[AMRadio] mike hu-m-m-m-m problem

Jim Candela jcandela at prodigy.net
Mon Dec 5 13:42:16 EST 2005


   If the problem is the mic element picking up stray
hum, then moving the mic around will vary your
symptoms. Do you see that?
If so, you can use the mic to zero in on the source of
magnetic flux. Steel, or preferably Mu-metal (the
stuff around oscilloscope CRT's) can offer shielding
from stray flux. 

   Here is a silly thought; remember an old photo in
the ARRL handbook where a car distributor cap was
shielded with a large tin can, and then copper braid
was added to shield each plug whire, and was soldered
to the same tin can? Doing something similar to your
prize mic might help, or go back to an Astatic D-104.
That crystal element is immune to this problem. :-)

   More seriously, maybe you can find the source of
flux, and eliminate it at it's source. For example
with my QRO 20A I had hum in the nulled carrier (120
hz) that was much worse after adding a filter choke
(open frame)to the power supply. The flux from the
choke was coupling through the metal chassis, and into
the two modulation transformers. The fix was to
replace the choke with another that was enclosed in a
steel case. Problem was solved. I could have also
tried to change component orientation, but limited
real estate to do so did not allow this option. I
still had some hum in the carrier null but this was 60
hertz. I added a hum neutralizing circuit to buck out
the hum in the proper phase, and magnitude. In the end
with a 6.3 vCT  filament circuit, I used 20 ohms to
inject current into the balanced modulator diode
support bracket. The carrier null was now without a
measurable 60 hz component.

 I hope you resolve the huuummmm issue with that nice
Heil microphone.

Jim Candela

--- W3CRR <w3crr at aerialacts.com> wrote:

> I'm having tons o' fun with my "new Collins 32V-2,
> BUT, I've got a hum 
> problem. According to several listeners, my Heil
> Classic microphone -- 
> while sounding great otherwise -- may be picking up
> EMI from the nearby 
> transmitter iron.  The mike has no internal
> humbucking coil.
> Has anyone else experienced this problem with
> dynamic mikes and heavy 
> iron?  If so, how did you solve it, please?
> Many thanks and 73,
> Craig
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