After 8 years on CBS, Scooby-Doo moved to ABC as part of a revamped Saturday morning lineup that saw Scooby's creators, Joe Ruby & Ken Spears, return to Hanna-Barbera after a 4 year absence, during which time they'd worked at DePatie-Freleng and 20th Century Fox, the latter on a live-action primetime series (Planet of the Apes), and develop two series for their original employers in Jabberjaw & Dynomutt, the latter of which was coupled with Scooby in a hour-long block.
The format was pretty much the same, with Mystery Inc. at a random location to solve a randomly developing mystery. You might as well have been stocking up on color-by-numbers books, since they were actually more entertaining.
However, the producers added to Scooby's family by introducing two cousins--Scooby-Dee, an aspiring actress, and Scooby-Dum, a Mortimer Snerd-inspired dullard who was about as sharp as stale cheese. Unfortunately, Scooby-Dee wasn't used as often, as there were only one or two movie-centric stories out of the 40 episodes produced over three seasons. Dum & Dee, oh by the way, seem to have faded into obscurity in recent years, since the writers of the DTV movie series have not seen fit to bring them back.
Following is the open used in syndication during the 80's. Shaggy (Casey Kasem) tries to sing. Stress tries.
Now, while most people seem to think the franchise jumped the shark with the debut of Scrappy-Doo in 1979, it may have actually happened with Scooby-Dum. Like, did you really think a great dane with the IQ equivalent of an average person's shoe size actually fit in with his cousin and his friends? The sad part is, when Scrappy---and later, Yabba-Doo---came along, Dum & Dee were sent far, far away, and so we never saw the whole Doo family together until much later.