Magnetron Destruction

[Charlie]

I have a Furuno 1831 Mark 2 Radar and it has stopped working. With an hours worth of work the tech was able to get the CRT to function on a simulator. I saw it it worked. Plugged the CRT into the cables on the boat and it boots up but then shows little signs of now but no hard targets. The tec said that the chances are that the magnetron is not functioning. There are only about 500 hours on the 1996 vintage radar. He said that old age, corrosion, or getting pinged by a military radar could cause the magnetron to be destroyed.

I was in a crossing situation with a submarine off of Point Loma a few months back. My radar was not on but I think his was. A friend who is a ex-navy pilot was at the helm. When I saw that we were in a situation where we were not going to cross I told him to make a sharp alteration to starboard and take the subs stern. Yes we have the ROW but I give people the guns the ROW even if they do not ask for it. The ex-pilot argued that we were a sailboat and had ROW and I said yeah but he has a machine gun. We laughed and my friend started to take the subs stern when we noticed the bow of the sub come out of the water and we were no longer in a colision situation. The sub crossed our bow and then slowed back down. Our closest point of approach was maybe a 1/4 mile.

Could the subs radar have destroyed the magnetron on my radar even though my radar was not on?

[LtBrett]

The power put out by surface search radar is miniscule. No way the sub is responsible. In fact, I'm scratching my head at how any military radar could fry someone else's magnetron (and only the magnetron). I guess it is conceivable if you blast one radar with another of the same frequency you could overload one magnetron. Or focus a continuous wave fire control radar at your radar. But the most likely culprit is wear and tear or a manufacturing defect.

Brett

[Lodesman]

Agree with Brett - the sub wasn't the culprit. Magnetrons should never be transmitted where there's no load, ie. no antenna or no connection to the antenna; that causes them to fry. I would check your cabling and antenna before firing up the replacement.

[Charlie]

Thanks for the info guys. I guess I will want to be there when the guy works on the radar then.

[amytom]

The magnetron is a high power device and would take a lot of power to fry it. For outside energy to fry anything it would be your receiver chain or T-R tube switching device. As you said the unit is old you probably had some corrosion or moisture in the tube and it arced itself to death.

[amytom]

Originally Posted by LtBrett

Or focus a continuous wave fire control radar at your radar.
Brett

Not going to happen. Even the 48e in pencil beam mode would fry the rcvr before the tube.

[ZS1MM]

It is very easy to check if the magnetron is still OK.The most likely thing to get damaged by a very powerful radar at close range is the receiver mixing diode. This is also very easy to test and very simple to replace.

[GordMay]

Originally Posted by ZS1MM

It is very easy to check if the magnetron is still OK.The most likely thing to get damaged by a very powerful radar at close range is the receiver mixing diode. This is also very easy to test and very simple to replace.

Greetings and welcome aboard the CF, ZS1MM.

Please describe the 'easy' procedures for testing
- Magnetron
- Rx mixing diode

[ZS1MM]

It seems that my previous reply was not logged. To test the magnetron all you need is a portable broadcast band AM radio . With the radar switched to operate mode and warmed up , hold a portable broadcast band AM radio about 10 meters or so in line with the scanner and you should clearly hear the sharp tone bursts as the RF lobe passes over the radio. This confirms that your magnetron is working and that the radar is transmitting. Checking the receiver mixer diode on the older radars requires you to open the scanner and look for the diode in the receiver front end . It is normally a 1N23WE type cartridge diode which you can easily take out and measure with a digital multimeter. On the more modern radars this is not so easy ,as the receiver front end consists of a single module . You need to have the correct test equipment setup to test it. You can however do a quick check by running the radar with all the anticlutter and other noise reduction devices switched off. Slowly turn up the gain and you should be able to see quite a lot of snow/noise on the screen. This is a good indication that the receiver is OK.

[bill good]

There will be a test point where the Transmitting current can be checked. I would also be going for the mixing diode.

[s/v 'Faith']

Wow,

What a factual thread with good info / advise.

The replies I was going to make were already made... so there is no need to restate them.

[s/v Jedi]

Often the magnetron doesn't die suddenly. My scanner is still transmitting but at a fraction of the normal output power. I can only see close-by targets with the gain at manual and high up., just before saturating the screen with snow/noise.

Measuring the transmit current can determine this problem and is the best way to go... but only useful for people who know how to do that or a documented test-point.

cheers,
Nick.