I was recently given a new dvd/vcr combo, but I have an old TV. I bought an RF Modulator so that I could hook them up. What I can't figure out is how to run my TV through the dvd/vcr combo. I used to have a vcr that I would run my TV through so I'd switch channels on the vcr and it would change the TV channels. Is this not possible with a Dvd/VCR combo?

Sure its and its no different. You just run the antenna/video source into the combo unit and then run the output from the combo unit into the RF Modulator which then goes into your TV.

The VCR will need a built in tuner, or you need an external tuner.