After some research http://www.tacp.toshiba.com/tacpassets-images/models/51hc85/docs/51hc85_om_e.pdf
There is no coaxial input for this tv. Also as far as the RF Modulator you unfortunatly are trying to use that incorrectly. The RF Modulator is used to convert the RCA or Red Yellow White cables down to Coaxile, not vice versa. However if you have an old VCR or better yet a cable box you would want to hook up the cable box then enter the TV using one of the RCA jacks, HDMI Jacks, or other inputs such as Svid. But easy enough if you have a vcr you can access cable through that as well, and make sure the tv is sat to the correct channel or input to access the VCR or other item.