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

[Richard Corfield]

I've done some further tests and proven the problem is with the MH3. So
I'll have to take that apart and look for the problem.
The radio is fine with little earbuds/mic plugged in via splitter cable.
Swap out the mic on the earbuds with the MH3 and leave all the rest the
same and it goes wrong.

 Thanks
 - Richard

On Wed, 5 Feb 2020 at 15:29, Nr4c <nr4c at widomaker.com> wrote:

> 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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