Thanks very much for that prompt reply A&C. I'm still really puzzled by this. The King KMA-20 operating guide says this:
"When you press the mic button, the output of all aircraft recievers is electronically muted so there is no cockpit feedback in your transmission". Note their use of the word 'all' suggesting that crosstalk between radios could not reach the headset. Their specification further states: "
Input Muting: Audio Muting is 40db or better when a transmitter is keyed"
The original installation of 2x KX170 worked fine with no interference during box 2 operation. I've swapped the KMA20 for an identical unit with no difference in results.
So my questions are:
Is it true that the KMA20 mutes 'all' receivers? If so, where does the sidetone come from? Is that the 40db referred to in the KMA20 specification?
If it's true about the muting, how could RF breakthrough possibly produce deafening interference in the headset? I take your point about shielding of the leads to the Sigtronics but I reasonably sure my installer has looked at this and in any case I don't see how a shielding problem could produce the deafening breakthrough that I experience when using either radio.
I note that the SPA-400 wiring diagram
here shows only the aircraft mic key and mic audio signals are passed back to the KMA20, not the individual radio mic key and audio. Could this be anything to do with it? (Noting of course that it worked perfectly well with the original KX170 radios).
Finally, I understand that the MX170 may not have the best reputation (the company are certainly very hard to contact) but is there any specific way in which it could cause my breakthrough problem? I've had no problems with it at all and it's a clearer and more reliable radio than the 430.