Sometime during the Congressional Black Caucus' gala dinner on Saturday night, when the faulty sound system made it impossible to hear either Chicago Mayor Harold Washington or singer Lou Rawls, Rep. Charles Rangel and former vice president Walter Mondale were head to head talking politics--specifically, the latest entry in the Democratic presidential sweepstakes, George McGovern, the would-be entry, the Rev. Jesse Jackson, and the impact of both on Mondale's chances.

Earlier in the day McGovern had spoken to several hundred participants of the weekend meeting. At the dinner, Jackson was a few tables away from Mondale, wooing potential campaign workers.

"First of all, no one is serious about a black candidate. No one is running for president, no one is seriously campaigning," said Rangel. However, he added, "No one should be critical of Jackson with McGovern in the race. McGovern is the joke."

The Caucus' legislative weekend, now in its 13th year and running five days, from Wednesday forums to Sunday brunches, has come and gone. It is the largest black political gathering of the year. It is an institution with its own drama and contradictions, and its own life, complete with power plays, deal-making, issue debates, absurdity and fellowship.

Rangel, a New York congressman since 1970 and an elder of the 21-member group, is a good case study of how a congressman gets through this packed schedule. He not only embodies part of this year's theme, "The Black Agenda for the Eighties: Toward Political and Economic Empowerment," but he is skilled in meeting the serious, as well as the social, demands. The all-Democratic Caucus itself, with eight members having served more than a dozen years, is an illustration of that political power. Rangel was right in the thick of the political debate, with his ties to Mondale and his heated refusal to acknowledge Jackson's activities as a presidential campaign.

Since his election to Congress to replace the powerful and controversial Adam Clayton Powell, Rangel, 53, has built a reputation as an important and progressive politician. He is now the third-ranking member on the Ways and Means Committee and chairman of its oversight committee, one of four deputy whips and chairman of the Select Committee on Narcotics Abuse and Control. His influence encompasses the inner circle of the House Democratic leadership and the country's financial policies, as well as critical urban issues. His word counts nationally: When Mondale damaged his reputation among some blacks by endorsing Richard M. Daley over Washington in Chicago's mayoral primaries, Mondale asked Rangel to intercede in his behalf.

This year the Caucus added two days of issue forums, which attracted at least 1,000 people. Many of the Friday workshops had to relocate or get additional seating because of the crowds. Another new element, an exhibition area at the Washington Hilton, was financially successful. "We had 150 exhibitors, and $800 of the fees per exhibitor goes right to the Foundation," said Rep. William Clay (Mo.), chairman of the weekend events. The Caucus Foundation supports a fellows program. This year, said Clay, the Caucus returned "close to $100,000 in dinner reservations" because of lack of space. He estimated 10,000 to 15,000 people came to town for the various events.

Several things happened:

* When Curtis Sharp, the winner of $5.6 million in the New Jersey lottery last year, walked down with his now familiar gray bowler, people stopped talking long enough to clap. Mondale walked up the same aisle and got the benefit of the turned heads.

* Though no one seemed to be listening to the congressional emcees, the audience sat still for Rawls and Julius Erving, the basketball great who was one of five honorees. Other awards went to Randall Robinson, the executive director of TransAfrica; Steven Pruitt of the public employe division of AFL-CIO; Mayor Washington; and Rep. Sala Burton (D-Calif.)

* Near pandemonium erupted in the audience during several skits at the fashion show. In one, a woman, rejected by her man, drools over his gift of jewelry; in another, a model garnishes a whip in a bedwear scene. If the scenes had been part of a television show the audience would be among the first to raise a protest.

Rangel developed a way to deal with the weekend that permitted him a leisurely pace and ample time to work with the New Yorkers, who probably formed the majority of the guests. Here's what Rangel saw, and some moments he didn't see:

* The Official Opening: Rep. Julian Dixon (Calif.), the Caucus chairman, told 500 people at Friday's plenary session to tell their senators to vote for the Martin Luther King Jr. holiday and jobs bills, their representatives to vote for the export administration act, which would prohibit new foreign investment in South Africa. Rangel, one of five Caucus members present, added, "without your support, we are all just out of business."

