The televised Iran-contra hearings brought about a partial restoration of President Reagan's personal credibility but did little of lasting impact to bolster support for aid to the contra forces opposing the Sandinista government in Nicaragua, a Washington Post-ABC News poll indicates.

The poll also indicates that the hearings went on too long for the public taste and left too many questions unanswered.

The survey shows that fewer Americans think that Reagan lied about the arms sales to Iran and diversion of funds and that more blame his advisers and consider his mistakes minor than was the case before the hearings.

But his overall approval rating is at the 50 percent level, where it has hovered since the issue became public last November.

The three-day survey of 1,205 adults, completed Wednesday, showed little enthusiasm for the more than 11 weeks of joint public hearings by select House and Senate committees.

A 58-to-39 percent majority agreed with the statement that Congress spent too much time looking into the Iran-contra affair, and a 58-to-38 percent majority said many important facts were not uncovered. Only about one-third of those sampled took both views.

Only 10 percent said they felt that they understood what happened "very well," compared with 35 percent who said "not too well" or "not well at all" and 55 percent who said "fairly well."

The star witness, to no one's surprise, was Marine Lt. Col. Oliver L. North, while his former boss on the National Security Council, Rear Adm. John M. Poindexter, was the only one of six featured witnesses given negative scores on credibility and favorability.

Of the four current and former senior administration officials who testified, Secretary of State George P. Shultz received the top marks, followed by Defense Secretary Caspar W. Weinberger. Former White House chief of staff Donald T. Regan and Attorney General Edwin Meese III drew mixed reviews.

Reagan's overall approval score, 50-to-47 percent positive, is identical to that of last January -- well below the mid-60s scores typical in 1985 and 1986. Disapproval of his conduct of foreign affairs has ebbed slowly from 64 to 53 percent but not swung to the positive side.

But on a series of specific questions relating to the hearings, his rating has improved.

The number saying Reagan has not told the truth about the Iran situation has dropped to 52 percent from a peak of 69 percent in early June. The share saying he participated in an organized cover-up has dropped from 51 to 43 percent in the same span.

The portion saying he made major mistakes has dropped to 39 percent from a March peak of 52 percent. And 68 percent now say his advisers were more to blame than Reagan himself, an 11-point increase since March.

And, while 60 percent still think that Reagan knew about the diversion of money for the contras earlier than he has acknowledged, that suspicion was held by 73 percent in early June.

However, 60 percent of those surveyed said they think that Reagan will continue to have serious problems because of the affair -- almost identical to the March reading -- and 51 percent said they think that information now public hurts his ability to lead the country during the rest of his term.

The Reagan administration was cheered by spot polls indicating that North's dramatic testimony in early July had boosted public support for contra aid. But that may have been a short-lived phenomenon.

The latest reading is 59 to 36 percent against military aid. A poll July 21 had it 49 to 41 percent against; immediately after North's testimony, it was 46 to 43 percent. During much of 1986 and the beginning of this year, fewer than 30 percent supported such aid.

Sentiment on prosecution or pardon for North and Poindexter continues to shift. Those surveyed oppose prosecution of North by 68 to 28 percent and approve a presidential pardon to prevent such prosecution by 51 to 46 percent. On both questions, pro-North sentiment has slipped since July by 7 to 11 points.

Sentiment on prosecution of Poindexter is 50-to-42 percent negative, but a 58-to-35 percent majority opposes an immediate pardon to prevent prosecution. In the last month, sentiment has shifted against the admiral by about 11 points on both questions.Polling director Richard Morin and polling analyst Kenneth E. John contributed to this report.