The Source Telecomputing Corp., McLean-based operator of The Source, an on-line information network for personal computer users, has named Charles Major director of product development and design. Major will retain his responsibilities as STC's managing editor. In addition, Leslie C. Seeman, formerly an associate in the Washington law firm of Wilmer, Cutler & Pickering, has been appointed the company's general counsel. And Randy Stratt, previously director of legal affairs, has been named director of the new market development department.

Scope Inc. has named David Bess director of marketing for its industrial products division. Bess joins the Reston electronics equipment manufacturer from Hughes Aircraft Co. in California.

Howard Mappen, formerly a vice president of Comsat's manufacturing subsidiary, Telesystems, has been named vice president for manufacturing at Spectrum Digital Corp., the Herndon manufacturer of telecommunications products.

Federal Data Corp. has elected Daniel R. Young president of the Chevy Chase computer systems integration company. He succeeds C. Robert Hanley, who remains chairman and chief executive officer. Young had been executive vice president of the company since 1976.

Computer Dynamics Inc. in Virginia Beach has named Betty Desjardins senior vice president for operation support and David C. Staley director of employe relations and administration.