This Saturday the members of the D.C. Youth Orchestra will be in South Korea beginning a 10-day, four-concert tour, taking with them an uncompromising program and the fruits of a lot of hard work.

They put the program through its paces Friday night at Coolidge High School in the sixth and last of this season's Summer Series Concerts. The performance showed a program on the verge of coming together, with glimpses of musical excellence and technical security and the promise that the rest is just around the corner.

Pianist Glenn Sales, a student at Harvard who has been performing in this area since he was 10, was the soloist in a muscular performance of the Mendelssohn Concerto No. 1, and violinist John Williams, a District native who studied his instrument at Howard and at the New England Conservatory, was a self-assured soloist in the Mozart G Major Concerto. Student conductor David Mathers led both concertos with care and with progressively deeper involvement in the music. His musical instincts are good, and to realize them fully, he will need to pay as much attention to his beat preparation as he does to the beat itself.

Youth Orchestra conductor Lyn McLain opened the concert with a well-balanced reading of the Barber "Adagio for Strings" and closed it with Beethoven's Seventh Symphony and the overture to Mozart's "Marriage of Figaro," choosing slow tempos on the whole, and paying particular attention to the clarity of inner lines.