I have a Magnavox DVD/VCR combo model # DV225MG9. The manual says that it has has no internal tuner. I want to know to hook it to my digital cable box so I can record off the TV. I hooked it up it will play tapes and dvds. But when I push record on the vcr it lights up. But I try to play the tape back there's nothing but a blank screen.What am doing wrong? Can someone help me?