Comcast digital box to analog tv problem

Comcast just swapped out an old box which had gone bad which was connected to an old analog tv with a new digital box. I get On Demand thus the need for the box and I am getting a strong signal through the cable but am having to use an rf modulator from the box to the tv via a 75/300 connection because the tv does not have a cable connection. The problem is that my picture is fuzzy. Help!