Dynalectron Corp., a major electrical contracting and technological services company based in Northern Virginia, reported yesterday "very large and unexpected losses" last year.

Blaming escalating costs of completing four large wastewater treatment plants being constructed by a subsidiary, Dynalectron Chairman Jorge Carnicero said his firm suffered a net loss of $1.5 million compared with 1977 earnings of $2.7 million (38 cents a share).

Although Carnicero said the "overall financial posture" of Dynalectron continued to be favorable in 1978 despite the loss, directors of ths firm took no action on annual dividend payments yesterday.

The McLean firm has declared cash dividends each of the past five Marches, the most recent being 7 cents a share on common and 70 cents on preferred a year ago.

"No dividend was declared yesterday because of unsatisfactory 1978 financial results," a company press release stated. A spokesman said yesterday's action does not mean necesarily there will be no dividend in 1979. Payouts could be considered at future board meetings, the spokesman added.

Dynalectron's loss in 1978 came on revenues of $290 million compared with $285 million the previous year. In the fourth quarter alone, the McLean company listed a net loss of $500,000 on sales of $75.7 million compared with profits of $1.6 million (24 cents a share) on revenues of $82.9 million in the same 1977 period.

According to Carnicero, the wastewater treatment plants are being built by a subsidiary, AFB Contractors Inc. Write-downs of the losses on these four plants totaled about $10 million before taxes, more than offsetting profits contributed by other activities.

Without the AFB losses, Dynalectron's profits last year would have been higher than in 1977, Carnicero stated. He said the firm's working capital rose 10 percent in the year to $33 million at year's end, while a backlog of contracts rose to $244 million from $219 million at the end of 1977.