[Scott Dorsey]

Bob Simon wrote:
When I connect audio out from my cable box to my pre-amp, I get hum. I was=
able to eliminate it for a couple of years by inserting an isolator (ISO-M=
AX VRD-1FF) just ahead of the Contour cable box.

Because there's a lot of 60 Hz trash on the cable shield, possibly because
it's not properly bonded.

Two days ago, the cable company disconnected my cable TV service because th=
ey determined interference was coming from my house. Yesterday, a Cox tech=
nician looked at my connections and stated that the source of the interfere=
nce was my isolator. He directed me to remove it, performed a measurement,=
and said that everything was ok now, so he reconnected my cable TV service=
. No surprise - the hum is back.

Get a real technician out. Likely the "interference" they are referring to
is 60 Hz trash on the cable shield, and it's caused by the cable not being
properly bonded to ground at your house. When the ground isolator is removed,
the system is being grounded through your TV set (which also provides a
path to discharge lightning and other inadvertent transients), and so the
cable folks don't see the 60 Hz trash. This is an unsafe condition.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."