Dynalectron Corp. of McLean reported yesterday that profits were down one-third in the 1983 second quarter compared with 1982, despite an increase in revenue of 17.5 percent. Still, second-quarter earnings made up nearly all of the company's profits so far this year, since mid-year earnings were only half of the near-record levels shown in 1982.

Net earnings for the quarter at Systematics General Corp. of Sterling, Va., almost quadrupled over last year, but fell sharply at Electronic Modules Corp. of Timonium, Md., and pulled the company into the red.

Dynalectron reported profits of $2.2 million (23 cents a share) in the 1983 second quarter, down from $3.3 million (34 cents) for the same period last year. For the half, profits were $2.5 million (28 cents), down from $5 million (54 cents) in 1982.

Revenues rose to $131 million in the second quarter from $111 million in 1982, and were $238 million in the first half, up 9 percent from $218 million in 1982.

Systematics General, which makes electronically secure computer products, had second-quarter profits of $307,000 (20 cents), up from $66,000 (5 cents) a year ago. Quarterly revenues increased to $4.5 million from $3.3 million.

For the first half, profits increased to $584,000 (38 cents) from $106,000 (7 cents) in 1982. Sales went up to $8.1 million from $7 million.

Electronic Modules, a manufacturer of microprocessor-based products, reported a loss of $351,455 (8 cents) in its 1983 third quarter, compared with net earnings of $849,645 (20 cents) in the same period a year ago. Sales slipped to $10.5 million from $13 million.

Over the nine-month period, the company showed a 1983 loss of $144,350 (3 cents), compared with 1982 earnings of $2.3 million (55 cents), despite sales increases in 1983 to $35.5 million from $35.0 million a year earlier.