Hum when cablevision is plugged in to tv

It is best to use the simple solution of isolation with two cheap 75 ohm to 300 ohm transformers of the capacitor -- not transformer -- type back to back. You can chech this with an ohm meter. There should be infinite ohms between the 300 ohm side and the coax side. If your house wiring is connected to the cable TV company's ground and not hardwired directly to your power service ground, you have set up alternate paths for lightning all through your house ( besides the hum problem ). Lightning will seek the path of least resistance. Then if the cable company's ground is better than your own service entrance ground, lightning will go all through your hose and out the cable to ground.
Don Scott

This approach works to block the DC offset but it does not eliminate it. Get the cable company to fix the ground.

Kal

Glad to hear it, Romad.

Ncarv I tried your suggestion and it worked! Thanks again for everyones help.

This question has come up numerous times. Here is a copy of my solution which I have posted before:

I had this same problem. Here are the two parts you need, total cost $11:

http://www.radioshack.com/product/index.jsp?productId=2103063&cp=&kw=300+ohm&kwCatId=2032057&parentPage=search

http://www.radioshack.com/product/index.jsp?productId=2062059&cp=&pg=1&kw=ohm&kwCatId=2032057&parentPage=search

Connect the cable coax wire to the transformer/splitter.
Connect the wires from the transformer/splitter to the 75-ohm coax combiner/splitter.
Connect the 75-ohm coax combiner/spltter to the input on your TV.
This was done for me by the cable TV guy. Worked like a charm. Why? I don't know. I guess it breaks the ground loop.

I called the cable company they are going to come out on the 3rd, hopefully they can ground it better or I will try some of your suggestions. Thanks very much Agon members for your help.

I heard that you might need to check to see if your Cable connection Outside your house is properly grounded.

This is a common problem. You have a ground loop caused by your cable connection. You need to isolate The cable from your system. Audio Advisor caries a product from Mondial called the Magic Block. AT $99 it is expensive, but it works.

Enjoy,

TIC

The cable isolator is the same thing as a ground loop interrupter, which Radio Shack sells for around $10 or less. I installed one on my son's TV cable and it did the job just fine.

Get a cable isolator from Audio Advisor ($99) or make your own for about $4.