* Kaypro Corp., suffering through a second year of slumping sales and inventory problems that led to large write-offs, said yesterday that its founder, Andrew F. Kay, will step down as president and be replaced by his son David. David Kay, 40, who holds a bachelor's degree in mathematics, has been vice president for marketing and new product development for the computer maker since 1980.

Under David Kay's guidance, the company has diversified to become a supplier of high-precision electronic instruments. His father, 65, founded the company as Non-Linear Systems Inc. in 1952 and has been its president since 1953.

The company manufactured a line of aerospace instruments until 1981, when it changed its name to Kaypro and quickly carved out a niche for itself in the personal computer field. However, the company fell on hard times starting last year as its sales sagged in the intensely competitive personal computer field. Since then, it has had to take large write-offs because of problems with missing and obsolete inventory.

Earlier this month, the company said it expects to post a loss for its fiscal fourth quarter, which ended Aug. 31. Those figures are due to be released later this month. For the first nine months of its fiscal year, the company lost $6.2 million on sales of $60.2 million. That loss included a $5.4 million inventory write-off in the third quarter.