail. For example, if marks and legends appear, but no noise nor echoes appear it could be the MIC; if marks, legends and noise appear but no echo it could be the magnetron. Other things it could be are a failing CRT/LCD, or a PCB component.
You could have the unit sent into our service/repair center, or have your local dealer send it in. Otherwise, the 1623 radar replaces the 1621s and the 1623s have been an excellent standalone radar now in production almost 20 years.
Hope this helps, :sail
- Maggy