component, s-video and composite newbie

I found the solution for $6.99 plus shipping. Its Calrad part number 75-504 DC Ground blocker and Isolation Transformer. Go to
http://www.calrad.com/calrad/cat-59page3.htm

I guess Mondial sells a more sophisticated unit for $99; it must be MAGIC.

Ivanj, In my system the buzz was the result of two different
causes - both easily corrected.

1)for the cable, I bought from Radio Shack, two $3.99 converters. (a) 75ohm F connector to 300ohm 2 wire (b)the reverse - 300ohm 2 wire back to 75ohm F connector. I hooked them up in series. Then simply plugged the last F connector to the back of the TV - hum gone. I have no idea why but it works - the picture is as sharp as ever.

2)I discovered that a second hum was caused by the volume
pot in my TV. The hum disappears when the TV volume is at 100%, then I get a nice clean signal to my hifi. Caution: you must MUTE the TV audio (which affects the TV speakers only). The volume is now controlled by your hifi. Be sure to turn down the TV volume when done. Good luck.

Solution found, finally. I called my buddy who sold me the Elite who made some recommendations. I broke down and went to the digital cable box so I can see old Charlie Chan movies and SpeedVision. I changed all the relevant connections to Toslink. Much better but still not good enough.
Comcast sent a techie who said that the signal coming into the house was too low. The grounds and all the rest were ok. He removed the cheap filter. He went to the pole and fiddled with that, still wasn't satisfied and added an amplifier in my basement where the signal came in the house, that brought the signal up to levels he was happy with. Now the analog channels are as clear as the so-called digital. He tried to charge me for the amp and I told him that I shouldn't be expected to pay for Comcast's inability to deliver an adequate signal to my area....not too pleased with CC.