The Metro Transit Authority decided yesterday to abandon its $52 million computerized fare collecting equipment on the 4th of July only and let passengers using the new subway throw 50 cents each into a barrel.

That pragmatic decision was made by the board on the recommendation of Metro general manager Theodore Lutz. Lutz, new to the job since the now-famous transit fiasco after the Bicentennial fireworks last July 4, is anxious that history not repeat itself, especially on July 4.

"We have had this nightmare," said one Metro staffer, who insisted on anonymity, before the meeting. "Hundreds of thousands of people descend on one small Metro station at 10:30 p.m. after the fireworks and all try to use that new computerized fare system. We knew that would never fly."

Metro plans to open 12 more miles and 17 new stations in the regional subway system on Friday, July 1. The system will not run on Saturday and Sunday, but will open again for 12 hours at noon Monday, July 4. Thus, July 4 will be only the second day of operation for the new line, running from National Airport to the RFK Stadium area through the Mall.

Supplementary buses also are planned for people who want to get to the fireworks on the Mall from parking lots at the Pentagon and the Stadium. Regular nonrush hour fares will be charged. There will be no busrail transfer July 4.

Subway riders will be encouraged to board at one of several stations within walking distance of the Mall so that not everybody tries to enter at the same place. The fare will be 50 cents for everybody, and will be deposited in large barrels.

"They will have concrete bases and grids on the top, and there will be security forces there," Lutz promised Metro board members.

"Do you think these barrels will ever replace the automatic fare collecting equipment?" Metro Board Chairman Francis White asked to much laughter.

That equipment starts to work next Monday on the five miles of the subway already operating, but its use will be optional until July 1. Then, except for July 4, the subway rider must insert into a machine and retrieve a magnetically encoded, wallet-sized ticket when he enters and when he exits the sytem.

Last July 4, Metro publicized that special buses would be available to handle the Bicentennial crowds. The crowd was enormous, the buses were blocked in some cases, did not show up in others, and many people waited up to four hours to get home after the fireworks.

Lutz said he had coordinated with police, national park officials and others to make sure buses have clear routes and are headed in the right direction this year.

The board also took two actions yesterday designed to deal with what has come to be known as the "Fairfax City problem."

First, the board voted to appeal a decision by the U.S. District Court in Alexandria that Metro owes Fairfax City $2 million, plus interest, because it breached an agreement to build a Metro subway station close to Fairfax City. That appeal to a suit brought by Fairfax City will not interfere with Metro's plans to build a truncated 60-mile system.

Second, the board approved the presentation of "transit service agreement" contracts for the Northern Virginian jurisdictions. Under those contracts, each jurisdiction purchases transportation services - whether bus or rail - from Metro.

Under the transit service agreement, largely the creature of Arlington County Board Chairman and Metro Board Member Joseph Wholey, Fairfax City can call its contribution to Metro operating costs anything it wants, just so long as it pays.

The contract also will contain provisions for holding the necessary public hearings and other actions leading to the withdrawal of transit services from Fairfax City if it does not pay.

In another development yesterday, the regional task force studying possible alternatives for Metro subway lines adopted a resolution rejecting a "request" by Transportation Secretary Brock Admas that a proposed Metro line between Silver Spring and Glenmont also be studied.

A cover letter seeking further negotiations between regional officials Adams is planned.

And on Wednesday, the Senate Transportation Appropriations subcommittee added $12 million to an appropriations bill to pay for the debt service on bonds due July 1. If the bill is not paid, Metro technically will be in default.