There have been times in Judy Collins' recording career when her pristine voice lacked the necessary emotional punch to put a song across; a particularly unfortunate situation considering the strength of her repertoire and her special gift for arranging material.

At first, it appeared Collins would skirt that problem altogether at Merriweather Post Pavilion last night. She opened her concert with a few pleasant, if not demanding, tunes by Danny Keith, Steve Goodman, Hugh Tirestwood. It wasn't until she dismissed her backup band and her trio of singers that she became more than just a very pretty voice. Seated at a piano with a dozen roses nearby, Collins had no trouble convincing the audience she was serious about her own compositions.

On songs such as "My Father," she displayed little of the polite restraint that characterizes many of her popular interpretations, and none of the lush orchestration that occasionally softens her impact. The solo set apparently boosted her confidence. When the band returned, she dealt more effectively with songs by Stephen Sondheim, Joni Mitchell and Leonard Cohen, than she has on records.

Aztec Two-Step, a last minute addition, opened the show. Although they have a large D.C. following, last night's set of soft rock was nothing short of mediocre. CAPTION: Illustration, Judy Collins