A Northern Virginia bingo promoter, accused of keeping most of the more than $1 million his bingo games supposedly raised for three charities, pleaded guilty a second time yesterday to charges of illegal gambling.

Speaking almost inaudibly, Alva Ford (Fordie) Thompson told Fairfax County Circuit Court Judge Thomas J. Middleton he was guilty of violating Virginia's felony gambling law. It was the second time in eight days that Thompson, a baldish, heavyset man, has pleaded guilty to gambling charges.

On Jan. 8 he entered a smiliar plea in Alexandria Circuit Court to two gambling charges that grew out of his bingo operations there. Thompson, 48, faces a maximum sentence of 10 years in prison and a $20,000 fine on each of three gambling counts.

Although Fairfax's lucrative bingo games have been the subject of active investigation by county prosecutors for several years, yesterday's plea and the conviction of a Thompson associate last month represent the first prosecutions there.

Yesterday, Deputy Fairfax Commonwealth Attorney Steven A. Merril told Middleton that had the Thompson case gone to trial, evidence would have shown that the three charities received only $22,000 out of total receipts of $1,117,942 that Thompson's company, United Charities, grossed from the bingo games.

Middleton set Thompson's sentencing for Feb. 23, two weeks after he is to be sentenced in the Alexandria case.

The Fairfax bingo games were conducted at the Bonanza Playhouse at 7520 Richmond Hwy. between April 1977 and May 1978 for three groups, the Mount Vernon Youth Association the Pioneer League Baseball Club and the Lee District Basketball Club.

A Fairfax Country auditor, Ron Cohen, testified yesterday that Thompson kept two sets of books. In one Thompson understated the games' receipts by an estimated $346,000, Cohen said, but in the other Thompson overstated the amounts awarded in prizes.

"At times, sponsors were not even told that extra games were being held, and the exrra receipts were never reported to them," Merril said yesterday.

thompson's lawyer, Book Howard, declined to comment on the case.

Virginia law permits charitable organizations to conduct bingo games if the games are run by volunteer members of the organizaton. Apart from picking up weekly receipts, no volunteers ever worked at Thompson's bingo hall, prosecutors said.

In Richmond yesterday, a bill that would place major new restrictions on bingo games in the state neared final approval in the House of Delegates General Laws Committee despite protests that it will hamper fund raising by charities.

Paul Kelly, manager of the Annandale Boys Club, was one of those who told the committee that limiting to two a week the number of games sponsored by any one organization would sharply curtail charity revenue.

By sponsoring three games a week, as permitted by law now, Kelly said the Northern Virginia boys club has developed the largest soccer club in the country and is providing services to 4,000 youths.