(FORTUNE Magazine) – Even though belt tightening is the order of the day, big corporations are increasing their support of the Public Broadcasting System. In 1990 they pledged $56.6 million in production money to major PBS stations, up 7% from 1989. (See table for the FORTUNE 500 companies that gave $1 million or more, along with some of the shows they sponsor.) Why the increase? Noncommercial PBS has been selling itself harder as an alternative to the ad clutter on network and other TV stations. Says Steven Bass, director of PBS's corporate support department: ''PBS is a different way for companies to get into the mind of the consumer.'' Sponsors are allowed a 15-second promotional spot at either end of any program. Mobil, the largest and oldest (20 years) underwriter, sees an extra benefit. Peter A. Spina, general manager of corporate relations, says the ''halo effect'' of being affiliated with programs like the long-running Masterpiece Theatre helps the oil company's recruiting efforts. - S.S.

CHART: NOT AVAILABLE CREDIT: NO CREDIT CAPTION: BIG SPONSORS' PUBLIC TV SHOWS