* The Business: Next on Friday morning was a 45-minute session with a group from the Swedish Parliament to explain the American legislative process. This was followed by his workshop on drug problems, with an audience of about 75. Nancy Reagan had declined to join the panel, but Carlton Turner, director of the Drug Abuse Policy Office in the White House, William Pollin, director of the National Institute on Drug Abuse, D.C. Police Chief Maurice Turner and others participated. The debate was heated. Wendall Foster of the Bronx said, "I don't know many blacks or Spanish with airplanes or boats. I cannot accept the fact that we cannot stop the drugs, if you really wanted to. When I come through customs, they search my underwear."

* The Prayer Breakfast: Rangel, busy with guests who had lost luggage, skipped Friday night's round of parties and missed the Saturday breakfast. The Rev. Leon Sullivan reminded the 700 people in the audience, "We have our high positions and our little bit of corporation and government jobs. We want to remember . . . the same God who put you up can bring you down. It's about time we put our hand in God's hand."

* The Fashion Show: When Rangel arrived at the hotel for the fashion show Saturday afternoon, the counsel for the New York state NAACP nabbed him for a discussion of a job offer from the governor. "Well, at the governors' meeting . . . so Virgil said . . . so Tony said . . . and he called me back," said Rangel, the counselor in action. His wife, Alma Rangel, the chair of the Caucus' spouses group, said she predicted the fashion show would raise $150,000 this year.

Alicia Rangel, 12, had just sat down on her father's lap at the Caucus spouses' fashion show, when the lights went out. Shadows, definitely of the male variety, moved out toward the runway. As the lights went up, the women began to cheer, making noise that escalated to screams. For the hour he stayed, Rangel never took his eyes off the runway.

* The Debate: Rep. Gus Savage of Illinois had arranged a forum for the Caucus members to question some of the presidential candidates. Only five members participated. Rangel stayed away saying he was unclear about the program and surprised that any candidates showed up. The small showing by the members was another sign of the tremendous infighting the potential Jackson candidacy is causing. One reason, cited by Indiana State Senator Carolyn Brown Mosby, was "Other black politicians are mad he didn't come through the political ranks."

But the unofficial session with John Glenn, Gary Hart, McGovern and Jackson was productive. At least many of the nearly 1,000 people who crowded into the room thought so. McGovern got strong applause. Glenn's was not as firm. Jackson got loud "run, Jesse, run" cries. And Hart's applause was weak because half the room emptied after Jackson spoke.

* Evening: When the Rangels arrived for the Anheuser-Busch reception before the dinner, they were immediately thrust into small talk with David Cunningham, a Los Angeles city councilman; golf pro Lee Elder and his wife, Rose, a Washington businesswoman; Liz Robbins, a tax lobbyist for New York; Charles Manatt, chairman of the Democratic National Committee; and John Jacobs, president of the Urban League. Rangel was the last to leave the party.

At his table, the guests included the Mondales; Joseph Stewart, a senior vice-president of Squibb; John Amos, the president of a Columbus, Ga., insurance company; and Barbara Little, the widow of a longtime employe of the Ways and Means Committee. The table was center rear. When Rangel and Rep. Alan Wheat (Mo.) tried to call the dinner to order, no one was listening. Rangel was walking back and forth from the floor to backstage as Mayor Washington said the black community must be "totally involved in politics in 1984." When Rangel asked people to sit down for the awards ceremony, they were pouring out into the halls.

Afterward the Rangels went to a CBS Records party. Larry Friday, a law student, gave Rangel a warm hug. "One day I was down in the House basement, feeling kind of blue. We talked for an hour and he charged me right up," said Friday. Then Ronald Brown, general counsel to the Democratic National Committee, came up. Brown's father was the manager of the Hotel St. Theresa in New York, where Rangel worked to put himself through law school. Explained Rangel, slapping Brown on the shoulder, "Now I have to ask the son how I am going to do next year."

* The Brunch: For years, New Yorkers and others have come early and stayed late for the Rangel beans, potato salad, ham and chicken. Steven Rangel, 15, was videotaping the event. Emmett Rice, a member of the Federal Reserve Board, was talking to Judith Rogers, newly appointed judge on the D.C. Court of Appeals. Illinois Rep. Melvin Price, chairman of the House Armed Services Committee, was talking to a vice president of Consolidated Edison. Then Mondale stepped in line, asking Al Thompson, the only black wholesaler of beer in New York state, how the weather affected his business this summer. "Up 40 percent," Thompson said.

A number of guests noted Mondale's appearances with Rangel during the weekend. Had Rangel made up his mind about 1984? "I probably have," he said, swinging around to shake another hand.