I'm an old ret'd Marine
Electronics Tech. The Jrc 1000 came on the market about the time I was retiring,so I never had any personal experience with them. I did replace several of the 1st version (not the JRC 1000 MKII) with Furuno
or Koden because the 1000 failed early & comm. fishermen don't/can't waste time on troublesome eqpt.
However,they were probably fine for occasional recreational use. I found that magnetrons don't last as long as 3KW & up in most of the little 1.5 & 2KW radars of all brands in comm. use. Like everything,they are built down to a price
You need to check one more thing before you call it a defective mag. & that is Tuning.
If you have lighted screen (raster),range rings, that proves the display OK.
If you also have noise
speckles on screen on long range,with STC(Sea Clutter) turned OFF,FTC (Rain) turned OFF & Gain turned UP, & a heading flash-your antenna
is turning & receiver/IF board in the dome is working. The video coax built into the display to dome cable is also OK. A clean screen with rings but no speckle indicates an open video coax. Note: Turning the Gain up & down should make the speckle come & go.
If you still have no targets & no "fuzzy thick donut" rings in the first mile or less,check the tuning bar graph at top of screen on long range. It should be going up & down some as the antenna turns & hits targets. The closer the target the higher the graph-it is measuring the strength of echos rec'd.
If tuning indicator is low & not fluctuating, you can try manual tuning per your owner/installation manual.
If manual tuning doesn't produce targets it's a good indication mag. is not transmitting. The driver ckt board (Modulator) could be defective but you need a Tech for this. Caution!! 5Kvolts at mag-modulator board will burn a nice deep hole in your poking finger.
Having to manually tune usually means the mag. is getting tired & will soon quit.
Fuzzy donut rings are another indication of a mag. on it's way out.
Turning radar to standby & waiting for a few minutes for mag. heater
to cool,& then switching back to transmit will sometimes shock a dying mag. into working for bit but it is dying.
The magnetron is a tube-the last tube still in use probably-& they wear out. They are rated for 1000 hrs but some go much longer though usually at poorer performance-more sea clutter & less distance.
I Googled for a JRC1000 manual/diagram to identify the mag. used but no joy. Lots of MKII manuals
but I don't know if mags were the same.
Remove the dome cover & ID the mag.
Here is a x-ref list,but it won't surprise me if it's NA as JRC also make mags & have discontinued others Radar Replacement Guide
JRC built small radars for Raytheon
for yrs & some of those are mags are NA.
Best to stick with Furuno
or Koden-they build the majority of small radars regardless of whose logo is on them.
I can't recommend you drop $US500 for a new mag for that radar-it is quite a gamble. Shop Ebay.