SAN FRANCISCO (AP) _ An appeals court reversed convictions for murder and manslaughter stemming from a 1987 slaying because jurors visited the crime site on their own to assess the evidence.

''Unauthorized visits to the crime scene are condemned,'' the 1st District Court of Appeal said in a ruling released Friday. The state court, in a 3-0 decision, said the observations by two jurors contradicted important defense testimony in the case.

The court granted a new trial to Dana Deweese of Castro Valley and his sister, Deanna Joao. Joao's lawyer, Kyle Gee, said she already has served most of her four-year sentence for manslaughter. Deweese was sentenced to 17 years to life for second-degree murder.

They were convicted of the August 1987 killing of Kenneth Tyrone Jackson after a brawl between the Deweeses and a neighbor family at a birthday party.

Prosecution witnesses said Jackson was not holding a weapon or making threatening gestures when Deweese, standing 15 to 20 feet away, shot him with a gun his sister allegedly had given him. They said Deweese was standing between his brother Duane and Jackson.

But Deweese testified that Jackson was pointing a knife at Duane, five or six feet away, and attempting to stab him when Deweese fired into the ground to scare Jackson. He said the gun accidentally went off a second time and hit Jackson, about 40 feet away.

After the verdict, two jurors, Margaret Westberg and Daniel Freeman, told a defense investigator they had visited the site during the trial. Freeman said he had concluded that Deweese had been less than 25 feet from Jackson.

Freeman's unauthorized observations contradicted Deweese's testimony, supported prosecution testimony that Deweese's brother was not in danger, and caused the juror to question Deweese's credibility, said the opinion by Presiding Justice Harry Low.

''By wrongly viewing the crime scene, the juror was improperly influenced against defendant's version of the shooting,'' Low said.

Alameda County Superior Court Judge Stanley Golde ruled that the declarations did not require a new trial, but was overruled by the appeals court.