Birtcher Medical Systems: The Irvine manufacturer of electronic medical instruments posted earnings of $351,000 for its second fiscal quarter, contrasted with a net loss of $946,000 for the corresponding quarter last year. Sales for the quarter ended Dec. 31 more than doubled to $7.8 million from $3.1 million. For the first six months, the company reported earnings of $678,000, contrasted with a loss of $668,000 a year earlier. Sales rose to $15 million from $5.3 million. William E. Maya, president and chief executive, said the sales and earnings gains were primarily attributable to the acquisition of Electro Medical Systems Inc. in November, 1989.