Six persons were convicted by a federal jury yesterday for their roles in running an illegal street corner "drugstore" at Seventh and T streets NW that sold thousands of dollars worth of heroin and narcotics pills to its patrons last spring and summer.

The jury deliberated for six days - one of the longest deliberations in a federal criminial case here in recent years - before convicting members of the drug ring, allegedly headed by John Robert (Stampede) Johnson, of 312 16th St. SE.

Johnson's brother, Odell, and four other persons also were convicted of the illegal narcotics activity. One member of the alleged conspiracy - Mary Ann Jones - was acquitted of various counts but the jury could not reach a verdict in her case on the conspiracy count.

he convictions were the first to come out of investigations by a special Drug Enforcement Administration-D.C. Police Department task force investigating alleged drug conspiracies here.

The task force used videotapes as well as voice tapes to detect what it contended in court were drug transactions in the 1800 block of Seventh Street NW, and to attempt to knock out a whole drug-dealing apparatus instead of focusing either on its top or bottom members.

Investigations by the drug task force have resulted in two other drug conspiracy indictments in federal court here.

In the other one to come trial so far, the alleged ringleader - Joseph Jackson - failed to show up for trial this week. Several other defendants in the Jackson case pleaded guilty, and only one, Irvin hall, on trial before U.S. District Judge Joseph C. Waddy.

Members of the Johnson conspiracy were indicated by a federal grand jury last October after a 5 1/2 month investigation. The Johnsons were charged with operating a criminal enterprise, a count that carries a life prison term, but the jury instead found them guilty only of a more routine narcotics conspiracy charge, which carries a maximum prison term of 15 years.

Also convicted in the narcotics conspiracy before U.S. District Judge George L. Hart Jr. were Arthur Watson Jr., John Roscoe Slade, George Norman Marshall and David Redds.

An eight person, Donald Larry Baxter, was indicted but did not show up for trial. Assistant U.S. Attorney Barry Leibowitz of the major crimes division said in earlier court proceedings in the Johnson case that informants had told law enforcement officers that Baxter had been murdered.

The alleged murder of Baxter, whose body has not been found, came up just before Johnson's trial was scheduled to begin. Law enforcement officials said they heard from an informant in D.C. jail, where John Johnson was being held pending trial, that Johnson was writing his friends and directing them to commit perjury and pin the blame on Baxter.

On the eve of the trial, police officers obtained search warrants for Johnson's jail cell and other locations and recovered letters that they said substantiated their allegations that Johnson was trying to fabricate an alibi defense. An investigation of this is continuing.