A former employe of a Metro subcontractor pleaded guilty yesterday to charges stemming from allegations that he found a way to produce $20 Farecards and sold them to federal employes for $10.

The defendant, Patrick Wright Chandler of Suitland, pleaded guilty before U.S. District Judge Gerhard Gesell to one count of wire fraud.

His father, Joseph George Chandler Jr. of Waldorf, pleaded guilty to one count of receiving stolen goods, a misdeameanor.

The father, a retired employe of the federal Office of Personnel Management, helped sell the free Farecards to his former colleagues, according to evidence presented by Assistant U.S. Attorney Judith Hetherton.

According to court papers and other sources, Chandler obtained 340 free Farecards and sold them for $3,400. He was employed at the time by a company that maintains the Farecard machines for Cubic Western Data, which has a contract with Metro for Farecard machine maintenance.

Chandler was employed by the Cubic subcontractor from December 1978 to March 30, 1979, as a repairman for the Farecard vending machines. According to court evidence, he discovered how to manipulate the vending machines so they would produce valid Farecards free.

On the last two days of his employment with the Cubic subcontractor, he produced 300 free Farecards, according to Hetherton. In May 1979, he went to work for Metro itself as a technician working on the subway control system. He was fired by Metro in March of this year, according to Metro officials.

By that time, Metro police, working on a tip, had a full-scale investigation under way into reports that a Farecard sales ring was operating in the Metro building.

Dennie W. Stewart, deputy chief of Metro's police, said that as a result of the investigation, "We think that this case is not only the tip of the iceberg, but the entire iceberg."

The Farecard machines themselves are being modified in an effort to prevent other technicians from doing the same thing, Metro officials said.

According to evidence presented in court, Chandler's father acted as a go-between with employes at the Office of Personnel Management.He would call and see how many cards were needed, then deliver them. He himself realized no financial gain from the operation but turned the money over to his son, authorities said.

The Metro Farecard, about the size of a wallet photo, has a magnetic tape strip on one side that is electronically encoded with the amount of money a rider puts into the vending machine. The cost of each subway ride is deducted automatically by electronic equipment in Metro's exit gates until the full value of the card is exhausted. Farecards can be purchased in amounts up to $20.

The Farecard system is Metro's high-technology answer to the question of how to charge varying amounts of money for subway rides of varying distances. Metro paid Cubic Western Data $53 million for enough Farecard equipment to fill the stations on the first 60 miles of the subway system.

Reliability of the equipment has been improving in recent months, according to Metro General Manager Richard S. Page, but Page is known to be unhappy with the complexity and cost of maintaining the Farecard system.

This is the first known inside job of a Farecard system theft, but a number of computer-wise Metro riders have discovered ways to evade or reduce their subway fares through manipulations with the cards.