Help me not do something stupid. I have an old Fender PA100 that I just love for practicing at home, taking to the occasional jam (and letting my kids sing along with YouTube). These days, it starts to hum after it has been on for 20 minutes or so. On the back is a "Hum Balance". It is down in what looks like a 1/4" jack, but it's a slot head like a screw. What is that? I don't want to just start turning it without knowing what it does. Please let me know if I haven't given you enough info. Thank you!