It often happens that the magnetron breaks down during the voyage. It is advisable to estimate the remaining resource in advance and have a magnetron in stock for replacement. On average, the work of the magnetron on transmission should be 3000 hours. By experience I will say that S-band radars have a resource of up to 4000 hours, and x-band up to 10,000 hours. On each modern radar, the time of operating the magnetron is displayed. It is also important to monitor engine performance. manufacturers guarantee work up to 10,000 hours, according to experience 20000-30000 hours.
If you are in the sea or to the nearest engineer very far I give the instruction on replacement on an example of radar JRC:

1 Completely de-energize the radar (indicator, scanner and possibly a separate automaton on the motor) on the power supply board of the navigation equipment.

2 Place a sign on the indicator and machines that work is being done on this station, also on the second one, if there is, in order not to get excess radiation

3 Go up to the scanner of the required radar, always with a safety belt.

4 Turn off the antenna rotation switch (usually on the side of the casing)

5 Use a wrench to remove the radar cover and make sure that the scanner does not receive any voltage

6 If a protective cover is mounted on the modulator - we remove 

7 Unscrew or solder the contacts of the magnetron (yellow and green wire) - always remember the polarity. We unscrew the magnetron itself (4-9 bolts) and install a new one in its place.

It should be very careful when connecting the magnetron wires, you need to cut off excess length so that there are no bare wires visible from the connector.

8 Twist all in place in the reverse order.

9 Turn on the engine tumbler on the scanner.

10 Turn on the power supply of the indicator, scanner, motor on the power panel of the navigation equipment.

11 Turn on the radar and leave in the STBY mode for 20-30 minutes
12 set the gain control, the interference from the sea and rain to a minimum and set the smallest Range.

13 gradually add GAIN before the appearance of the image

14 gradually add Range to 12 nm and evaluate the quality and stability of the image. If the targets disappear - turn off the radar in STBY mode for another 5-10 minutes and try again. With a stable picture, we leave the radar on the radiation at a 12 nm scale for 15 minutes.
15 Let's go to normal mode of operation.