I inadvertently shorted out something in my Pathfinder by plugging in a speaker extension cable where the separate speaker leads on the other end touched. Now the amp just hums and won't produce any guitar audio on output. It does the same when I plug headphones in. Also, when I turn the power switch off, the indicator light stays on a few seconds before dimming and shutting off. Any idea what might have got shorted out? I'm not sure if I can fix it myself, and I'm hoping it doesn't cost a lot to repair (especially compared to the low price of the amp).