There was little fanfare around “ Scooby-Doo, Where Are You!” when CBS launched the Saturday-morning animated series about four crime-fighting teens and their talking Great Dane, Scooby-Doo.

That was Sept. 13, 1969. Fifty years later, “Scooby-Doo” remains one of TV’s most iconic animated series, spawning a slew of spinoffs, five big-screen movies and a merchandising empire — while adding Scooby’s “ruh-roh” to the American lexicon.

“I’ve tried to figure out what made people like ‘Scooby-Doo’ so much,” says Frank Welker, who’s voiced square peg Fred Jones on the series since Day 1 (and later added the voice of Scooby-Doo). “I’m totally blown away that we’ve been on for 50 years. If you put a show like ‘SpongeBob SquarePants’ next to ‘Scooby’ it’s like, ‘Whoa! Time warp!’ ”

Maybe that’s the secret to “Scooby-Doo,” since its template and characters have changed little, with ascot-wearing Fred Jones, Daphne Blake, Velma Dinkley, hippie dude Norville “Shaggy” Rogers and Scooby tooling around in their psychedic “Mystery Machine” van solving crimes and (inevitably) unmasking the bad guy, often disguised as a ghost or monster. Creators Joe Ruby and Ken Spears based the characters on the 1959-63 CBS series “The Many Loves of Dobie Gillis” — think of Shaggy as an animated version of Bob Denver’s beatnik Maynard G. Krebs.

“What’s appealing is that the animation [in ‘Scooby-Doo’] is slower-moving. There’s time to digest it and the characters are fully rounded,” says Welker, who was a 23-year-old aspiring standup comedian sharing an LA stage with Steve Martin when he was hired for “Scooby-Doo.” “I don’t know if it’s the friendship between Scooby and Shaggy — they’re complete goofballs, yet they totally respect and love each other — or the solving of mysteries that has people intrigued. I totally believe that keeping it as original as possible through the years seems to have been the key [to its longevity].”

In addition to Welker, the original series cast included Casey Kasem as Shaggy, Nicole Jaffe as Velma, Indira Stefanianna as Daphne and Don Messick as the beloved Scooby-Doo, a role he voiced until his death in 1997 at the age of 71.

“Casey was already huge in radio at the time … and he was so busy that he would always refer jobs to me,” says Welker. “We would go out for lunches and I would just sit there in awe and listen to his stories about Dick Clark and the wars between radio stations. It was fascinating.

“Don was kind of quiet — he was more of an intellectual type,” says Welker, who tears up talking about his late co-star. “I remember thinking, ‘Wow. He knows so many big words. He plays this dog yet he talks so well.’ ”

The “Scooby-Doo” franchise moved from CBS to ABC in 1976. Kasem, who died in 2014, played Shaggy from 1969-1997 and from 2002-2009, while Thelma and Daphne have been voiced by different people in the many “Scooby-Doo” iterations. The live-action 2002 movie version featured Freddie Prinze Jr., Sarah Michelle Gellar, Matthew Lillard, Linda Cardellini and Neil Fanning as Fred, Daphne, Shaggy, Velma and a computer-generated Scooby.

Welker took over as Scooby in 2002. “You can’t capture somebody’s soul, but you can do a voice and bring your own qualities to it,” he says. “I just hope I do honor to Don and to the fans.”

And, after all these years, there’s still one thing about Fred that bothers Welker.

“The guy still wears an ascot and that’s beyond me,” he says. “I never quite got that. Of all the ‘Scooby-Doo’ mysteries, that one’s never been solved.”