My Princeton Reverb is on the fritz. A couple days ago it started humming badly. It was a low freq hum that remained even with the volume off. Just a little while ago, I was showing off a new guitar to a buddy (I'll talk about the guitar in another thread). I was noodling around when all of the sudden the amp started humming very loudly. I abruptly turned the amp off. My friend, a mad-scientist type familiar with electronics said he thought it was a filter cap going. He picked up the guitar and played a low Bb and announced "that was definitely 120 Hz we were hearing". We pulled the chassis and could see no signs of anything having exploded. Of course we couldn't see into the cap can. My friend advised I should undo the leads to the cap can and check the filter caps with a capacitance tester (which he has). Or I could undo the leads, power up the amp and read the voltage from the caps. First question: Are the symptoms consistant with blown filter caps? Second: any tips for dignosing this problem?