Is there such a thing as a co-ax cable to input converters?

Before you go that far, realize that the cable itself may be the problem. Usually poor picture quality can be traced to poor cable connections. Also, another possibility is poor connections inside the tuner of the set. I assume that you are using a cable box. Some of the newer boxes have direct video connections which will connect as you describe. Check the condition of all of the cables and connectors before yo invest in anything.

Keep us posted.
Dan