Morton Lapides, the tenacious and ambitious chairman and chief executive officer of Allegheny Beverage Corp., has relinquished the title of president of the newly expanded national food service company to his longtime colleague and confidant, Edward A. Weisman.

In a management reorganization designed to guide the company after its recent doubling of revenue, Lapides also has recruited David Barr, a young banker who was formerly with Perpetual American Bank, as the final member of the company's management troika and the designated heir to the Lapides empire.

The appointments follow a recent expansion of the Cheverly company, best known for its Macke vending-machine business, into a food-service empire with more than $1 billion in annual revenue.

Allegheny Beverage's purchase last year of Servomation Corp., combined with its existing operations, made it the largest "business and industrial" food-service company in the country and placed it among the 10 largest publicly owned companies in the Washington area.

"We needed more support at the top," said Lapides, who has held the combined title of chairman and president for many years.

Lapides founded the small Pennsylvania soft-drink franchise in 1960 and built it by acquiring other, often much larger, operations, such as the Macke Co. in 1981.

Weisman, 56, an engineer who has been with the company since 1972, most recently as executive vice president, and Lapides, also 56, will continue with their previous responsibilities, Lapides said.

The addition of Barr, however, is expected to "take some of the pressure off Weisman and Lapides," one of the executives said.

Barr was recruited after a 10-month sales pitch by the company to lure the former banker to Allegheny, Lapides said. Lapides and Weisman met Barr, 34, when he was second in charge of Perpetual American Bank and was designing the financing that would help Allegheny with its purchase of Servomation.

Barr "strengthens the management team," said Weisman. He added that Barr, who holds the title of executive vice president, fits into the company's plans to groom a successor for either of its top two positions.

Allegheny Beverage also announced that William Avella, 50, formerly president of Servomation Corp., will head the Service America Corp., the firm's food and vending subsidiary created from the combination of Servomation and Macke.

And recently, Allegheny announced that John E. Baker, who had been with City Investing Corp., the company that sold it Servomation, was named chief financial officer. COMPUTERS

Micros Systems Inc. last week announced the expected resignation of J. Robert Schultz as president and chief executive officer of the Beltsville developer and manufacturer of electronic terminals for cash registers and data processing. Schultz's work "is now complete," the company said of the business consultant who was hired a year ago to help in organizing, financing and marketing the $221 million, publicly owned company. Replacing Schultz, who will remain on the firm's board of directors, is Louis M. Brown, a founder and director of Micros and the president of Ideas Inc., a major shareholder of the company. COMMUNICATIONS

Kenneth F. King, a veteran Washington printing executive, has been named president and general manager of Editors Press Inc., a subsidiary of Kiplinger Washington Editors Inc. King, a former vice president, succeeds Herbert Morrow, who will remain with the Hyattsville company in the new post of vice chairman of the board of directors.

MCI Communications Corp. has appointed as its treasurer, C. Thomas Faulders III, the former treasurer of Satellite Business Systems Inc., the McLean business communications firm that MCI acquired from IBM last year. Faulders will oversee cash management, insurance, leasing, bank relations and employe stock plans. PROFESSIONAL SERVICES

At a board meeting last month, directors of Genex Corp. elected Robert F. Johnston chairman of the Gaithersburg biotechnology company. Johnston, who cofounded Genex and now is president of Johnston Associates Inc., has been on the board since 1977, and served as its chairman from 1977 to 1983. He succeeds J. Leslie Glick, who remains president and chief executive officer of the company. Johnston will continue to help Genex -- which lost a major part of its business last year -- to find "corporate partners" who can help sell Genex's products. Through genetic engineering, Genex creates new biological products, including therapeutic proteins, biosensors and immobilized biocatalysts. INDUSTRIAL, MANUFACTURING

Chesapeake Corp. has named the presidents of two of its operating divisions as officers of the parent corporation, a paper products company based in West Point, Va. George P. Mueller, president of Wisconsin Tissue Mills Inc., was named group vice president for tissue products. Samuel J. Taylor, president of Plainwell Paper Co. Inc., was appointed vice president for printing and specialty papers. Both will retain their current duties as presidents. In a separate development, R. L. Hintz, executive vice president of CSX Corp. in Richmond, and William D. McCoy, chairman and chief executive officer of Koch Label Co. in Evansville, Ind., have been named to Chesapeake's board of directors.

Occidental Chemical Corp. has promoted Julie Archuleta to director of government affairs from manager of government affairs. She succeeds Harris M. Miller, who retired after 44 years with the company.

Directors of Martin Marietta Corp. have elected Janice K. Henry secretary of the Bethesda-based aerospace corporation. Henry, who has served as controller, director of business management and, most recently, assistant secretary, succeeds Mary Jane LaBarge, who is retiring after 23 years. BANKING, FINANCIAL SERVICES

Riggs National Bank recently announced the election of three new members of its board of directors, who will take office at its annual organizing meeting on Wednesday: Donald J. Heim, chairman, president and chief executive officer of Washington Gas Light Co.; George C. Williams, chairman and chief executive officer of the Allied Capital Corp., a venture capital financing company in Washington, and Frank L. Langhammer III, senior executive vice president and head of Riggs's corporate and commercial banking.

D.C. National Bank has appointed Henry E. Catto Jr. to its board of directors. Catto founded and became chairman of Washington Communications Corp., publisher of Washington Journalism Review in 1979 and was assistant secretary of Defense for public affairs from 1981 to 1983. He is currently vice chairman of H & C Communications, which operates network affiliated TV stations. RETAIL, WHOLESALE TRADE

Bert I. Helfinstein, a veteran computer industry executive, has been promoted to president and chief operating officer of Entre' Computer Centers Inc.'s North American operations, the Vienna-based computer retail outlet franchiser announced. Helfinstein, who became vice president of operations in 1984 after stints as president and chief executive officer of Source Telecomputing Corp. and as president of the computer system group at Planning Research Corp., both in McLean, will continue to have full responsibility for planning and daily operations.