During the two weeks that threatened to be its last, the financially failing Philadelphia Bulletin exploded a bombshell -- a series of articles posing startling new questions about the sensational Yablonski murder case.

The Bulletin charged that former United Mine Workers president W.A. (Tony) Boyle had been railroaded into prison for the murders of Joseph A. (Jock) Yablonski, the reformer who challenged him for the union presidency in 1969, and Yablonski's wife and daughter.

Claiming it had turned up new information on the case, the paper demanded that the imprisoned Boyle, 79, now appealing his 1978 conviction, be granted a new trial. The series also made charges of serious misconduct by Richard A. Sprague, the hard-nosed special prosecutor who won the conviction against Boyle.

The Bulletin's series has created an uproar in Pennsylvania, but it also is taking on the air of a backfire, raising questions about the paper's rush to print the articles before all the loose ends had been nailed down.

The Bulletin may have been going down the tubes, but it was attempting to go down in the great tradition-- battling for justice to the end, keeping the authorities straight, righting wrongs, keeping its dwindling readership informed.

"The investigation concludes that regardless of his guilt or innocence, Boyle . . . was unfairly convicted of originating the murder plot because he was prevented from presenting certain information in his defense that might have created a reasonable doubt in the jury's mind," the first article said.

A principal doubt, according to the Bulletin, might have come from further disclosure about a "conspiracy" to kill Yablonski in 1968, a year before Boyle had been implicated in the contract murder. But confessions and testimony in various of the conspiracy trials already had largely discredited this theory.

The series went on from there, through thousands of words over five days, seemingly one last powerful blast by a troubled newspaper suffering so severely from declining advertising and circulation that its publishers planned to shut it down Aug. 16 unless employes made contract concessions. The employes did make the concessions, saving the paper from immediate extinction.

On the surface, this was potent stuff. Bulletin editors salivated over its prospects as a Pulitzer Prize contender and a possible rejuvenator of the paper's sagging fortunes.

But the series, front-paged throughout and backed up with trenchant editorials, has drawn a bitter attack from Sprague and a public denunciation from Yablonski's sons, who called for a page-one retraction by the Bulletin. Sprague went to court last week to halt efforts of officials in Washington County, where the Yablonskis were slain, to have him deposed as prosecutor.

Executive editor Craig Ammerman says the newspaper stands behind the substance of its stories, but he declined Thursday to respond to charges by the Yablonski sons --Joseph (Chip) and his brother Kenneth -- that the Bulletin had "maliciously" and "recklessly" distorted the record "solely to sell newspapers." Ammerman said a formal reply would be made within a week.

"If that was what we were trying to do with a 12-year-old murder case, that wasn't the way to do it," Ammerman said. "Our circulation was not up during that period when the series ran ."

There were other questions about the Bulletin's investigative efforts: whether its information was really new or relevant; why the series was published without a formal response from Sprague; whether the paper misled readers by placing key items that contradicted some of the series' main points in obscure lower paragraphs of the long articles.

For example:

* The apparent launching pad for the series was a personal note written to Boyle last March by Paul Gilly, one of the convicted gunmen, who apologized for "lying" about Boyle's involvement in the conspiracy. Gilly testified at the first Boyle trial in 1974 (Boyle's conviction in that trial was overturned on appeal) but not at the second, a fact that the Bulletin did not mention until the next-to-last paragraph of its main piece on Gilly. The stories left the impression that Gilly's testimony played a part in Boyle's second conviction.

* Although the paper accused Sprague of gross misconduct, it did not confront him before publication. Sprague was vacationing in Europe, not returning telephone calls from the Bulletin, and not scheduled to return home until Aug. 17, the day after the paper faced closure. The Bulletin's editors could go with their series as planned on Aug. 2 without comment from Sprague, or they could wait until he returned, and possibly have no paper in which to print the series. They went ahead without Sprague.

* The Bulletin charged that Gilly was coerced by Sprague into implicating Boyle in the murder scheme and that Sprague's conduct tainted the verdict against Boyle. However, public records show that after his brothers appealed to him to tell the truth, Gilly confessed before FBI agents, state police and his own attorney. Sprague was not present during the confession.

Notwithstanding this information, the Bulletin quoted Gilly as saying that a "cussing and screaming" Sprague had pressured him to confess. Ammerman conceded last week that this was an editing error and that the Bulletin intends to publish a correction.

* The paper reported that a conspiracy to kill Yablonski existed in 1968, a year before federal investigators implicated Boyle in the plot to eliminate his rival. The 1968 conspiracy had been reported publicly before and, in effect, was discredited by the jury at Boyle's retrial in 1978.

The Bulletin alleged that the story of the earlier conspiracy never was presented adequately to a jury and, if it had been, it might have altered the outcome of the Boyle trial.

In fact, the Bulletin's source, a Cleveland burglar named Robert G. Tanner, did testify in the second Boyle trial, saying he had been contacted in 1968 about killing Yablonski and an official from a rival union in Tennessee. In 1973, however, in a statement to FBI agents and in a Cleveland newspaper interview, Tanner made no mention of Yablonski as a target in the 1968 plot.

* A convicted forger, submitting to a lie-detector test financed by the Bulletin, said that Albert Pass, a high UMW official from Kentucky also convicted in the conspiracy, had told him in prison that Boyle had nothing to do with the Yablonski murders. This raised further questions about the Boyle "railroading," the Bulletin said.

But Pass was saying the same thing to law enforcement officials at the time -- that neither he nor Boyle had a role in the murders or the conspiracy. He later confessed and implicated Boyle, and he remains imprisoned in Pennsylvania.

Some of these points were outlined last week by the Yablonski brothers and by Sprague in a petition filed in Washington County, where he went to court to attempt to retain control of the case. County officials, apparently spurred by the Bulletin's series, "fired" Sprague and said they were contemplating a suit against Sprague to recover $800,000 in court and prosecution costs since the 1969 murders. The officials had long been at odds with Sprague, mainly because of the expense of the prosecution.

When Sprague went to court to keep his job, a Washington County judge temporarily ruled in his favor but set another hearing for Sept. 15 on a petition to halt the county from taking over Sprague's files. The judge also agreed with a Sprague request to direct the state attorney general to investigate the Bulletin's allegations of misconduct by the prosecution.

Sprague, meanwhile, is considering a libel suit against the Bulletin. He said last week that he had planned to publicly answer the charges in the series upon his return from Europe, but that was postponed when Washington County officials went on the attack against him.

"When the county officials jumped wildly into this, I felt I had an obligation to step in and protect the case, which is still on appeal," Sprague said.

The prosecutor, calling the series "an outrageous disgrace to the profession of journalism," said that had the Bulletin talked to him before printing its stories, he would have made available to its reporters the same documents that he has in the past made available to other journalists.

On that one, Sprague knows he has the Bulletin over a barrel. Ammerman conceded he was in "an impossible situation" because of the paper's pending death. It was publish or perish, and the Bulletin published.