I have a Furuno 1623 radar on my sailboat. Unit started to lose targets beyond 2 miles. Good target display on first sweep, 50% reduction on second sweep, then nothing. Local authorized Furuno service technician tested unit on the boat, said the magnetron was probably failing. He replaced the magnetron and adjusted heater voltage (was low by 0.5 volts). Re-installed radome on mast. Still has same problem as before. Works great at 1.5 miles. When changed to 3 miles, good target display for first sweep, second sweep much degraded, third sweep, nothing. What should I do?