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

[Richard Corfield]

The metal shield cover on the 2m module was not flat but slightly raised at
the edge. I have pressed this back on and the problem seems cleared. So I
guess it was contacting or coming close to the circuit board on the control
module, perhaps that giving the feedback loop for RF. Question is - could
any damage have done if it had contacted the vias/pads on the board there?

 - Richard

On Wed, 5 Feb 2020 at 21:01, Richard Corfield <richard.corfield at gmail.com>
wrote:

> 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
>> > ______________________________________________________________
>> > Elecraft mailing list
>> > Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> > Help: http://mailman.qth.net/mmfaq.htm
>> > Post: mailto:Elecraft at mailman.qth.net
>> >
>> > This list hosted by: http://www.qsl.net
>> > Please help support this email list: http://www.qsl.net/donate.html
>> > Message delivered to nr4c at widomaker.com
>>
>>