Sen. Harrison A. Williams Jr.'s lengthy and halting effort to explain away incriminating statements on the FBI's Abscam videotapes backfired on him in the trial that ended in his conviction on bribery and conspiracy charges.

After Friday night's verdict in Brooklyn, a number of jurors said they didn't find the senator's explanations convincing. And earlier, during a break in Williams' testimony, the judge in effect had predicted the outcome.

U.S. District Court Judge George C. Pratt told Williams' attorney then, in an exchange out of the jury's hearing, "You may convince the jury of exactly the contrary of what the witness is saying, simply because he is saying it so often. And it may become in their view very artificial and very rehearsed."

The defense attorney, George J. Koelzer, admitted to the judge that the tactic was risky, but added: "It is something that has to be done because the tape is there."

The tapes. There in black and white on the courtroom television monitor for the jury to see. The senator and the FBI's phony sheik and his front men, time and again, discussing the power of Williams' office and getting government contracts for a titanium mine in which he would hold a hidden interest.

The tapes were the crucial evidence in Williams' conviction, just as they had been earlier in the cases of the six House members who were found guilty for their dealings with the sheik.

Jury member James Caracappa, a phone company worker, told Associated Press, for instance, that the videotapes "were certainly a key." Shirley Tannenbaum, 56, a housewife, said Williams shouldn't have taken the witness stand. David Greenberg, 24, a paramedic, said he felt the senator's answers contained too many contradictions of his taped words and actions.

The 61-year-old New Jersey Democrat, who once wanted to use his deep, resonant voice to become a radio announcer, just couldn't talk his way out of the videotapes.

But the government still may have some talking of its own to do in explaining the conduct of its agents during the controversial undercover investigation. The House Judiciary Committee has been examining whether limits should be placed on such undercover investigations. And the convicted members of Congress all have appealed on the grounds their constitutional rights were violated by overzealous FBI operatives.

A federal judge in Philadelphia reversed the convictions of two city councilmen there on those grounds. And, in an unusual departure, he went out of his way to criticize the government's techniques in the Williams probe.

In this last of the congressional Abscam cases, troubling questions remain.

The senator was taped flatly refusing a cash bribe offer in one meeting with the phony sheik. And two Justice Department prosecutors testified for the defense because they were offended by the efforts of one undercover man to put words in Williams' mouth.

Juror Emil Zullo, a 55-year-old Brooklyn welder, told United Press International that he felt the FBI "snookered" Williams. "I was ready to say they trapped him," he said. "You just can't go trapping people. I'm a citizen. I don't want to be trapped."

Zullo said, however, that Pratt's re-reading of his "entrapment" charge helfway through the jury's 28 hours of deliberation "enlightened us." Pratt said the government could overcome an entrapment claim if the defendants seemed predisposed to accept the FBI inducement -- a $100 million loan from the sheik and a promise of later profits in a titanium mine.

The jury decided that the senator "would have walked away" from the undercover agents' offer if he hadn't been predisposed to accept them.

During the month-long trial, Koelzer and Harry C. Batchelder Jr., attorney for codefendant Alexander Feinberg, argued that undercover informant Melvin Weinberg, a convicted con man, and undercover FBI agent Anthony Amoroso manipulated the conversations and coached Williams and Feinberg so that only incriminating statements would be recorded.

The most outrageous example was just before a crucial June 28, 1979, meeting between Williams and the sheik, where the senator bragged of his influence and said there would be "no problem" getting government contracts for the titanium mine.

In the pre-meeting session, Weinberg told Williams: "It's all talk. All b------- . . . you gotta just play and blow your horn. The louder you blow and mention names, who you control . . .."

"You gotta tell him how important you are, who you are and what you can do, and you tell him [the sheik] in no uncertain terms: 'Without me, there is no deal. I'm the man. I'm the man who's gonna open the doors. I'm the man who's gonna do this and use my influence and I guarantee this.'"

Two New Jersey prosecutors testified they felt the tactic was improper and they criticized Weinberg. They said he responded: "If we don't do that, we won't have no cases."

Prosecutors trying the case acknowledged being troubled by that episode.

The last jury has now spoken on the conduct of Williams and the six House members caught in the undercover sting. The jury is still out on Abscam as a proper tool for exposing political corruption.