I have a Line 6 Spider III amp that makes a very loud humming/buzzing noise when I turn it on. It also does not amplify any input. The amp was working fine until I tried connecting an iPad in the aux-in port.

I want to repair the amp myself but have very little knowledge on how to do so. I am looking for some pointers from you kind people on what to look at first.

So far I have checked the output going into the speakers and it's about 20v (probably DC). I am not sure what to look for next. Please help.