Dick Cerri, a folk music authority and host of "Music Americana," has left his successful base, WLTT-FM (94.7), for the hit-oldies station WXTR-FM (104.1).

He starts on WXTR Sunday from 8 p.m. to midnight.

"You reach a point where you are still doing well but doing the same thing," Cerri said in explaining the move. The change gives Cerri, who has been identified with the folk music scene here for 25 years, an additional 30 minutes per show as well as more time within each show because there will be fewer commercials per hour.

"Four full hours gives you a lot of things to do," he said. This Sunday he will feature "Folk Music 101," he said, laughing. "The listeners of Extra are familiar with Gordon Lightfoot, Judy Collins and the Kingston Trio."

WLTT, which started carrying Cerri's show in July 1981, has not decided on a replacement for his time slot. On WXTR, Cerri is replacing "Sunday Night Thing of the Past." Dick Lillard's "Doo Wop" features, which were also on Sunday night, will be integrated into other shows, according to program director Bob Duckman. Windows on WOL

WOL-AM (1450), which moved into the H Street NE corridor last November, will unveil its deejay window tomorrow at 3 p.m. Nicknamed "RadioVision" by station owner Cathy Hughes, the street-level studios where seven WOL deejays will work will be visible 24 hours a day.

"People used to get excited in the '50s and '60s when radio stations had their deejays in the windows. I want younger people to see a career with high technology. RadioVision becomes a living billboard," said Hughes.

"RadioVision" begins with festivities at 2 p.m. at 400 H St. NE. Expected are Mayor Marion Barry and singers Chuck Brown, Stacy Lattisaw and Johnny Gill. Open Auditions

WMZQ-AM (1390) is holding on-air auditions for a man to team up with personality Jeannie Jackson. To qualify for the competition, a would-be deejay must submit a tape to program director Katy Daley. After the initial screening, the applicant will be tested on the air and the audience will determine the final choice. WMAL News Expansion

Besides the announcement that Ed Walker is leaving WMAL-AM (630) for WWRC-AM (980) this week, WMAL had other news to report: It has expanded its noon news from 30 to 60 minutes.

"The 12 to 12:30 p.m. news segment is very popular. I assume people will want more," said WMAL vice president Fred Weinhaus. The segment will include ABC Network News, Bob Ryan's weather reports and commentaries from Peter Jennings and Steve Bell, as well as business, entertainment and sports features. Paul Harvey will now be heard in the 12:30 p.m. segment.

The additional time will enable the station's award-winning news department of nine reporters and two sportscasters to showcase more of its own work. In addition, Tony Renaud has gained more time on the weekends. His Saturday show is now from 8 p.m. to 1 a.m., with the Sinatra portion still 10 p.m. to 1 a.m., and he has added a Sunday shift from 7 p.m. to 1 a.m. Simulcasting the Senate

The U.S. Senate takes a historic step Monday with the television broadcast of its proceedings. So WETA-FM (90.9) will simulcast the opening-day action with WETA/Channel 26 beginning at 2 p.m. WETA-FM, however, will follow the Senate for only 90 minutes. New Christian Station

WBZE-AM (1030), a contemporary Christian station, debuted from Waldorf last Sunday. Its musical offerings will range from Amy Grant to Sandy Patti to Andrae Crouch. The station is daytime only, filling in a Class 1A channel of Boston's powerful WBZ, which can be heard here only at night. Classic Additions

WCXR-FM (105.9), the new and successful classic rock station, has added a summer series called "Rock Connections," syndicated by CBS Radio, to air Sundays at 7 a.m. The show, which is hosted by Mike Harrison, former West Coast editor of Billboard Magazine, is one hour, except for holidays when it will be three hours of flashbacks. On Saturdays at 10:30 p.m. WCXR will air "Saturday Nite Spotlight," an array of concerts and documentaries compiled from three syndicators; tomorrow the show features Phil Collins. The station also has instituted a daily feature called "Soul Fix at Six," 10 minutes from the Motown treasury. It moved "Album Exchange," the feature prompted by listeners who lost or destroyed a favorite album, back to 5 p.m.