Contributors to Channel 26 who think their $15 a year helps support "Masterpiece Theater" and "Live From Lincoln Center" may be surprised to find out tonight that they have underwritten the telecast of a hockey game.

It's a surprise to the struggling Washington Caps hockey team as well, because until late yesterday the team thought it was footing the bill for WETA's telecast of the fifth game in the Stanley Cup hockey finals and that the broadcast would help build audiences for hockey in Washington:

Ward Chamberlin, president of WETA, decided it would not be "proper" for hockey team to underwrite a hockey game, however, and besides, he think the game will help build audiences for WETA in Washington.

And in order to present the game, starting at 8 o'clock tonight, Chamberlin has preempted until next Monday a special program that Lawrence K. Grossman, president of the Public Broadcasting Service (PBS) yesterday called "one of the biggest, most significant shows" on public television this season.

"TV on Trial," a two-hour documentary that includes edited highlights of the 1977 trial of Ronny Zamora - a Florida youth convicted of murdering an elderly neighbor despite his lawyer's plea that the boy was suffering "intoxication" from TV violence - is one of the public TV showpieces this year. Late yesterday, PBS personnel were busily trying to get "Channel 26" removed from newspaper ads that will run today promoting the telecast, still set for Channels 22 and 67.

In addition, Channel 26 will preempt another PBS special, "A Connecticut Yankee in King Arthur's Court" - the first American-made entry in the previously all-imported series "Once Upon A Classic" - until Saturday at 7 p.m.

All this is to present a hockey game that another Washington station, WDCA-TV, already turned down for fear of low ratings and lack of viewer interest Jim Reed, WDCA program director, said yesterday, "Hockey has never been a good performer in this market" and a spokesman for the Washington Caps conceded that ratings for Caps games that have shown on Channel 20 "weren't all that high."

Abe Pollin, the ubiquitous local entrepreneur who owns the Caps, wants Washingtonians "to see the kind of hockey we're eventually going to give them here," said Andy Dolich, director of marketing for the Caps. So when Channel 20, in the thick of a crucial ratings "sweep" period, turned down the Stanley Cup finals, Dolich thought, "Well, why not try public TV?"

And late last week, WETA picked up the puck. Officials there agreed not only to carry the game but to accept $1,500 from the Caps to pay for the rights and the costs of the telecast. Dolich also offered to underwrite the sixth game of the series, scheduled for Thursday, and a seventh and final game, on Saturday, if one is played.

It apparently didn't occur to those WETA officials that the Caps were in effect buying air time on public television to promote the cause of hockey in Washington. Dolich said he hoped to have Caps members in the studio at WETA to comment on the game between periods. There would be other time to fill as well, because most of the stations carrying the game, produced and supplied by the National Hockey League (NHL), are commerical stations, and the game comes with some commercial built in.

Public TV is not allowed to carry commercials and it is not supposed to accept underwriting of programs from those with a vested interest in the program content.

"We weren't trying to bring this game to the people of Washington for the purpose of selling season tickets," Dolich said.

"But we want hockey on TV in Washington as far into the season as we can get," he added. "If people are talking about hockey this far into the season, it can only help the Caps. We're going to be the Avis to the Redskins' Hertz in this town; someday we'll be the second sold-out team."

Dolich said that while attendance at Caps games this year was down slightly from last year, the hockey games still drew more fans than the basketball games played by the Washington Bullets. Of course Pollin isn't losing any sleep over that since he owns the Bullets, too, and the stadium they will play in, the Capital Centre. (Pollin was unavailable for comment yesterday.)

Yesterday Chamberlin and WETA Executive Vice President Gerald Slater decided that perhaps the "public" television station was being used for private gain. And so the day before the telecast, they nixed te deal with the Caps but gave the go-ahead for the fifth, sixth and possible seventh games of the finals to be shown on the station.

It will only cost the station $800 for the rights to each game, plus "a couple hundred dollars" in station expenses, Slater said. Members of the Caps will be in the WETA studios to help - not with promoting hockey but with drumming up still more contributions to WETA.

"We decided it was inappropriate for the hockey team to underwrite the use of local membership dollars to underwrite it, said Slater.

"When the matter got to what we call the top management level," said Chamberlin, "and even though it would have been perfectly legal, it didn't seem to me to be a good idea for the hockey team to underwrite the game."

Chamberlin said it will be worth it to delay national PBS programming like "TV on Trial" for hockey because "I'm an enthusiast about scheduling an occasional sports special, even if it means moving a few programs around. It's part of our effort to get different people able to interested in tuning to Channel 26. We hope that when they tune in for this, they'll come back for something else."

Slater said he didn't see anything incongruous about putting on a rough even brutal game like hockey on public TV. The fact that the commercial stations had passed it up gave WETA a chance to step in and fill a community need, said Slater. Told the commercial station passed it up because hockey doesn't draw many viewers here, he said, "Well. Okay. My impression is that there is some audience out there for hockey."

Two other public TV stations are among the 50 stations nationally who are carrying the Stanley Cup finals. One of them is, ironically or not, WPBT in Miami, the station that originated the tapes that were edited into the "TV on Trial" special. The Florida Supreme Court had opened the courts there to TV cameras as part of a one-year experimental project.

The other station is KQED in San Francisco, which actually has two broadcast channels - a VHF station, Channel 9, and a less-watched UHF station, Channel 32. The hockey game will be on the UHF station.

At neither station is the hockey game being sponsored by a hockey team. In Miami, the costs of the telecast will be paid by an apartment complex called the Charter club.

In San Francisco, the hockey game will be underwritten by a company that makes prosthetic devices.

PBS president Grossman, while extolling the importance of "TV on Trial," did not criticize Channel 26 for delaying it.

"If they've got an opportunity for a live sports event, I'd hate to look over their shoulder and try to say what's right," said Grossman. "You know the way public television is - every station is autonomous and we never force anything down their throats."