The General Assembly decided yesterday to soften the state's tough law on "fuzz busters," devices used by motorists to detect police radar.

It would still be illegal for motorists to use the devices, but under a bill that passed the House of Delegates, 72 to 9, and was sent to Gov. John N. Dalton, drivers who have a detector in their car but don't have it hooked up could not be arrested.

The bill, which passed the Senate earlier, also says that even drivers who are arrested and convicted could get their detectors back if they request it and pay the mailing expenses.

Virginia's current tough law on such devices -- only Mississippi has such a stringent statute -- has brought groans from some out-of-state drivers who found themselves under arrest once they crossed into the commonwealth. Furthermore they had to surrender their "fuzz buster." to. so to speak. the fuzz. Radar detectors cost from $100 to $150.

Virginia state police in enforcing the present law. have maintained that the only purpose of the detectors is to enable drivers to speed above the posted limit. "I don't see that the state has any obligation to a person bent on breaking the law," state police Maj. C. M. Boldin was quoted as saying in a story on the issue last August.

Signs have been posted at the border entrance to major roads in Virginia warning motorists that radar detectors are illegal and subject to seizure.

Defenders of the detectors say the devices actually promote safe-driving. "When you're driving down the road on long trips you're not always watching the speedometer," one person said in the August news story. "When your speed creeps up, the device goes off if it's in a radar field and you're automatically going to slow down. . ."

The bill that passed yesterday was sponsored by two legislators from Loudoun County -- Sen. Charles L. Waddell (D) and Del. Earl E. Bell (D), who runs an automobile dealership in the county.

The fine for drivers convicted remains the same -- $25 to $100. But the Waddell-Bell bill eliminates the provision for imprisonment up to 10 days.