Public TV stations totaled pledges of $ 38.7 million during their March pledge drive, representing a 15.9% increase over the same period last year, according to preliminary data compiled by PBS.Pledge volume was also up significantly, compared to March ’92, to 563,980, an 11% jump. Figures are expected to rise for the March 6-21 fund-raising period as stations report final results to PBS. Most stations conduct pledge periods in March, August and December. Viewer support remains the largest single source of revenue for public TV, at roughly 21%. Local stations totaled $ 282 million in fiscal ’92 on donations from more than 5 million viewers.