"Mystery Radio or Ground Loops
While working in Chicago's "The Second City" theatres, I came
across a mic input that was picking up hefty amounts of "WBBM
News Radio 78 AM". From my experience, radio interference
is usually caused by poor grounding or a ground loop. For those
unfamiliar with the term "ground loop", this refers
to a system or device's multiple paths to ground.
So, I knew there had to be a ground loop due to the blaring radio,
but where? Inspection of the system showed that the mixer was
properly grounded and all outboard and power amp AC cords were
lifted. Next I checked the video recorder and camera grounds.
This equipment was used for taping and dressing room cues. Nine
times out of ten, ground loops in theatres can be traced here.
I've even encountered ground loops caused by cable TV installations.
Cable ground loops can be remedied by installation of a
lifting transformer on the audio section, available at Radio Shack
(in the car audio area).
But none of the above was the cause of my radio problem. After
climbing thru the suspended ceiling of the theatre, I finally
found the cause of my headaches. Years ago during the original
mic snake installation, the installer grounded the case of the
XLR connectors to pin 1 (a practice that I don't recommend).
It turned out that the shell of one of these connectors was touching
against the metalwork of the suspended ceiling, which in turn
was touching the stage lighting grid... turning the entire ceiling
and lighting system into a giant radio antenna. One little snip
of the shell to ground connections solved all my problems with
"News Radio 78". The weird thing was that as soon as
I fixed this radio interference problem, in walked a reporter
from News Radio 78 to interview the cast!!