Washington no longer has a baseball team, but the people who run Robert F. Kennedy Stadium want to fix the stadium up-at a cost of up to $5 million-in hope that a team will be found to play here as early as next year.

Robert H. Sigholtz, general manager of D.C. Armory Board facilities, won approval from the board yesterday to explore ways to finance the improvements.

Mayor Marion Barry, who recently appointed himself to the Armory Board and who became its acting chairman at yesterday's meeting, agreed to the Sigholtz suggestion.

"Eventually we are going to have baseball, I hope," Barry declared. "We ought at least to anticipate."

Sigholtz said he did not know of any current attempts to get a baseball team for Washington, which lost the Senators expansion club to Texas in 1971. "I think it's just a matter of time," he told a reporter. "I'm surprised we don't have one this year."

The Armory Board operators both RFK Stadium and the nearby D.C. Armory, a National Guard facility on East Capitol Street near the Stadium.

Although the Senators and the Redskins football team shared the stadium up to 1971, Sigholtz said the current earlier start of the football season would rule out future joint use without renovations.

It takes 11 days to convert the stadium from football to baseball and 11 days to switch back again. The renovations he has in mind would permit the change to be done in a matter of hours, Sigholtz said, making joint use possible.

Sigholtz said he suggested the work at this time because nearly $20 million in bonds that were sold to build the stadium in 1959 will get paid off next December.

Sigholtz also disclosed that he has talked to an unidentified firm that would be willing to convert the stadium's baseball press box into a membership-only bar and restaurant, from which games could be viewed through large windows. The board did not act this.

The press box seats 250-far more than the dozen or so reporters who covered Senators games-and could well be sacrificed for a new club, Sigholtz said.

A smaller press box would be built below it.

"With 26 teams (in the two baseball leagues), you don't get an All-Star game very often" that draws hordes of reporters, Sigholtz said, "and a World Series-once in a while."

The original Washington Senators last played in a World Series in 1933, when their home park was the old Griffith Stadium on Georgia Avenue.

In other matters, the Armory Board approved:

A 10 percent rise in rents for use of the armory-the first since 1974-pushing the daily cost for exhibit events to $1,650 and for performance events to $2,200.

An 11:30 p.m. cutoff time for a rock concert planned at the stadium on July 7, a half hour later than the 11 o'clock curfew formerly in force.

Approval for two carnivals to be held in May and July in the stadium south parking lot, the first such events since 1968.

In a related matter, Barry announced the nomination of Stuart Long, a Capitol Hill restaurateur, to fill the one remaining vacancy on the Armoury Board.