buzzing on clean
buzzing on clean
A cold solder joint is basically one where the solder didn't quite reach the optimum heat. They'll often look brittle, cracked, dull, or beaded on top of a component, but they're not always visible to the naked eye. If you carefully wiggle individual wires within your electronics cavity in turn while your guitar is connected to your amp and you hear crackling (etc) that's a good way to find cold solder joints. Failing that, systematically resoldering the joints in the signal path is usually the simplest way to faultfind one.
That said, it is strange that you only get the buzzing on low notes when you pick hard. That sounds like fret buzz, even though I know you say it's not that. You can't always feel fret buzz in the neck, so if that's why you're saying it can't be fret buzz, I think that's a mistake. I'd have a close look down close at the string as it's struck to make sure it's not fret buzz before I got into resoldering the electronics (unless you're particularly au fait with soldering)
Get a continuity checker. They're invaluable when you're trying to diagnose a wiring issue and are part some multimeters. Radio Shack sells a continuity checker for $30. It's better than losing your sanity trying to figure out what's going on. Resoldering one contact is infinitely less time-consuming than replacing an entire pot and resoldering it (risking more cold joints.) You just attach one lead to the actual wire that you've soldered and attach the other lead to the pot casing or any other contact point that you're suspicious of.
A multimeter might be the most useful tool though since you can use it to check whether a pot is fried, among other things. It all depends on whether you do this type of thing on a regular basis or have another use for the tools besides guitar tech work. Digital multimeters vary in price and the upper-end ones will run you $70-80. Check your local Menards, Lowe's, Home Depot, or Radio Shack.
Probably either a bad switch or cold solder joint... I have noticed that my EMG's can get a little noisy when you get close to a large power source, ie amp, computer, etc.