MANAGUA, NICARAGUA -- Rep. Cass Ballenger (R-N.C.) arrived here last weekend with eight cartons of electronic parts, worth $35,000 he said, that he helped install at the recently reopened Catholic Radio.

"We wanted the Catholic Radio to come on the air full blast," the freshman congressman explained, adding that he collected the spare parts from private donors and still has more fund-raising to do back home to cover the cost.

These donations and a $20,000 grant to aid political prisoners, plus U.S.-sponsored visits by anti-Sandinista personalities, have delighted some Nicaraguans in the opposition and worried others who fear Managua could quash their efforts in response.

U.S. officials have encouraged more open American support for the legal opposition here -- as distinct from the rebel guerrillas, known as the contras -- since the five Central American presidents signed a peace plan Aug. 7 designed to amplify personal freedoms in this country.

The Catholic Radio's two transmitters deteriorated during a 19-month shutdown because the leftist Sandinista government did not allow the station to crank up its equipment, even without going on the air, according to its director, the Rev. Bismarck Carballo.

Since the official voice of the Archdiocese of Managua resumed broadcasting Oct. 2, the statical radio waves have failed even to reach across the capital. Carballo, who served in a suburban Maryland parish during a 14-month exile that ended Sept. 12, had made numerous public pitches for American donations for the radio before Ballenger came through.

Only a year ago, the government, in a crackdown that followed the approval by the U.S. Congress of $100 million aid for the Nicaraguan rebels, closed the opposition newspaper La Prensa. The daily had received a grant worth $100,000 in ink and other materials from the congressionally funded National Endowment for Democracy in Washington.

Sandinista leaders argued then that the grant made La Prensa a tool of U.S. foreign policy. La Prensa reopened Oct. 1.

Late last month, the endowment approved a new grant for Nicaragua, a spokeswoman confirmed in Washington: $20,000 for a fledgling group of relatives of political prisoners named the January 22 Movement. The funds are administered by Southern Legal Assistance, formed by three Atlanta lawyers, including former attorney general Griffin B. Bell.

Bell came to Nicaragua a year ago in a thwarted attempt to represent the downed American airplane crewman, Eugene Hasenfus, in his trial before Nicaraguan courts. Hasenfus was defended by Enrique Sotelo Borgen, an outspoken opposition lawyer, and was pardoned last December.

Sotelo, in an interview last week, said he asked Bell and other Atlanta lawyers he met through the Hasenfus case to help raise private funds for the January 22 Movement and for the legal defense of Nicaraguans jailed on political charges.

"I told them we didn't want U.S. government money and we weren't going to do anything anyone could say was illegal in Nicaragua," Sotelo said. As counsel for the movement, Sotelo hopes to open a legal services office with part of the endowment grant.

{Barney Haynes of Southern Legal Assistance said he, Bell and William T. Boone Jr. also had contributed some of their own funds to the effort. "The purpose of the National Endowment grant is to provide legal representation for people who otherwise would not be represented by lawyers," Haynes said.}

Nicaragua is holding at least 4,500 political prisoners, according to estimates of independent human rights groups. The January 22 Movement, named for the day of its founding this year, claims to represent 4,000 of them. Up to now, mothers and wives in the group met expenses from their own pockets and through small local donations. The government says the group is not legal. Before Aug. 7, its members often were harassed by state security police.

In September, the movement began to hold street marches for the first time. Its first march, Oct. 1, drew only six women, who paced in silence around the Calvario Church in downtown Managua with white bandanas, bearing the names of prisoners, wound around their heads. Later marches grew to several dozen.

Movement leaders said they were disconcerted by news of the endowment grant.

"We're not going to accept it if it will compromise our prisoners or make the government accuse us of being from the CIA," said the movement's treasurer, Ana Soledad Roman, 27.

Movement directors said they remain befuddled after discussions with U.S. Embassy officials about whether the grant is private or public. The nonprofit National Endowment for Democracy was created by Congress and works with U.S. government funds, but Director Carl Gershman said in a telephone interview that it is run like a private organization, autonomous of the Reagan administration.

In a separate initiative, the U.S. Embassy sponsored a brief tour by two former leftist writers, David Horowitz and Peter Collier, and historian Ronald Radosh, all critics of the Sandinista government. Horowitz extolled the rebel forces before several opposition party audiences in Managua.

Former ambassador Jeane Kirkpatrick, a backer of military aid for the rebels, was flown here by the U.S. Air Force to address opposition politicians at an embassy Columbus Day reception.

President Daniel Ortega has said that if the U.S. Congress approved new military aid for the contras, it would create "larger tensions" in Nicaragua that would force him to impose "new restrictive measures."

So far, the government has offered no reaction to the new American involvement in opposition efforts here.Staff writer Joseph Pichirallo, in Washington, contributed to this story.