I need some amp guys to chime in... I typically build guitars, so when it comes to amps I'm lost. I have a crate gxt100 sitting around I need to sell. The problem with it is if you wiggle the input jack while it's on channel A it pops and cracks through the speaker, but if you change to the B channel with the same input jack it's just fine. If this is a loose solder joint, how do you find loose joints?