Dynalectron Corp. of McLean last week reported first-quarter earnings of $315,000, an 82 percent drop from last year's $1,718,000, while revenues were up $200,000 to $107.4 million.

The company said a substantial part of the decline in earnings--which dropped from 20 cents a share in last year's first three months to only 4 cents this year--was due to a $1 million pre-tax provision "to cover anticipated, non-recurring costs associated with the forthcoming, previously announced relocation of the company's petrochemical engineering activities from Miami, Fla., to New Jersey."

That provision "magnified the severe adverse impact from the currently depressed petrochemical industry on the engineering workload and process licensing activities of the company," the announcement said. Severe weather problems also hindered the company's contracting work in California and elsewhere during the quarter.

Dynalectron's backlog of unfilled orders at the end of March stood at $354.1 million, up from $309.1 million a year earlier and $327.4 million at the end of December.

American Management Systems Inc., a Rosslyn-based computer service and consulting firm, had a 122 percent earnings increase in the first quarter, to $349,000 (21 cents a share), the company said last week.

Revenues rose 8 percent to $18,668,000, primarily because of a $2.3 million gain in the company's government consulting work. Revenues in the same quarter last year were $17,294,000 and earnings $157,000 (10 cents).

The company said profits rose in its computer services and federal consulting operations but dropped in its custom systems segment. Its losses in its packaged systems segment widened from $618,000 a year ago to $1,020,000 this year.

Columbia Gas System Inc., an energy holding company whose interstate pipelines supply most of the natural gas to the Washington area, last week reported a 28 percent drop in first-quarter earnings.

Earnings fell from $92.3 million in the first quarter last year to $66.6 million this year as the quantity of gas moving through its pipelines dropped sharply, as did production of gas by a producing affiliate. The volume of gas sales by its retail distribution companies was also down.

With a surplus of deliverable gas expected to last at least through 1986, the company said its level of capital spending will fall this year to $325 million, a 44 percent reduction from last year's level and a 10 drop from the figure reported in its recently published annual report.

Farmers and Mechanics National Bank, a $286 million bank based in Frederick, last week reported a 29 percent increase in first-quarter earnings.

The bank said earnings rose from $795,700 ($1.22) last year to $1,024,200 ($1.56) this year. The per-share figures were adjusted for a 5 percent stock dividend paid in May 1982.