The defense in the Abscam bribery trial turned a federal courtroom in Brooklyn into an electronic classroom of sorts today in an effort to show that a key government witness lied in his testimony last week.

Mark R. Weiss, an acoustics expert who worked on the Watergate tapes case, testified that tape recordings of two telephone calls made last July by Melvin Weinberg, a convicted con man who was working undercover for the FBI, were deliberately stopped and restarted. Weinberg testified that the FBI, were deliberately stopped and restarted. Weinberg testified that the gaps in the tapes were caused when he accidentally dropped the tape recorder on the floor.

Attorneys for Rep. Michael (ozzie) Myers (D-Pa.) and three codefendants have tried to focus the jury's attention on Weinberg rather than their clients' actions. Myers was secretly videotaped taking $50,000 last August from an undercover FBI agent posing as the representative of an Arab "sheik" after Myers promised to introduce a private immigration bill to help the phony foreigner.

Myers testified Monday that he didn't mean what he said on the videotape. He said he never intended to introduce the bill. He never did.

Weiss used an array of technical diagrams and slides of "magnetic development" and "wave form analysis" to show how he concluded that the gaps in the tapes of conversations between Weinberg and Angelo J. Errichetti were deliberate.

The defendants consistently have attacked Weinberg's credibility, saying he told them the scheme to take money from the "sheik" was only an act.

Richard Ben-Veniste, attorney for defendant Howard L. Criden, used Weiss' testimony to raise the possibility that Weinberg deliberately shut off the tape recorder so he could give directions to Errichetti, the mayor of Camden, N.J., without his FBI superior's knowledge.

Thomas P. Puccio, the government prosecutor, showed through telephone records that the gaps in the two tapes totaled no more than 56 seconds in one instance and one minute, 17 seconds in the other.

Before the jury began hearing evidence today, Puccio complained to U.S. District Judge George C. Pratt that allowing a defense that Meyers was "only acting" would "open the floodgates for every public official going into a meeting to cross his fingers behind his back. . . I have never seen a case where a public official gets up on the stand and brazenly admits he has violated the law, that he took the money, that it was easy money, that he made false representations to get it."

The judge said later that he planned to give the jury its legal instructions Thursday after summations by Puccio and defense attorneys. He said the jury will begin deliberations the same day.

The rest of today's court session was taken up by laborious defense questioning, mostly by Ben-Veniste, that finally tried Pratt's patience. By the end of the day he was sustaining objections Puccio hadn't even made.

After the defense rested its case late in the day, Puccio called a West Palm Beach car dealer as a rebuttal witness. Tuesday Errichetti's nephew, Joseph DiLorenzo, testified that DiLorenzo had given Weinberg's several gifts, including TV sets he put in Weinber's van in the late summer of 1979. This information was elicited in an effort to discredit Weinberg by showing that he had accepted gifts from the man against whom he was testifying. s

Stanley Fortner, the car dealer, testified today that Weinberg traded in the van at his dealership on June 23, 1979. Puccio was trying to discredit DiLorenzo's testimony, as the defense had tried to do the opposite Tuesday.