A federal judge in Alexandria yesterday ordered the limited release of Ronald L. Humphrey, the U.S. Information Agency employe charged with spying for Hanoi, so that he can confer with his lawyers and review documents before his trial May 1.

U.S. District Court Judge Albert V. Bryan Jr. said he approved Humphrey's release from 9 a.m. to 5:30 p.m. next Tuesday through Friday in part because his codefendant, Vietnamese antiwar activist David Truong, was released April 14. Truong was able to post his $250,000 bond. Humphrey is being held in the Alexandria jail and has been unable to meet his $150,000 bond.

U.S. Attorney William B. Cummings opposed Humphrey's lawyers' request that he be released because he said it might set a precedent. Neither Cummings nor Humphrey's attorney, Warren L. Miller, said they knew of another case where a judge allowed a jailed defendant to be released when he couldn't meet a bond requirement.

But Bryan said he was unconcerned about setting a precedent. "The circumstances of this case where one defendant has been released on bond . . . and there's substantial documentary evidence . . . and where the offenses are particularly serious as they are" warrant his release.

"I felt it was absolutely essential and the court agreed with me," Miller said. Humphrey must review "literally thousands of documents and tapes' his lawyer said. According to Bryan's order Humphrey will be under guard at all times during his release.

Bryan denied other requests made by Humphrey's and Truong's lawyers, such as moving the case to the federal court in the District of Columbia and dismissing several charges because the indictment is allegedly vague. Bryan also said he would not allow the defendants to have separate trials, as requested by Truong's lawyers.

Humphrey is charged with stealing classified U.S. diplomatic cables and delivering them to Truong. Truong is accused of having the cables and documents delivered to agents for the Vietnamese government.