My buddy has a Fender Frontman 212R solid-state amp that he bought used from Guitar Center, and it's been performing fine for almost a year now. He plugged it in a couple days ago and it produces this 120 Hz buzzing, even without an instrument or cable plugged into the amp, and even with all knobs set to 1. My first thought was that it was a grounding issue, but we checked out the inside and everything's grounded properly, and nothing's wrong with the IEC cable. I did a scan over the board to check for any damaged components and made sure everything was plugged in properly. After looking around online, some people are saying that there's either DC on the output of the amp due to a bad output transistor, or it could be caused by a bad filter capacitor. I haven't taken the cover off the front of the enclosure to see whether or not the speakers were moving, but I can feel them vibrate when I touch the backs of the speaker cones, so I don't think there's DC signal getting sent to the speakers. I'm also not the best when it comes to electronics, especially amps, so this one's really got me stumped.
I should also add that the buzzing doesn't change when any of the pots on the amp are turned and regardless of whether or not a cable or instrument is plugged into the amp.
If anyone could provide some insight as to what might be the cause of the hum and/or any possible way to fix the amp, that would be appreciated.
Here's a link to the schematic of the amp:
http://www.amparchives.com/Amp%20Archives/Fender/Schematics/Fender%20Frontman%20212R%20Schematic.pdf