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

[Nr4c]

Turn down TX Monitor volume. 

Sent from my iPhone
...nr4c. bill


> On Feb 5, 2020, at 5:23 AM, Richard Corfield <richard.corfield at gmail.com> wrote:
> 
> 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
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