[Elecraft] RF feedback into MH3 mic with KX3 on 2m

[Richard Corfield]

I had issues with RF feedback on the MH3 during the UKAC SSB 2m contest
last night. It seemed intermittent. Result was terrible noise in Monitor
output (my headphones) and to the other station, so it affects the audio
input. With experiments at the time and this morning:


   - Occurs when RF power is increased above 0 and when Mic Gain is
   increased. I could operate with Mic Gain set low, obviously at reduced
   power out. (Indicative of a feedback loop)
   - Occurs with a dummy load. (Also when I moved the antenna away last
   night). So is the loop internal or is it so sensitive the small leak from a
   dummy load is enough?
   - Does not occur if I feed a stage Mic In through a field mixer (kit I
   have to hand). The field mixer claims an output impedance of 150R and is
   running at mic level to match the MH3 allowing same gain setup in the KX3.
   Mic Bias is turned off in this case.
   - Does not occur on HF into the dummy load.
   - Manifests as buzzing on 2m FM
   - May result in phantom button presses on the KX3 Mic (I didn't think I
   knocked the dial)

As an aside - one station last night was loud enough to activate the "RX"
indicator which I take to be input overload. S meter was well over S9 but
not at the top. I didn't see anyone else at the site, unless they were
hiding the the woodland carpark but I saw no light there or on the summit
behind me. I hit ATT, should have swung the beam to try to null them. But I
guess the ATT and even maybe the input protection is after the 2m module's
input?

My thought is I need to inspect the wiring in the mic. Or could there be
anything else I need to check?

Thanks
  - Richard