REPORTER Ed Meyer this Friday winds up his series exploring the impact of AIDS on dating and social trends. "AIDS and the Big Chill" airs on WMAL (630) at 6:30 a.m., noon, 6 and 10. Friday night at 11:30 WMAL-AM will simulcast ABC-TV's four-hour "Nightline: A National Town Meeting on AIDS," with Ted Koppel.

Saturday, the ABC-owned station will broadcast jockey Chris McCarron's efforts to guide Alysheba to the winner's circle at the Belmont Stakes and capture horse racing's Triple Crown. Coverage begins at 5:06; post time for the 1 1/2-mile test on the flat is 5:33.

Jazz-oriented WDCU-FM (90.1) and gospel stations WBZE-AM (1030) and WYCB-AM (1340) will join urban contemporary WKYS-FM (93.9) this weekend and next in broadcasting live from Potomac Riverfest '87.

Rockville's all-hit WINX-AM (1600) has begun a weekly countdown of Washington's Top 40. The show, which debuted last Sunday, is hosted by Kraig Erickson Sunday noon to 3.

Dick Cerri hosts his last "Music Americana" folk program from oldies station WXTR-FM (104.1) on Sunday starting at 8 with a four-hour "wide-open folk free-for-all."