Accusing the network of refusing to bargain "in good faith" on a new contract, the National Assn. of Broadcast Employees & Technicians, whose 2,800 members at NBC are working without a contract, filed the charge with the National Labor Relations Board. The union, whose members include 700 West Coast NBC employees--200 of them at KNBC-TV in Burbank--primarily consists of technical and engineering staffers. But it also represents 350 local and network news writers. A strike "is a definite possibility," said John Krieger, spokesman for the union.