Have you tried it with a composite video or scart cable? You'll get a much better picture from the N64 that way. Any Gamecube cable should work fine. Not sure, but the Wii might use the same connector as well. Try your local video game shop.
I'm attempting to tune my Samsung 32" LCD to receive a signal from my Nintendo 64. It has to be plugged into the aerial socket but when I try the auto search for a signal it comes up with nothing. Is there an issue with this device being plugged in this way on HD ready TVs? Or is it something silly that I've missed...
Hope someone can help.