whom do i get to fix a ground loop hum?

You can get a CALRAD isolation transformer (CALRAD part 75-5040) for less
than $6 (not $60 !!!), works like a charm and is used by several people on
Agon and Asylum.

For some reason all previous threads where it has been recommended have
been removed. Anyway, I ordered mine from a local electronic shop and it
took 2 weeks to arrive. You can find several online sellers by using Google.

Good luck!

Sorry for the incomplete information earlier: From the Calrad manual: "
Although the CALRAD 75-504 can actually meet the basic frequency response
requirements ..., the relection coefficient ... is not low enough at the
frequency extremes to prevent long cable runs from introducing ripples in the
response that disrupt the digital communication. ... . For this reason, we do
not recommend trying to use the 75-504 for either Digital Set top cable
boxes or broadband cable modems. It will not harm the system in any way if
you do try it to see if it will work"

More from the CALRAD manual:

Insertion loss: 1.5dB
Frequency: 5 MHz - 1000 MHz

I only had it connected with regular (analog cable) where it worked great. I
have not tried it with digital cable. I am really not that much of a TV person, I
only get the very basic, local 10 channel package. Not even close to the
digital 100+ channel package.