A federal jury acquitted state Sen. John S. Joannou and former state senator Willard J. Moody today of all charges in a ticket-fixing case.

Joannou, 44, charged with one count of conspiracy and 12 counts of mail fraud, began to cry as a court clerk read the verdict, reached after about 11 hours of deliberation. "I just can't find words to express the emotional trauma I've been through," he said later.

Moody, 60, who faced eight counts of conspiracy, mail fraud, obstruction of justice and perjury, charged prosecutors and investigators in the case with prosecutorial misconduct. Asked if he felt bitter, he said, "Of course I would be."

The courtroom could have been the scene of a political victory party as dozens of relatives and friends filed up the center aisle to embrace and congratulate the defendants. Joannou, who has missed the first eight days of this year's General Assembly session, said he plans to resume his duties Monday as a Democrat representing Portsmouth. Moody, who built up substantial power during 27 years in the legislature as a Democrat representing Portsmouth, stepped down when his term expired in 1983.

During the 10-day trial, defense attorneys seized on the unsavory character and seemingly inconsistent statements of Clarence Mixon, whose testimony was the cornerstone of the prosecution's case.

Mixon, Moody's partner in a Portsmouth car dealership, testified that Moody and Joannou knowingly used phony calibration certificates from the dealership, purporting to show inaccurate speedometers, to help friends, relatives and political cronies get their speeding charges reduced or dismissed.

He was the only one of more than 70 witnesses for the prosecution to testify that Moody and Joannou knew the documents were false. Defense lawyers attacked him as a "vile" character who, according to the testimony, repeatedly lied under oath, embezzled from the dealership, stole potato chips off trucks and siphoned gasoline from customers' cars.

Wayne Lustig, an attorney for Moody, said Mixon was so thirsty for revenge and so eager to avoid prosecution for his many crimes that he would say whatever he thought was most damaging to Moody and Joannou. "You just push Clarence Mixon's button," he said.

"Clarence Mixon's story," said defense attorney James C. Roberts, "depends on which day of the week it is." Mixon lent some credence to that contention during the nine hours he spent on the stand. At one point he admitted giving two versions of how the scam began, telling a grand jury that Moody came up with the idea in 1977 or 1978 and testifying during the trial that it was Joannou's suggestion in 1975 or 1976.

Two witnesses directly contradicted Mixon, testifying that he was distributing phony certificates years before he said he, Moody and Joannou agreed on the scheme. One said Mixon told him the certificates were "my hustle."

Defense attorneys characterized the rest of the prosecution's case as "a hodgepodge of innuendoes." What prosecutors called "common-sense" inferences, defense attorneys called "speculation and guesswork."

Assistant U.S. Attorneys Joseph Aronica and Robert Seidel did not directly question Mixon's conflicting stories, but they tackled head-on the issue of his character.

They admitted on the first day of the trial he was a "scoundrel." But Seidel told the jury his testimony was just one piece of a "mosaic of guilt" that pictured the two lawmakers as systematic lawbreakers.

The attorneys conceded that some of the evidence was circumstantial and vigorously pursued a line of argument that a defense attorney said boiled down to "How could they not have known?" that the calibration certificates were fakes. They argued that it was obvious to two members of Moody and Joannou's law firms that the certificates were phony.

"There was the 15-mph special and the 30-mph special," Aronica said.

He argued that it was "total nonsense" to believe that two well-educated lawyers and legislators "accustomed to asking why" couldn't figure out the certificates were false.

Both Moody and Joannou testified that they thought the documents were valid. Joannou's attorney said that as a busy legislator Joannou didn't have time to investigate the documents.