https://www.youtube.com/watch?v=QNtsTKVPO7I
at about 7:00 it talks about setting up purity.
Before doing anything else I recommend you look on ebay for an RCA color service manual by carl babcoke.
http://www.ebay.ca/itm/RCA-COLOR-TV-...3D311360906136
oh and start looking for a degaussing coil.