A Connecticut superior court judge has ruled that one of the heirs to a publishing fortune was not quilty of willful misconduct when he bought an ailing newspaper over the objections of other heirs.

The judge ruled for Lionel S. Jackson, president and publisher of the New Haven Register Publishing Co.

In 1973, the Register company paid $7 million to the Gannett Co., Inc., for the Hartford Times, and afternoon daily in the state capital. On Oct. 20, 1976, the 159-year-old Times ceased publication after nearly a decade of ailing circulation.

The suit against Jackson, filed in 1973 shortly after he bought The Times, was brought by two of his three sisters, a brother and the heirs of another brother.

All are heirs to the John Day Jackson Trust, established in 1956 by the founder of the Register company who died in 1951 at the age of 83, leaving more than $61 million to his heirs.

The trust owns the stock of the Register company, which publishes New Haven's morning and afternoon papers.

The suit charged that Lionel Jackson and other had acted him prudently in buying The Times without making a careful inspection of its financial condition.

In a separate suit filed in federal court, the Register Publishing Co. is seeking $15 million in damages from Gannett for allegedly inflating circulatn in damages from Gannett for allegedly inflating circulation figures of The Times. Gannett has countered with a suit claiming the Register company still owes it $1.3 million of the agreed purchase price.

In deciding for Lionel Jackson and the other trustees, in the family suit, Judge Thomas J. O'Sullivan stated: "It could certainly be argued that these trustees should have examined the circulation figures The Times and its revenues and expense more carefully . . . "

But he concluded that to find them guilty of willful misconduct "requires a greater proof."

The judge refused to remove the trustees, as requested by the plaintiffs, because he concluded that the trust fund was not in jeopardy. As proof, Judge O'Sullival cited testimony in the trial that the publishing chain controlled by Samuel I. Newhouse had offered $32 million in 1973 fo the the Register company stocks.

An attorney for the plaintiffs said earlier this week that Judge O'Sullivan's decision will be appealed.