January 20, 1977 saw the airing of an episode of Barnaby Jones entitled "Testament of Power" which guest starred Susan Dey. Susan Dey portrayed Linda Jason, daughter of the secretary of a very wealthy man. When the rich man dies and the secretary dubiously commits suicide shortly thereafter, Barnaby Jones begins his probe into the complex web of events.
This was Susan Dey's last guest appearance on an hour-long television drama until 1999! She spent much of her time following the end of the Partridge Family doing these types of guest spots on various shows. However, after this Barnaby Jones episode she would abandon these guest roles in favor core casting (Emerald Point N.A.S., L.A. Law, etc), TV movies and feature films.
Here are some images from the episode (click for full size):