Congressional tax writers yesterday introduced a 500-page bill making what they called "technical corrections" to last year's tax-revision law, vowing not to undermine or substantively alter the new law's provisions.

"Whenever legislation as large and comprehensive as last year's Tax Reform Act is passed by Congress, some mistakes must be expected in drafting under such extreme time pressures," said House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) in introducing the package in the House. "Clearly, last year's statute is not drafted with technical accuracy in every respect."

Congressional staff aides said the giant corrections package contains only one substantive change in the new tax code, a set of provisions designed to make a new tax credit for low-income housing work better than had been agreed on by tax writers in both houses.

The plan also includes so-called "transition rules" intended to ease the law's provisions for certain companies and interest groups, but all the rules had already been agreed to last year. They were included in a smaller technical-corrections package that failed to pass Congress in the waning days of the 1986 session.

The bill also wipes out several controversial transition rules that had been part of the 1986 tax act, purportedly by mistake. Among them: A tax break for eight investors in a Colorado coal mine and a special exception for patrons of the sports booster clubs of Louisiana State University and the University of Texas. The exception had made the entire costs of their contributions tax-deductible, a benefit not enjoyed by booster-club members at other universities.

The technical correction that probably will apply to the largest number of individual taxpayers clarifies the law's crackdown on interest deductions on home-equity loans. Under the law, mortgage interest is only deductible on loans on the first two homes if it is less than the original purchase price of the home plus the cost of improvements, unless the loan money is spent on medical costs or education.

The technical-corrections bill makes clear that taxpayers can refinance their homes to get a lower mortgage interest rate, as long as the amount refinanced does not exceed the outstanding amount of their mortgages as of August 1986.