Tiny KCSN-FM at California State Northridge may not be big enough for an Arbitron rating, but it sure has some huge fans helping it raise money. Tom Petty and the Heartbreakers are playing what the Los Angeles Times calls “a rare small-theater show” as a benefit for the station. U2 has donated a guitar signed by all four band members that could go for upwards of $150,000 during the fall pledge drive. Also contributing: Mick Jagger, Coldplay, Sheryl Crow and others. Why? “It is because of stations like KCSN that music will always come first,” Coldplay said through a spokesman. The station moved its former all-classical programming to an HD channel on March 1 and switched to an automated Triple-A format. But since July 15 it’s been playing what it calls “smart rock,” determined by music-passionate local DJs. And recent hires include Triple-A veteran Nic Harcourt, former music director and host of the influential Morning Becomes Eclectic program at KCRW and an early supporter of Coldplay; and Sky Daniels, a DJ and programmer at influential rock stations in L.A., San Francisco, Chicago and Detroit — as well as a former promotion exec at U2’s record company.