Lockheed Aircraft Corp.'s internal investigation of improper payment is nearly complete and has so far uncovered no evidence of domestic violations, chairman Robert W. Haack said today.

Haack said he had no idea what the committee of seven outside directors which have been looking into payoff allegations had come up with in the international area where the company has admitted questionable payments approaching $25 million for the 1971-75 period, claiming he was keeping himself separate from the probe to keep it independent.

But, addressing San Francisco's Commonwealth Club in his first speech since he took over as chairman of the beleaguered aerospace company nearly a year ago, Haack said it was his "understanding that the committee has uncovered no evidence of any domestic commercial improprietites, nor has there been any evidence of any improper domestic payments or any illegal campaign contributions," he did not say how he learned this information.

Haack said Lockheed has "not paid a dime" in foreign payoffs since a strict new policy went into effect in the fall of 1975.

He voiced support for a proposal by Treasury Secretary Michael Blumenthal that American industry establish a national board or council to prepare a business code of ethics.

But he said the question of overseas payments was a problem for which there was no "simple 'black or white' solution," and that American firms could suffer from their international competitors by unilaterally adopting a no-payoff policy.

He said Lockheed had lost some business recently when a government official abroad demanded a Mercedes-Benz as the price for approving a multi-million dollar purchase, and Lockheed turned him down.

But Haack also conceded that Lockheed has just wound up the next year of foreign aerospace sales in its history, despite the strict controls on payments and use of agents abroad. He said "the jury is still out" on how his company will ultimately be affected.

The investigation by outside directors is required as part of a consent agreement reached by Lockheed and the Securities and Exchange Commission last year. The SEC charged Lockheed with improper accounting and failing to disclose the extent of its payment activities to shareholders.

The committee, which is working with outside counsel and an accounting firm, has asked for and received several time extensions from the SEC for completion of the probe, Haack indicated today that it was now in its "final phases" and would probably be finished in early April.

After completion, Lockheed has 30 days to go to court and try to get confidential treatment for parts of the report which it might claim are damaging. Then it must mail a summary out to shareholders as part of a proxy statement in preparation for the company's first regular shareholder meeting since 1974.

Haack said he now expected that meeting to be held in July or August, and he hoped that by that time Lockheed would convince its 24 banks to waive the government guarantee on the remaining loans it holds under the controversial guarantee program.