A key detail of House Armed Services Committee Chairman Les Aspin's alternative to the defense authorization was incorrectly reported yesterday as a result of an editing error. Aspin's plan would cap MX missile deployment at 40.

The Senate last night overwhelmingly approved a scaled-back Defense Department authorization bill for next year after passing a compromise urging continued but modified compliance with the unratified SALT II arms control treaty.

Final passage came on a 92-to-3 vote, and the SALT II resolution was approved, 90 to 5.

With President Reagan due to decide by Monday whether to continue recognizing SALT II when it expires at year's end, arms control advocates pushed for a resolution that would put the Republican-controlled Senate on record as advocating compliance at least through the end of 1986.

But they had to compromise with conservatives balking at action aimed at influencing Reagan toward sticking with SALT II, which they say the Soviet Union has been violating, undermining U.S. security.

An agreement after hours of back-room negotiations said the United States should continue through Dec. 31, 1986, to "refrain from undercutting the provisions of existing strategic offensive arms agreements" as long as the Soviets do so.

But it also specified that "proportional responses" to Soviet violations and continued development of the small, mobile Midgetman missile would not be barred. "Proportional responses" was not clearly defined.

The nonbinding language apparently would not block the administration from a "gray-area approach" that would avoid outright dismantling of a Poseidon nuclear submarine in order to meet SALT II missile limitations when a new U.S. Trident submarine enters service later this year.

"It gives the administration the flexibility to choose that as a possibility," said Sen. James A. McClure (R-Idaho), a SALT II critic, who said the administration could use the "gray-area approach" to reduce Soviet SALT II violations.

Sen. Dale Bumpers (D-Ark.), sponsor of the original compliance proposal, said nothing in the resolution prevents the United States from doing what it intended to do. He said continued compliance would restrain the Soviets far more than the United States.

Sen. Edward M. Kennedy (D-Mass.) described the resolution as a "major victory in the United States Senate for the forces of arms control," but Sen. Pete Wilson (R-Calif.) called it "mealy-mouthed."

McClure indicated that the administration neither embraces nor opposes the compromise. "I haven't been able to get from them what they want . . . . They have the language, and the roof is still on the White House," he said.

Voting against final passage were Democrats John Melcher (Mont.), Howard M. Metzenbaum (Ohio) and William Proxmire (Wis.). Voting against the SALT II compromise were conservative Republicans Chic Hecht (Nev.), Jesse Helms (N.C.), Gordon J. Humphrey (N.H.), Steve Symms (Idaho) and Malcolm Wallop (Wyo.).

The defense authorization bill reflects earlier agreement between Senate Republican leaders and the White House to cap defense spending to help reduce deficits.

The bill anticipates a record $302 billion allocation but would level off Reagan's four-year military buildup by limiting additional spending to no more than the cost of keeping pace with inflation.

This contrasts with inflation-adjusted increases of about 7 percent in earlier years of Reagan's administration, reflecting increased impatience, even in the GOP-dominated Senate, with Pentagon handling of nearly $1 trillion allocated for defense since 1981.

A budget approved by the Democratic-controlled House anticipates even more constraint: a freeze at current spending levels, with no allowance for inflation.

While giving Reagan most of what he wanted in weaponry, the Senate earlier forced the president to agree to halve the number of MX missiles sought for deployment in existing Minuteman silos, limiting them to 50. Reagan agreed to the reduction in the face of pressure for deeper MX cuts.

Similarly, the Senate agreed to go ahead with Reagan's Strategic Defense Initiative (SDI), trimming the funding request from $3.7 billion to $2.9 billion while rejecting several amendments to slow or reorient the controversial "Star Wars" missile-defense program.

The Senate cracked down in several ways on Pentagon management of weapons procurement and imposed several reforms, including requirements for more competitive bidding and stricter reporting on contracting and tougher rules for management of unspent funds.

Meanwhile, House Armed Services Committee Chairman Les Aspin (D-Wis.) outlined to a closed session of the Democratic caucus yesterday an alternative to the defense authorization approved by his panel. His plan would cap MX missile deployment at 50, reduce spending for the SDI and suspend testing of antisatellite weapons.

It would increase funding for conventional weapons, the Midgetman missile and research and development of some space-based programs. It would also signal a U.S. intention to continue adherence to SALT II.