Both ONE STEP DOWN and EXCALIBUR are Washington jazz clubs, open late on weekends and offering good food, yet in mood and atmosphere they're more than a couple of miles apart.

One Step Down sits on Pennsylvania Avenue, just this side of Georgetown. Little has been done to the club's interior since 1963, when brothers Joe and Ken Cohen bought it. Back then it was "a regular drinkin' bar," according to Ken, who runs the day side of the operation. "It's a homey atmosphere, nothing fancy, nothing sloppy," he says. The cigarette machine continues to dispense non- filtered Camels, one example of a bygone day.

There's no stage -- musicians are squeezed among the crunch of tables at the end of the small bar. It's a cozy effect. Dark wooden booths line the opposite brick wall, which is decorated with pictures of jazz players and posters of jazz festivals. A black light shines on the wall menu written in glowing orange and green. Patrons sit close together, making for some interesting eavesdropping between sets.

The One Step Down has been described as a classic smoky jazz bar, but one insider paints a more accurate picture: "It's a dive where the music somehow works."

And weekend crowds who pack the jazz club are proof that the music works, thanks to such big names as David (Fathead) Neiman, James Mooney, Nat Adderly, rchie Shepp, the Dewey Redmond Quartet and Washington's home-grown headliner, Shirley Horn and her trio. The tiny, 72-seat club with additional seating on the outdoor patio is starting to earn a national reputation -- not up there with Blues Alley or Charlie's of Georgetown but still with a wide appeal, attracting college students, jazz fans and "regular steadies," says Ken Cohen. The jukebox, offering the big band sounds of Benny Goodman and Artie Shaw and jazz standards by the Modern Jazz Quartet, George Shearing, Stan Getz, Dizzy Gillespie and others, has a following of its own.

Cover ranges from $2.50 to $6.50, depending on the entertainment. There's a two-drink minimum. Live music is heard Friday through Monday evenings. The featured Friday and Saturday night performers change once a week; sets begin at 10. The Shirley Horn Trio is featured this weekend. There are afternoon jam sessions every Saturday and Sunday with the Lawrence Wheatley Quartet. The Reuben Brown trio performs Sunday and Monday evenings.

y In Adams Morgan, the Excalibur takes another approach to jazz. Performers change once a month and are billed as "the best of Washington jazz artists." Washington's best may not be enough for the keenest of jazz followers, yet for most folks it'll do just fine. The music is heard on the second floor of a rooming house-turned-nightclub on Columbia Road. With its Tudor decor, carpeting and white stucco walls, Excalibur is fresher and cleaner than One Step Down, yet somehow missing that cozy ambiance. The stage is off to the side, in the middle of the long, narrow room. Unless you're sitting directly in front it, you usually can't see anyone but the lead performer. But good seats are generally available and the acoustics help make up for the setup. "It's a pleasant place and you can have a good time," says Carlos Guivar, radio host of WAMU's Jazznight, "but you're not likely to see anyone who'll break national anytime soon." The Jamie Broumas Quartet and Nap Turner and The Wild Bunch, two talented local groups who play there often, are some of the club's better performers, says Guivar. Owner Henry Williams says that, compared to other jazz clubs, Excalibur offers music that is "lighter, with more of a mix of jazz, blues and some reggae." The audience gets a broader range of music and more recognizable tunes than at traditional jazz clubs, where musicians tend to "jam" more. The cover, between $2 and $5, changes with the talent, and there's no minimum. The first set is scheduled for 9:30 on weekend nights but often gets a later start. The Dohn Nunnley Quartet plays Friday night and Buck Hill and The Rhythm Section on Saturday. The Glenroy Jones Trinidadian Band plays Wednesdays and Thursdays through this month. ONE STEP DOWN -- 2517 Pennsylvania Avenue NW. 331-8863. EXCALIBUR -- 1834 Columbia Road NW. 462-0415. graphics/1 illustration: Jazz. By Terri Diamond Lawernce.