A paid FBI informant testified yesterday that one of the Cuban exiles charged with the 1976 murder of former Chilean ambassador Orlando Letelier bragged to him that he made the bomb that killed the diplomat, but that investigators were "too stupid" to link him to the slaying.

Ricardo Canete, who has been convicted of larceny and counterfeit money charges, told a U.S. District Court jury that in mid-1977 Cuban exile Alvin Ross Diaz "started to brag about his work -- making bombs. He said he once made a bomb out of a coffee pot and that he made another explosive device recently that worked very well.

"'I made Letelier's bomb,'" Canete quoted Ross as saying, "and I said (to him), 'So what?'"

Canete's "so what?" remark was greeted by laughter in the courtoom from a couple of relatives of the three Cuban exiles on trial in connection with Letelier's killing. But Ross sat stony-faced as Canete, himself a Cuban exile, continued to calmly recount conversations he said he had with Ross in 1977 and 1978. Canete's testimony was by far the most damaging heard yet against Ross, who once was imprisoned by Cuban President Fidel Castro for two years for his antigovernment activities.

Canete, who said he had been paid $2,000 by the FBI between May 1977 and April 1978, recalled that in March 1978 he got together at a Union City, N.J., restaurant with Ross and another Cuban exile charged with the Letelier killing, Virgilio Paz Romero, a fugitive in the case.

Canete said he told Ross and Paz that he was worried that he soon would receive a subpoena to testify before a U.S. grand jury in Washington that was investigating the Letelier slaying.

"Mr. Paz leaned over the table and said, 'Look, we did it. They know it. We know it. But let them prove it. Mr. Ross nodded his head" to confirm Paz's statement, Canete testified.

"Mr. Ross said they even had some papers of (his) but they are too stupid to figure out what they have," Canete said.

Canete said he was told by Paz not to say anything to the grand jury and, to emphasize the point, was blindfolded by Paz and taken for a drive. Canete said nothing further to the jury about what happened on the drive, but only because Judge Barrington D. Parker refused to let him say more.

Ross braggadocio later turned to anxiety, Canete said. By last April, Chilean officials had rejected a request for a $25,000 loan to aid the Cuban nationalists who allegedly had helped Chilean secret police agent Michael V. Townley kill Letelier.

Canete quoted Ross as saying: "If I have to. I'll lay this all the way into the hands of Manuel Contreras," the former head of the Chilean secret police once known as DINA. Contreras is one of three Chileans waiting for a Chilean Supreme Court decision on whether they must be extradited to stand trial for Letelier's killing.

Canete also testified that he made elaborate false identification papers for Ignacio Novo Sampol, who is charged with lying to the grand jury investigating Letelier's Sept. 21, 1976, death and failing to tell authorities about the crime.

Canete, who will be cross-examined today by defense lawyers, said that the government promised not to prosecute him in connection with a theft from an interstate shipment in exchange for his testimony in the Letelier case. He currently is being cared for under a government witness protection program and his identity was not made known to the defense until last Sunday night.