The New York Rangers expected to revive their sagging Stanley Cup hopes tonight amid the adulation of their manic fans in Madison Square Garden, where they had lost only once in their last 17 games.

The New York Islanders showed, however, that neither fans nor building can overshadow talent. They easily disposed of the Rangers, 5-1, to build a3-0 lead in their best-of-seven semifinal, with the possible clincher set here Tuesday night.

The Islanders scored two power play goals in the first period, then scored two more goals during a second period in which they outshot the Rangers, 17-4. Bob Nystrom, always a superb playoff performer, led the Islander attack with a goal and two assists. s

"We wanted the power play to click, because we knew they'd be hitting and taking it to us tonight," Nystrom said. "The power play worked right away and I think they figured they couldn't risk hitting too much and getting more penalties."

The Islanders had been beaten on their last five visits here, the last game of the 1979 semifinals and four games in the regular season. This time they were ready.

"A lot of the difference was in the way we approached it," said defenseman Denis Potvin, the target of boos whenever he touched the puck. "We came in here this time with a little more confidence. We were excited about playing the Rangers, because it's the playoffs and it means so much. You can't come in here every two months during the season and get way up."

The Islander power play, successful in every playoff game and now 24 for 61 after ranking No. 1 during the regular season, has been a key factor in the Ranger downfall. Of almost equal importance, though, is the failure of the Ranger power play, which ranked 20th in the NHL and is only nine of 54 in postseason action.

The Rangers, as expected, came out hitting hard, with fan hero Nick Fotiu dumping Potvin on his first shift of the series. An early Islander power play was killed without difficulty and after seven minutes the Rangers had a 6-1 edge in shots. Then Islander Clark Gillies was chased for holding.

The Ranger power play was so inept and disorganized that it obviously deflated both Rangers and fans. Then Bryan Trottier flattened Ranger captain Barry Beck with a solid check and the momentum belonged to the Islanders.

Fotiu was banished for elbowing Mike McEwen and the Islander power play was successful in freakish fashion. Mike Bossy, cutting across the slot from the right-wing corner, tried to pass to Trottier near the left post. Ranger defender Carol Vadnaisw blocked the pass, but the puck caromed off his skate directly inside the right post as goalie Steve Baker, moving to counter Trottier, could not get back.

Bob Bourne, who had missed three games with a groin pull, made it 2-0 on a rebound of a McEwen shot with Ranger Ron Duguay in the box for tripping.

In the second period, the Rangers survived a stretch of 1 minute 39 seconds in which they were two men short, Baker blocking five shots. Ken Morrow eventually made it 3-0 on a 50-footer and with five seconds left in the period Nystrom made it 4-0 after the puck took a crazy bounce off the boards as Ranger Chris Kotsopoulos tried to pass it around behind his net.

"When they've got a 4-0 lead, it's a hell of a job even to come back within a sniff of them," Beck said. "We tried -- if you don't try, you ought to pack up and go home, but they're playing damn good hockey and we never got close."

For an added sour note, Beck lost his balance trying to check Wayne Merrick and slid into the boards, injuring his back. He did not miss a shift, though.

"I just snapped my neck," Beck said. "Then when I got up my neck was all right, but I pulled something in my back. I'll be all right for Tuesday."

Peter Wallin spoiled goalie Bill Smith's shutout bid with 14:47 left, but it was strictly a last gasp. When Wayne Merrick concluded the scoring with 5:06 to play, many of the 17,381 fans already were elsewhere.

Asked whether the fans bothered the Islanders, Gord Lane laughed and said, "I guess we bothered them."