I have a 51 inch Sony HD TV. Turned it on and

Most likely you have a bad RED CRT. Because of the age of the set Sony does not make the CRT assembly available. You must have a Qualified Service Facility come out and remove the CRT and send it to Sony to be refurbished. The cost of this runs around $300.00 for the CRT, $ 270.00 for Labor, plus any Service Call Fee. As you can see it get real expensive. I would invest in a new LCD Product instead of in this set.
I hope this helps,
Skyassoc