Dynalectron Corp. of McLean reported a second-quarter gain which, with a strong first-quarter advance, boosted the diversified electrical engineering company's first-half earnings 83 percent above those of a year ago.

Second-quarter net income rose to $801,000 (11 cents a share) from $787,000 (11 cents) a year ago as revenues increased to $105.9 million from $86.9 million.

A spokesman said conversion of debentures increased the number of shares outstanding in the latest period, leaving earnings per share unchanged despite the overall gain in profits. The conversion also reduced interest costs, he added.

First-half net income was $1.8 million (25 cents) on $203.2 million in revenues compared with 1979 first-half net income of $989,000 (14 cents) on $162.1 million in revenues.

Dynalectron's backlog of contracts and orders was $319.6 million at the end of the second quarter compared with $325.5 million at the end of the first quarter and $283.1 million at the end of the 1979 second quarter.

Student Loan Marketing Association net income for the second quarter was 227 percent higher than depressed levels a year ago but 50 percent below the first-quarter level, according to the Washington-based corporation, which provides a secondary market for guaranteed student loans.

Program assets were a record $2.1 billion at midyear, 89 percent above those of a year ago and 23 percent above those at the end of the first quarter.

Earnings a year ago were affected adversely by a ceiling on the special allowance paid by the federal government to lenders participating in Sallie Mae's guaranteed student loan program, the organization said.

The decline from the first quarter's results was attributed partially to a very rapid drop in Treasury bill rates in the latest period. Sallie Mae's variable-rate assets and its debt costs are indexed to T-bill rates.

Second-quarter net income was $1.2 million ($7.04 a share) compared with $359,000 ($2.15) a year earlier and $2.4 million ($14.17) in the first quarter. $3

First-half net income was $3.5 million ($21.21), a 58 percent increase over 1979 first-half net income of $2.2 million ($13.40).

HGIC Corp. of Richmond, the parent corporation of Home Guaranty Insurance Corp., reported record earnings and revenues for the second quarter and first half after reducing its expectation for losses and increasing its investment income.

Second-quarter net income rose from $78,981 (8 cents a share) a year ago to $102,252 (11 cents) as revenues rose from $506,820 to $537,968. First-half net income was $123,431 (13 cents) on $1.07 million in revenues, up from 1979 first-half net income of $117,659 (12 cents) on $928,921 in revenues. f