Government Technology Services Inc., the Chantilly-based computer seller that has tried unsuccessfully to break into the market for large-scale federal sales, has found a new president to replace one who stepped down this summer after a year on the job.

Richard M. Rickenbach, 50, a 28-year veteran of Control Data Corp., assumes the title Monday.

He succeeds Ray Lunceford, who remains at the company as executive vice president for federal procurement.

Greg Layton, GTSI's vice president for marketing, declined to discuss the reasons behind the change of presidents.

He noted that Lunceford remains an executive vice president, a job he described as advisory.

The new president, Layton said, "has extensive knowledge and experience that nobody else in the company had. We think he'll be helpful to GTSI in many ways."

Layton said the company, which employs about 450 people, was in good health financially.

"We're going to have our best year ever" both in terms of profits and revenue, he said.

Ulric Weil, a Washington computer consultant, said Lunceford had taken the job with an eye on retirement and made it clear in industry circles that he did not intend to stay for the long term.

Efforts to reach Lunceford directly by telephone were not successful.

Founded in 1983, Government Technology Services grew quickly as a major "reseller" of computers and related equipment, acting as a middleman between federal purchasers and manufacturers. The privately held firm had sales of $288 million in 1989, rivaling Falcon Microsystems Inc. of Landover and Bohdan Associates Inc. of Gaithersburg.

Information Strategies Group, a research firm, estimates that the government personal computer market is approaching $2 billion a year.

But it's a highly competitive market, one that for now is stagnant because of federal budget constraints.

GTSI makes many of its sales through the General Services Administration, which runs a program by which agencies can buy small volumes of equipment without going through competitive bidding.

Many computer purchases seek agency-wide procurements. In that system, an agency awards a jumbo contract to fill most of its needs at once rather than making a series of small purchases. The large contracts, in theory, allow volume discounts and standardization of equipment.In recent times, GTSI has tried to expand into this big-contract work, bidding for three or four such jobs. It won none of them, though a $500 million-plus contract for software and computer support equipment that the Navy had awarded to Zenith Data Systems last year remains up in the air, tied up as lawyers argue whether the procurement was conducted by the book.