[CQ-Contest] Audio hum

[Gilmer, Mike]

Late last night, while setting up for the ARRL SSB contest, I noticed a fair
amount of hum in my monitored microphone audio, while hooked up through a
DVP.  I hear the hum by having the monitor on, and keying the radio (PTT
button).  I'm not "on-the-air" - 940's output power is turned down all the
way - so it's not RF.  Processor in or out doesn't matter (unless you turn
the appropriate gain way down - then of course the hum goes away - and
everything else!)

I've noticed this hum before (months ago), but I had lots of other trouble
with the station, so I never addressed it.  It's not a HUGE hum (like when
you are fooling around in the back of stereo and half-plug-in a RCA jack)
but it's more than I want to hear, although I'm not really sure how much
gets out on the air.  Now that it's almost contest time and (nearly) too
late, it's the perfect time to fix it!

The conditions:
I have a TS-940SAT, Heil headset, DVP, "official" Kenwood DVP cable (from
LZ).
I actually have three different headsets all with Heil type 4 elements for
the mike (old style Heil, new ProSet, and cobbled-up Telex with a Heil
element).  All tests have same results for all three headsets.  I also have
two DVPs (older US, newer LZ). Same same.  Only one official DVP cable,
however.

The test:
If I plug a headset directly into the 940 - no hum.
If I, instead, connect a headset through the DVP and its cable, I get the
hum.
I unplugged the DB connector of the DVP cable from the DVP board, and
jumpered the connector for mike audio (pins 1 and 3 was it?) - hum is still
there - so it's not the DVP board or its PC environment.

I suspect a ground loop or missing ground/shield in the DVP cable but don't
really want to hack up the "factory" cable.  Could the problem really be in
the radio?

Any ideas?

Mike N2MG
n2mg at contesting.com


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