Bound to be a bad connection somewhere. Something too loose, having surface corrosion
, an internal break in the wiring
Pay very careful attention to the connectors for the control cable. They are junk....totally unsuited for a marine environment
, but Icom has chosen to use them anyway. Those little pins tend to go astray, get bent, get corroded, come loose or come out easily.
I agree that one would not think first of the control cable, given the symptoms first described (intermittent SWR), but I suppose the tuner could somehow be attempting to retune while transmitting. Anyway, rule
that one out.
Next, I'd put a good power/SWR meter in line near the transceiver box. And replace the antenna
coax at the back of the radio with a dummy load. Use the low or mid power setting first, the FSK mode, and press the mic button. Leave it depressed for several seconds and see what power output you get.
Then, kick up the output to high power (Func High). Repeat the process and see what you get. You should see a steady output...no wavering and certainly no flashing "SWR" indicator. This assumes that the two pigtails are properly terminated with PL-259 connectors....no shorts or opens.
Now, replace the dummy load with the coax-to-tuner connection. Try the output power/swr tests again, picking a clear frequency.
If you get the intermittent SWR problem now, and didn't get it with the dummy load, then you have a problem somewhere in the antenna system
, meaning coax and the control cable to the tuner, the tuner itself, the RF ground system, the GTO-15 wire to the antenna, or the antenna itself.
Some detective work should find the problem.
Sometimes coax goes bad with age, mistreatment, misuse, and just plain use....especially when it hasn't been terminated correctly and protected from water
and moisture intrusion in the first place.
If you suspect the coax, replace it and be sure the connectors are put on correctly. If the run to the tuner is very difficult, use a longer length of NEW coax....hey--it's not that expensive --... and run it thru the cabin
to the tuner to see if that fixes the problem.
Good luck, and please let us know what you find.