WNTR-AMs (1050), Washington's only all-news/talk station, has been put up for sale at auction by its owner Gary L. Portmess.

The auction, which is being conducted by Michael Fox Auctioneers Inc. of Baltimore, will begin at noon May 2 at the Bethesda Holiday Inn. The terms call for a cash deposit of $75,000. Portmess, a Hagerstown businessman who bought the station for a reported $950,000 in 1984 with hopes of carrying forth the popular talk and news format of the old WRC, could not be reached for comment this week.

In other news from the station: WNTR will have an interview with extremist political leader Lyndon LaRouche today from noon to 1 p.m. Ed Graham will take calls from listeners. Ember's Departure

Steve Ember, an announcer of classical and popular music in Washington for the last 20 years, including 14 at WETA-FM (90.9), has left the station. Both sides called his departure a "mutually agreed-on separation."

Ember said he was initially told by WETA Program Director Tim Owens that the station wanted to increase his on-air time, but that after Ember returned from a vacation Owens told him his "programming was weak." Said Ember "I recognized this 180 degree turnaround for what it was: an invitation to leave."

Owens said programming was "completely separate and apart" from their discussions about Ember's departure. Owens said Ember had indicated a desire to do more documentary and "production-related programs" but didn't present any acceptable ideas.

"He wanted to do what he wanted to do. WETA needs him to do what WETA needs done," said Owens.

Ember had been the host of "PM" three times a week, "Ember at Large" on Sundays and a weekly feature on advances in recording technology and home entertainment. In 1980 his programs won an Armstrong Award in technical achievements for WETA.

Ember said he isn't bitter over severing his ties with WETA but was "dismayed by their actions." Ember will continue as commentator for the Library of Congress concerts and work part time at the Voice of America and WGMS-FM (103.5), where he started his Washington career. He said he hopes to syndicate a show of American popular songs. One of Ember's old slots, Sundays at 3 p.m., has been filled by "Potpourri" with Bill Cerri. New Commercials, New Slogans

Tonight during "Miami Vice" WBMW-FM (106.7) will debut a commercial called "Voyeur." Ernie Anderson, who does voice-overs for "The Love Boat," does the dialogue, including the line that WBMW "is so hot, it's cool." In the commercial a woman dancing in front of a window gets drenched by a tidal wave . . . WAVA-FM (105.1) quietly unveiled a slogan, "Power 105 WAVA," which is tied to a current giveaway of a Porsche but will last past that promotion. " 'Power' is a pulling together of our total image," said station spokeswoman Kathy Kolodin. As a test of its power, deejay Mike Beach has started S.O.R.E., a campaign to "Save Our Riggo from Extinction." The station and 16 bars around 19th and M streets NW will be circulating petitions to get John Riggins a tryout for his old slot with the Redskins. Grants

The Corporation for Public Broadcasting has announced three local recipients of its Minorities/Women's Training Grants. The local grantees are Gailya Osborne, marketing data coordinator at WAMU-FM (88.5); Patricia Watkins, vice president of national affairs at the National Federation of Community Broadcasters; and Cory Nightengale, a technician at National Public Radio. A Beckett Tribute

WETA is airing "All That Fall," a play Samuel Beckett wrote especially for radio, Sunday at 6 p.m. on the author's 80th birthday. Music Education

Secretary of Education William J. Bennett has set up a Monday luncheon date with Jeff Baker and David Burd of WCLY-FM (95.5). Bennett won the duo's challenge to Bennett's self-proclaimed title of "Cabinet Expert on Rock 'n' Roll" by winning a trivia contest on the station. Charity Dates

WGAY-FM (99.5) is a cosponsor of the E.F. Hutton Tennis Championship starting tonight at the George Washington University Smith Center. This competition for seniors features Rod Laver, Virginia Wade and Fred Stolle; the proceeds go to Children's Hospital . . . Ken Beatrice of WMAL-AM (630) has signed on again as the grand marshal of the 10-mile Canal Trek tomorrow to raise money for the National Kidney Foundation. Guests

Tina Yothers of "Family Ties" will host the annual Henny Penny Playwriting Contest broadcast tomorrow on WPFW at 10 a.m. The winners include students from P.E. Williams Elementary School in Upper Marlboro . . . Actor Robert Blake, a promoter of the troubled March for Peace, will discuss the effort Sunday at 9:15 a.m. on WINX-AM (1600) with Susan Raitt, host of "Get Involved."