Horiz locking problem
 

On my '50 RCA KCS47A the picture is off center. I've adjusted the magnets and such and is still off center. I can adjust the horiz hold and it will center nicely but when I change channels the horiz goes out of lock. Have adjusted the locking range cap but it won't quite work. (min capasity) How does the locking curcuit work on this set?
Terry