Tandon introduced an upgraded series of PCs.

The Chatsworth company said its new Targa personal computers use Tandon-designed chips and take up less desktop space than competing equipment. Meanwhile, Tandon’s stock, traded over the counter, rose 75 cents Tuesday to close at $3.50 on the unusually heavy volume of 2.1 million shares. Another 2.1 million shares were traded Monday, when the stock also climbed 75 cents. The company said it could not account for the stock activity but suggested that it stemmed at least partly from the new product.