The call sign was changed to WLUP on March 14, 1977.The station adopted an album-oriented rock (AOR) format and was rebranded as "The Loop", referring to the nickname of Chicago's central business district. Jay Blackburn was Program Director and Tommy O'Toole served as the station's first morning host and was the first Loop DJ to sign on the air.
The first song O'Toole played on "The Loop" was "Morning Has Broken" by Cat Stevens.The original Loop air staff included Tommy in mornings, William "Captain Billy" Martin in middays, Lester "Les" Tracy in afternoons, and Bob Shannon evenings.Other early Loop DJs included Greg Budell, Ford Colley, Don Davis, and Glory-June Greaif.
Heftel Broadcasting Corporation, owned by Hawaiian Congressman Cecil Heftel, purchased the station effective March 5, 1979, for $3 million. Lee Abrams was hired as consultant.The rock format was kept in place, but emphasis was put on harder-edged rock.Jesse Bullit was the program director. Steve Dahl, who had been unemployed since WDAI (94.7 FM) went to a disco format in December 1978, began hosting mornings on March 19, 1979.
On July 12, 1979, Dahl staged one of the most famous promotions in radio and sports history, Disco Demolition Night.Garry Meier, known over the air at the time as "Matthew Meier", was an overnight DJ, and was later teamed with Dahl as the morning newsman.Soon thereafter, Meier became Steve's full-time sidekick, and used his real name, Garry Meier.
In 1979, Jeff Schwartz became head of sales and Mitch Michaels was hired as afternoon DJ. Michaels became program director shortly thereafter.Sky Daniels was also hired in 1979, and served as disc jockey and music director. 
 On February 6, 1981, Steve Dahl was fired for what WLUP's management and Dahl said was a contract dispute, though Heftel Broadcasting said it was for violating "community standards". Garry Meier left the station as well, to remain Dahl's sidekick. A series of morning men including Matt Bisbee, Mark McEwen, and the team of R.J. Harris and Pat Still tried their hand until Jonathon Brandmeier was hired in late 1982.
 Kilman remained as newsman, and Bruce Wolf joined the station as a sports reporter in 1982.
Greg Solk became program director in 1983. Dr. Demento was carried on WLUP from 1978 through 2010, just prior to when Dr. Demento ceased over-the-air broadcasting.
 

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