9's are about the best early color set RCA ever made, in my opinion. Reliable, cool running and easy to work on. That chassis/schematic design is what all the rest would be based on till the end of the roundies, and that's saying something. I myself had an Atherton, it was my first vintage color set and I loved it till I had to sell it in order to make room for others.
Enjoy it!
Wild guess on the issue you're having- one of the controls is dirty, have a look at the schematic diagram to see where it is:
(look for the control called 'blue screen' near the picture tube)
http://earlytelevision.org/pdf/RCA-CTC9-Sams-459-1.pdf
Its actual location is at the rear of the set/chassis, probably under a metal panel held in by a pair of screws. If twiddling that a little bit doesn't have any effect, you likely need some more in-depth troubleshooting. For starters the power supply caps will need to be replaced, for reliability's sake. Mine worked on all original parts, but did have a slight 60hz wave in the picture. Caps fixed it. After that, you may find some worn tubes or tubes loose in their socket. Shouldn't be a tough fix, 9's are as easy to work on as they come.
