A federal appeals court today overturned the conviction of Frank P. Quattrone, a former technology investment banker who came to symbolize the excesses of the 1990's boom, ruling that the judge presiding over the case gave the jury erroneous instructions.

In an unanimous decision, the United States Court of Appeals for the Second Circuit in Manhattan ordered a retrial of Mr. Quattrone, who was convicted on obstruction of justice and two other charges in 2004, and said the case should be assigned to a different judge than the one, Richard W. Owen, who originally presided over the case.

It would be Mr. Quattrone's third trial; the first jury to hear a case against the banker was unable to reach a verdict. After his second trial, Mr. Quattrone was sentenced to 18 months in prison. He has been free pending the appeal.

The jury had convicted Mr. Quattrone, who had been head the technology investment banking practice at Credit Suisse First Boston, for encouraging the destruction of documents that were being sought by a grand jury and the Securities and Exchange Commission.

The case against him was built largely on e-mail messages in which he endorsed another colleague's instructions to associates in his division to "clean up those files." Federal investigators were looking into how Credit Suisse First Boston allocated shares in hot initial public offerings.

In writing for the court, Judge Richard C. Wesley said Judge Owen's instructions to jurors were wrong because jurors were not required to determine that Mr. Quattrone knew the documents he was asking associates to destroy were the same ones being sought by investigators.

The instructions "removed the defendant's specific knowledge of the investigatory proceedings and the subpoenas/document requests from the obstruction equation," Judge Wesley wrote. "It left a bare-bones strict liability case."

Mr. Quattrone and his lawyers have argued that he was following the company's documents retention policy and did not know that the files in question were being sought by federal officials.