A Northeast Washington man was convicted of manslaughter yesterday in Prince George's County Circuit Court for the fatal stabbing last year of a Fort Washington woman after he admitted the act, but blamed it on a mental disorder he said resulted from his traumatic combat experiences in Vietnam.

Orvie W. Chapple, 36, used a rare defense against charges of first-degree murder, robbery and theft, claiming that he was affected by post traumatic stress disorder, a vestige of battlefield experience. When the victim threatened him with a butcher knife, he said, he wanted only to defend himself and did not realize he was striking a fatal blow.

The victim, Juanita Robinson Harris, 70, of the 2400 block of May Place, died on July 25, 1984. She was found on the floor of her home with cuts on her throat, face and hands. Chapple turned himself into police three days later.

Chapple could have been sentenced to life in prison if the jury, which deliberated two hours, had found him guilty of first-degree murder and robbery. Instead, he faces a maximum of 25 years in prison, 10 for the manslaughter conviction and 15 for theft.

Dr. Neil Blomberg, a psychiatrist who testified for the defense, said Chapple suffered from the disorder, which has been formally recognized in psychiatric circles since 1980.

Chapple said that while in Vietnam, his best friend was shot and killed, a land mine destroyed a truck he was driving and he once was ordered to shoot a child who was carrying a grenade.

Chapple, who was with the Army for 18 months in Vietnam, testified that he went to Harris' home to ask her to help him get trucking contracts. Harris lived with one of his former employers.

Harris "went berserk," Chapple said, when he asked about her daughter, who was in a drug rehabilitation program, and told him to leave her home. Chapple said he tried to leave and was struggling with a locked door when she came after him with the knife.

"I grabbed for the knife. As I grabbed for it I swung. I guess I hit her," he testified. Chapple said he remembers hitting Harris with the knife once. "After that, it was blank," he said. Chapple said he took a television and videocassette recorder to make it look as if a burglar had killed Harris.

Assistant State's Attorney Thomas McManus argued that Harris' death was premeditated murder, and that Chapple returned from Vietnam in 1969, but hadn't sought counseling for his illness until Harris' death.

A psychologist and a psychiatrist from the Clifton T. Perkins Hospital in Jessup, who helped evaluate Chapple, disputed the claim of post traumatic stress disorder.

Sentencing was set for Oct. 29.