A District homicide detective was fined $1,000 yesterday after a police trial board found him guilty of attempting in 1978 to set up an unathorized "safe house" in D.C. for a man who had supplied investigators with information on the assassination of former Chilean ambassador Orlando Letelier.

The detective, nine-year veteran Stanley A. Wilson, 38, was acquitted of a second charge that he had misrepresented himself as acting for the State Department during a 1978 bond-extradition hearing for Jorge A. Zimeri in Miami.

Zimeri, a 35-year old Guatemalan businessman who supplied D.C. police with information about the 1976 assassination, was facing extradition to Guatemala on a separate murder charge when Wilson appeared on his behalf in Miami.

Wilson, who had been working with the State Department on the Letelier case, said he offered to take Zimeri into his personal custody.

State Department officials have said that Zimeri had given them information about terrorist activities in South America. Wilson said at the hearing that Zimeri's deportation would have meant certain death for him at the hands of his political enemies. Since that hearing, Zimeri has remained at an undisclosed location in Miami.

The D.C. police trial board found that Wilson did not have prior approval to provide a "safe house" for Zimeri. Wilson was fined $500 on that charge and an additional $500 for failing to tell the Justice Department that he was appearing on Zimeri's behalf at the hearing.

Federal officials including agents of the Alcohol Tobacco and Firearms Bureau, had decided to deport Zimeri after he was arrested in Washington and convicted of illegal possession of a handgun in 1977.