Furuno 1721 misbehaving

[fudge]

We have a Furuno 1721 radar which has been a faithful friend for many a year, a reliable companion on the night passage showing up the behemoths that gross our path in the lonely hours of mid-watch.
Until now, when switched on prior to crossing the shipping channel into the Dardanelles all worked except for an absence of echoes.
Our manual suggests a naughty magnetron and proposes some tests. However, it does not specify whether the unit should be switched on or switched on and transmitting in order to perform the tests. Reading the dire warnings about getting anywhere near the scanner whilst transmitting I am dubious about venturing near it with my multimeter.
Do any budding electronics buffs who may be reading this know if if needs to be switched on and transmitting in order to test it. Oh! and just how easy is it to replace a magnetron?

 

[alan]

Depends on what they are asking you to measure. Typically a radar transmitter would draw a small amount of current in standby which would be a heater current to the magnetron; then when transmitting there would be high voltage applied to the anode and "large" current drawn; the heater current is not needed any more as the cathode is excited by stray electrons getting back onto the cathode.

So, it depends on what they are asking you to measure.
I would imagine you could make the measurements from behind the scanner; that is stop the scanner rotating and work from behind it being careful not to put your face/eyes or hands anywhere near the front of it.

I would suggest that there are two possibilities here:
One is that there is no transmission (could be the magnetron but not necessarily).
The second is that there is no reception (could be the Low Noise Amplifier, the I.F. amplifier or the video amplifier as the likely culprits).

Maybe the measurements they are suggesting you make is to try and eliminate whether the problem is Tx or Rx??

Alan.

 

[MarkJohnson12345]

Its possible that the drive belt has come of the motor and antenna.

Same happened to me last year. The electronics and screen show the screen being refreshed as if the antenna is rotating.

To allow this to happen the antenna will have stopped with its rotating sensor over the static sensor in the radome. Otherwise you would get an error message.

Second time it happened to me, I managed to refit the belt whilst the radome was still up the mast. Just a matter of undoing the radome cap, easing the cap up until you can reach inside and refit the belt.

But then you will have to find out whey the belt came off in the first place.

Worth looking at in the first place.

regards

Mark J

 

[hightech]

If you turn the gain right up and get snow on the screen, then chances are it is the magnetron. If you get no snow, check the video cable (the screened one) has good continuity as this will appear like a duff magnetron or front end problem. If your on time is of the order of 2000+ hours then your magnetron is likely to be gone.