Sun 26th Mar 2017, 20:29
RF breakthrough. - your output is getting back into the radio, being demodulated and is the RF version of a feeding back microphone. What you need to do is to work out how it's getting in. First thing I'd try is to look at what you have plugged in. You have the speaker, but is anything else in? Data cables, keyers etc.
Does it happen with the external speaker removed, using the radio internal one? I suspect it doesn't? This shows the route into the radio is via the speaker cable. If you have a meter, see if the sleeve of the connector jack is linked to the metal back panel of the radio. Not certain if this external socket 'floats' or has a grounded sleeve, but most have the sleeve connected to ground and the case. If you discover there is no short to that sleeve, it can be a good route into the radio for the RF causing the problem. Check the speaker cable to see if it is simple twin core, or thin coax.
You could always swap twin conductors for coax, which may help - but I suspect the instability might...