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

[Richard Corfield]

That didn't fix it. Found this lunchtime with a real antenna:

   - Worked OK with case open, testing the idea it was coupling from the 2m
   module itself.
   - Worked on other mic (SoundMagic earbuds and mic)
   - Touching the case with metal made interesting noises on receive.
   - HF (20m) seemed OK, if I take the response "America only please" as
   indicative the other station heard me and my signal was good enough for him
   to tell I was not in America.

 - Richard

On Wed, 5 Feb 2020 at 22:07, Richard Corfield <richard.corfield at gmail.com>
wrote:

> 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
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>>>