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RF feedback into MH3 mic with KX3 on 2m

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Richard Corfield

RF feedback into MH3 mic with KX3 on 2m

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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Nr4c

Re: RF feedback into MH3 mic with KX3 on 2m

Turn down TX Monitor volume.

Sent from my iPhone
...nr4c. bill

> On Feb 5, 2020, at 5:23 AM, Richard Corfield <[hidden email]> 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:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
Message delivered to [hidden email]

Richard Corfield

Re: RF feedback into MH3 mic with KX3 on 2m

Even with monitor running into headphones?

I was running the contest with monitor off at first, but I turned monitor
on only in response to my contact telling me my audio was garbled. That
said, I initially wondered if the garble was due to frequency drift not
feedback. Though if it was a monitor problem it would also manifest on HF
and through the vocal mic?

I can try though. I know the headphones leak, closed as they are, as I've
picked up notification beeps when using the field recorder and monitoring
on headphones while recording my voice.

- Richard

On Wed, 5 Feb 2020 at 15:29, Nr4c <[hidden email]> wrote:

> Turn down TX Monitor volume.
>
> Sent from my iPhone
> ...nr4c. bill
>
>
> > On Feb 5, 2020, at 5:23 AM, Richard Corfield <[hidden email]>
> 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:[hidden email]
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> > Message delivered to [hidden email]
>
>

Richard Corfield

Re: RF feedback into MH3 mic with KX3 on 2m

In reply to this post by Nr4c

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 <[hidden email]> wrote:

Richard Corfield

Re: RF feedback into MH3 mic with KX3 on 2m

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 <[hidden email]>
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 <[hidden email]> wrote:
>
>> Turn down TX Monitor volume.
>>
>> Sent from my iPhone
>> ...nr4c. bill
>>
>>
>> > On Feb 5, 2020, at 5:23 AM, Richard Corfield <
>> [hidden email]> 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:[hidden email]
>> >
>> > This list hosted by: http://www.qsl.net
>> > Please help support this email list: http://www.qsl.net/donate.html
>> > Message delivered to [hidden email]
>>
>>

Nr4c

Re: RF feedback into MH3 mic with KX3 on 2m

In reply to this post by Richard Corfield

Yes. Headphone to mic distance gets short and feedback can happen from leakage.

Sent from my iPhone
...nr4c. bill

> On Feb 5, 2020, at 10:44 AM, Richard Corfield <[hidden email]> wrote:
>
>
> Even with monitor running into headphones?
>
> I was running the contest with monitor off at first, but I turned monitor on only in response to my contact telling me my audio was garbled. That said, I initially wondered if the garble was due to frequency drift not feedback. Though if it was a monitor problem it would also manifest on HF and through the vocal mic?
>
> I can try though. I know the headphones leak, closed as they are, as I've picked up notification beeps when using the field recorder and monitoring on headphones while recording my voice.
>
> - Richard
>
>
>> On Wed, 5 Feb 2020 at 15:29, Nr4c <[hidden email]> wrote:
>> Turn down TX Monitor volume.
>>
>> Sent from my iPhone
>> ...nr4c. bill
>>
>>
>> > On Feb 5, 2020, at 5:23 AM, Richard Corfield <[hidden email]> 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:[hidden email]
>> >
>> > This list hosted by: http://www.qsl.net
>> > Please help support this email list: http://www.qsl.net/donate.html
>> > Message delivered to [hidden email]
>>

Richard Corfield

Re: RF feedback into MH3 mic with KX3 on 2m

In reply to this post by 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 <[hidden email]>
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 <[hidden email]>
> 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 <[hidden email]> wrote:
>>
>>> Turn down TX Monitor volume.
>>>
>>> Sent from my iPhone
>>> ...nr4c. bill
>>>
>>>
>>> > On Feb 5, 2020, at 5:23 AM, Richard Corfield <
>>> [hidden email]> 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:[hidden email]
>>> >
>>> > This list hosted by: http://www.qsl.net
>>> > Please help support this email list: http://www.qsl.net/donate.html
>>> > Message delivered to [hidden email]
>>>
>>>