[Elecraft] Heil Pro 7 headset with KX3 - RF feedback?

[Drew Arnett]

Ignored this for quite a while, but came back to take a look this
morning.  Sharing in case it helps someone else out in the future.

I think the jack on the headset itself has an intermittent and/or
faulty connection.  That's why trying the straight cable (which I
never use) as Heil suggested didn't help me narrow it down before.  (I
always used the curly cable and left the straight one in the box.)

Symptoms I see today:

KX3 on battery.  RF power level set to 0 watts (so no RF feedback of
course).  Mon set to 4.

Gear:  Pro 7 headset.  Heil Y cable (for mic/foot switch) but using
the little hand/button switch Heil includes, a cheap pair of Walkman
style earphones, and a homebrew electret handmic (that also requires
5V bias).

No difference between curly cord that I normally use and the straight
cable that I never took out of the box.

In all cases, I use the Y cable.

Notice the combinations that work and don't work:
* walkman head phones & homebrew handmic - works OK
* walkman head phones & pro 7 mic - works OK
* pro 7 headphones & homebrew handmic - works OK
* pro 7 headphones & pro 7 mic - ***PROBLEM***

I suspect the feedback path is due to wiring fault causing the
headphone audio return to share path with microphone return causing
IxR coupling.  (I hate the term ground loop, but there you go.)

Wiggle testing.  Nothing at the jack/plug interfaces on the radio
(mic, headphone).  Nothing on the jack/plug interfaces on the Y cable.
Ah, the jack/plug on the headset definitely has some sort of
mechanical intermittent at the least.  And it does for both the curly
cable and the straight cable.  So, it's the headset and not the cable.
:-(

So, off to Heil for repair.  (Looks like snap plastic/spudger tool
work, which I'm not really interested in doing.)  Or to perhaps shop
for something more reliable?

Best regards,

Drew
n7da

On Sat, Jul 3, 2021 at 9:05 PM Drew Arnett <arnett.drew at gmail.com> wrote:
>
> No luck sorting this out in the field during FD, especially with a
> workaround available.  (Alternative mic.)  I'd like to resolve it if
> possible before replacing the Pro 7, the KX3, or both.  :-O  So, now
> that I'm unpacked at home, I'll try a simple setup for debugging.
>
> Trouble shooting setup now:  KX3 (with new, charged internal NiMH),
> Pro 7 with Y cable and that little clip on PTT switch they include, 3
> FT coax to (shielded) dummy load.  Electret mic element.  MIC BIAS on,
> CMP 0, PWR 5, 14 MHz, SSB.
>
> With MON 0, no RF output indicated while transmitting and speaking or
> whistling for MIC GAIN 0 to 80.
>
> With MON 3 (a level I like for CW sidetone) and PTT on, doesn't matter
> if I'm speaking/whistling or not.  The RF output indicates nothing
> until about MIC GAIN 70 when it abruptly comes up with a harsh sound.
> Other electret microphone sounds fine at MIC GAIN 60.
>
> This is probably something simple.  I just haven't found it, yet.
>
> Thanks,
>
> Drew
> n7da
>
> On Thu, Jun 17, 2021 at 3:36 AM Drew Arnett <arnett.drew at gmail.com> wrote:
> >
> > Shield of mic jack goes to bare metal Elecraft thoughtfully left on
> > the chassis metal.  10 nF shunt to ground.  (Need to see where that
> > is, but hopefully SMT and with very small loop area.)  Sheet 9:
> > https://ftp.elecraft.com/KX3/Manuals%20Downloads/KX3SchematicDiagramDec2012.pdf
> >
> > Heil suggests on their tips page generous ferrite choking of all 3
> > leads of their Y adapter cable:
> > https://heilhamradio.com/support/tips/
> >
> > Drew
> > n7da
> >
> > On Thu, Jun 17, 2021 at 3:23 AM Drew Arnett <arnett.drew at gmail.com> wrote:
> > >
> > > Hi,
> > >
> > > In the KX3 for Field Day department, we plan to use it for the GOTA
> > > station for phone on HF.  (Yes, will have antennas not on top of each
> > > other and bandpass filters on ALL radios.)  Would be great to setup
> > > with a headset and foot switch.
> > >
> > > Last weekend, I used my KX3 barefoot on 6 and with a
> > > transverters-store.com transverter (also barefoot) for 2.  6m antenna
> > > was 2 elements about 15 ft above the ground and 2m antenna 6 elements
> > > at 10 feet above the ground.
> > >
> > > I worked a couple of contacts on 2m phone with the headset.  Later,
> > > when I tried to use the headset for phone on 6, seemed like I was
> > > having a feedback problem.  Cabling was the stock cabling that comes
> > > with the Heil Pro 7 headset and a footswitch.  Monitor was on and set
> > > to 4.
> > >
> > > So, ALC didn't indicate anything at the normal level (60), and
> > > increasing it very abruptly caused horrible sounding audio.  (Yes,
> > > acts like positive feedback.)
> > >
> > > I have a small electret mic on a 1/8 plug thing I bought some time ago
> > > from the computer store.  (Meant at the time for adding mic to
> > > laptops.)  I have a 1/8" audio extension cable for it.  I plugged that
> > > in instead of the headset mic and it seemed to act normally.
> > >
> > > Is this purely an RF from the TX antenna getting into the mic audio
> > > problem or is it something else?
> > >
> > > Best practice to avoid this problem?
> > >
> > > On my todo list:
> > > - ask here
> > > - look at KX3 schematic to see if it has RF ingress mitigation on the mic jack
> > > - ask Heil
> > > - unplug the Heil mic element and gator clip my cheap electret mic on
> > > instead and try on 6m in driveway with identical antenna/mast/radio
> > > setup
> > > - look for or assemble an RF filter to inline with the mic jack (tough
> > > to do with this little time
> > > before field day)
> > >
> > > Thanks and best regards,
> > >
> > > Drew
> > > n7da
> > >
> > > ps.  Other than this, KX3 was great in the VHF test.  Worked great
> > > with the transverter on 2 and was a lot of fun running barefoot with
> > > the sporadic E opening Saturday afternoon & evening.