How to kill a push-pull pot
In short, solder your grounds to it with a pitiful cheap iron, where you heat the casing too long and burn the little sucker to death.
Here's my story, I recently wired my M-II so my two humbuckers could be tapped via the push-pull volume pot. Worked great. Unfortunately, I was so focused on wiring up the right coils I totally forgot to rough up the surface of the pot. So a couple weeks later the solder blob let go during some David Lee Roth acrobatics and my ground wires popped off the pot with the solder. Well, I fixed it, but unfortunately my cheap soldering iron didn't deliver enough juice to get in and out quickly, so I think I killed the pot. It must be extra sensitive; I've never done that before.
So, since then, I pitched the crap iron, and bought a sweet Weller setup, some good 60/40 solder, and I have a new pot on the way.
HERE is my question:
Any tricks or recommendations on soldering ground wire(s) to the outer casing of the push-pull pot? It is a Dimarzio 500K, if that helps. I'd rather not kill another one if I can help it.
In light of the overwhelming response on this topic, I thought it would be a good idea to follow up with my own findings, in case it benefits another solderer of Electric Sound Products.
It turns out a push-pull is chock full of sensitive and burnable little goodies. Even more so than a tranditional pot. Additionally, the metal casing is not nearly the thickness (or heat capacity) of your usual pot. So add it all up, and if you get too liberal with your iron use, or worse, have a poorly soldering iron, it can be easy to kill that fancy new pot.
SO: I combined a couple tricks to compensate. For one, if you look at the bottom of the pot where you would normally solder your ground/earth connections, there is a thin metal overhang that protrudes past the edge. This is far enough away from the business inside the pot so you shouldn't hurt anything if you work efficiently with the iron. I just roughed this up a bit with some 100 or 200 grit and it made a good little surface for soldering.
The other thing is that my particular wiring configuration ended up with about 5 wires that needed to be grounded, and this was the only pot in the guitar, so it would be tempting to try to ground them all onto the bottom of the pot. Rather than do this, I ran a single lead from the bottom of the pot, and I gathered all 5 or 6 wires into a harness, twisted them tight, coated the bundle with solder, and then sealed it up with electrical tape. This saved me from having to solder a pile of ground wires to an already sensitive pot.
In closing, the guitar is perfect, better than ever, sounds killer, all the connections are solid, and no grounding or buzzing or humming whatsoever. Well, until I tap a humbucker and get the fun 60 cycle hum we all know and love.
If you have good soldering gear, and know what you are doing it should not be a problem. I usually take a flat edge screw driver and score the surface area where I need to solder. This will break through the plating (if there is any) and make it easier and faster for the solder to bond to the metal.
As stated previously you do not want to keep the heat on the back of the pot very long. I have used a 60 watt iron after using fine sandpaper to clean the area to solder to.
I have an 80 watt I have used which allows me to get on and off pretty darn quick (quick is a must here with this iron) as it heats the area to solder very quickly so you can get on and off quickly. But DO NOT use more than 25 or 30 watts on the wire lugs of a pot.