Leo M. Bernstein, Diplomat National Bank's new chairman and chief executive officer, yesterday pledged to broaden the institution's operations to include the entire Washington community. Mindful of the bank's original mission to serve the Asian-American population, he declared, "We'll do more for Asian minorities in this way than by being limited to one aspect of the community."

Bernstein took control of the financially troubled bank July 15 when his offer to purchase $2 million in stock was accepted by the board of directors. Another $1 million bid by 11 Korean trading companies and Korea's largest bank failed to win U.S. government approval in time. Bernstein's offer, which runs counter to the current trend toward foreign takeovers of U.S. banks, still must be approved by federal regulators.

Thus ends Korean domination of Diplomat National. In its short but turbulent history the bank has been plagued by political scandal and financial mismanagement. Two years after its establishment in 1975 to serve the Asian-American population, the Securities and Exchange Commission charged that Diplomat had been secretly and fraudulently controlled from the outset by evangelist Sun Myung Moon and lobbyist Tongsun Park. Through nominees Moon controlled 43 percent of the stock and Park, 10 percent, even though the bank's founders promised that no person would control more than 5 percent of the stock.

Moreover, the SEC alleged that Moon's Unification Church controlled or attempted to control Diplomat policies with its checking account at the bank which at times constituted 30 percent of the bank's total deposits. Two years later the SEC charged Moon still controlled one-third of the stock.

Asked yesterday how much the church now owns, Bernstein said he did not know. However, he said that National Savings and Trust Co. had made loans to Moon when Bernstein was its vice chairman, adding, "If he [moon] has money to put in the U.S.A. and Washington, he's welcome."

Diplomat has operated in the red from the beginning. Yesterday, John E. Ricchie, the bank's last president before Bernstein, said Diplomat wrote off $1 million in bad loans last year out of a total portfolio of $7.5 million. Sixty percent of the bad loans were made to Caucasians.

Diplomat was down to its last $200,000 of operating capital when the Comptroller of the Currency ordered it to raise $1 million or be merged. With Bernstein's infusion of $2 million. Diplomat is now back to where it started in terms of capital. The bank has about $7 million in deposits. News of Bernstein's acquisition brought in an additional $700,000 on the first day, he said. He predicted that figure would reach $25 million by May 1981.

Bernstein announced the reborn bank would resurrect a form of "all-purpose, open-ended loan" used many years ago in the District by savings and loan associations. The loan, called Bankstra, gives the customer the flexibility to get loans as needed for big ticket items and make repayments while paying interest only on the average amount outstanding.

He also announced plans for Sunday symposiums, beginning in September, devoted to subjets like money management, leasing procedures, contract writing, etc. Among the other ideas he is toying with are Saturday service and holding a contest to select a new name for the bank. He has also pledged to cut costs by eliminating company limousines and other "frills."

Bernstein, 65, is a native Washingtonian with a long career in real estate development and banking. He recently sold out his 15 percent interest in National Savings and Trust Company and resigned as vice chairman after NS&T president Joseph Riley, his junior, was named chairman. Bernstein's new board of directors at Diplomat includes only two Koreans-Americans, Joohn Rhee, founder of karate school, and Hyattsville physician Soo Young Oh, and one Chinese-American, Dr. William Chin-Lee of the District. Joining these holdovers on the board will be the bank's officers and contractor Joel Fernebok, and retired Army major general Kenneth J. Hodson, who has lived in both Korea and Japan.