You've probably got a slightly older set that only decodes MPEG2. For most channels the sound is in MPEG2 but the picture is in MPEG4. Hence you get sound but no picture. There are a few channels where both the picture and the sound are MPEG4 so you should get nothing at all on those channels. There is one channel, ART, where both sound and picture are in MPEG2 so you should get that one completely.
The solution is to buy a set top box that decodes MPEG4 and connect it with SCART or HDMI to the TV. They aren't expensive. If you are in the Chania area there is a shop on Anagnostou Gogoni that is very helpful. From the Mournies junction of the National Highway go down between Carrefour and Lidl to the traffic lights and turn left. Go through the next lights and stop near the Jet petrol station. The shop is almost opposite near a shop with lots of chimney hardware outside.