The Prince William County Board of Supervisors last week unanimously approved a modified version of County Executive Robert Noe's proposed $137.4 million budget that will mean a 3-cent real estate and personal property tax rate increase and a salary increase for 1,200 county employes.

The owner of a median-priced home of $78,600 will pay $1,140 in taxes next year. Last year, before a general reassessment increased county house values, the owner of a median-priced home valued at $75,200 paid $1,068 in taxes. This represents an increase of $72 or 6.7 percent.

In addition, the board abolished the merchant's capital tax and set rates for the business license tax it approved last month. Casting the lone dissenting vote on the new business tax was Woodbridge Supervisor Donald Kidwell, who opposed the license tax when it was established.

The real estate and personal property tax rate of $1.45 per $100 of assessed value included the second 3-cent increase in two years. County officials estimate that the increase, which is a penny less than Noe recommended, will generate about $1.5 million next year. The board made up that penny by transferring $500,000 -- the amount raised by one penny per $100 -- from a cash reserve fund to the general fund.

In addition, the board adopted the $69.8 million school budget requested by school superintendent Richard Johnson, thus guaranteeing the 10 percent pay rause Johnson recommended for the county's 2,000 teachers. The school budget and the proposed salary increase have received almost unanimous support at hearings held by Johnson and his staff at both ends of the county during the last month.

The business license tax was energetically opposed by merchants and the Prince William-Greater Manassas Chamber of Commerce at hearings last month. It was approved over merchant's objections, the board said, because the old method taxed only businesses that carry inventories, such as automobile dealerships and clothing stores and was unfair. The new tax, which is expected to generate $1.9 million in revenue, will tax the gross receipts of nearly every type of business in the county.

The new tax rates for each $100 of gross receipts include

* Thirty-five cents for professional, financial and real estate services.

* Thirty cents for public utilities.

* Twenty-two cents for business, personnel, repair and other services.

* Twelve cents for retail merchants.

* Five cents for wholesale merchants.

In a move to stop a relatively high employe turnover rate, the new budget will provide a 4 percent cost-of-living increase for county employes and will lift the salary cap on certain staff positions.

In other business, the board voted to send to public hearing a proposal that would allow fireworks to be sold in the county, as they have been in the past. In adopting a new fire code ordinance this year, permission to sell fireworks was inadvertently omitted. According to county attorney John Foote, public hearings will be concluded and a vote taken in time for July 4 fireworks stands to be set up in the county. The amendment to the fire code would require anyone selling fireworks to obtain a permit from the fire marshal's office.

The board also issued a public commendation to budget officer Blaine Aiken, who had agreed to delay taking his new position in Fairfax County until the end of the budget process. Aiken has worked in Prince William for three years.

Neabsco Supervisor John Jenkins, who succeeded James McCoart on the board two years ago, appointed Roger Fitzgerald to fill McCoart's unexpired term on the Northern Virginia College Board. McCoart, who resigned from his board position citing ill health, died suddenly two weeks ago. The college board terms ends June 30.