With all the holes drilled, and the components located, it’s time to line the cavities with self-adhesive copper foil. This is to create a Farraday cage, inside which the pickups sit – isolated, (or that’s the theory anyway), from all the nasty electrical interference which constantly surrounds us.
There’s usually a low hum at about 60Hz from the electric main, which is annoying. None of us want to experience that Spinal Tap moment where the guitar picks up radio transmissions from the local taxi company. We want our pickups to work properly. The Farraday cage is supposed to conduct any interference around and away from the pickup coils – sending it to the ground side of the pickup circuit.
The copper foil used has a conductive adhesive – so that continuity is achieved, and maintained between all the separate pieces of copper used. Where separate cavities exist – such as between the pickup cavities and the jack plug cavity – continuity is achieved either by relying on a conducting sheet on the back of the pickguard or, as in this case, with the belt and braces approach of soldering a conducting piece of wire, which physically links the two compartments.
In order to run any wires between the two compartments, (and remember – there will be more wires to come – one taking the hot signal, and another linking the ground side of the circuit), a small hole has to be drilled between the compartments. It’s tricky to drill the hole horizontally, given the tight access – so it helps to have a very long drill bit to keep the angle of attack as low as possible, and to protect the face of the body while you drill through.
The cavities of the guitar are cleaned from any dust contamination, and then small pieces of copper foil are overlapped and stuck down. I leave a little extra, running up over the side of the cavities, and onto the face of the guitar. This contacts with similar foil on the back of the pickguard. Obviously – the foil shouldn’t be visible once the pickguard is installed – so it’s necessary to trim it back, although it’ll help if the pickguard screws fix through the foil, so they also become part of the cage. The black Fender pick guard came fully shielded on the reverse with aluminium foil – however I didn’t really have to copy the coverage, as shown on the photo above. The additional foil on the face of the body between the pickup cavities won’t show – it’s over engineering on my half, and a bit of a waste of copper. I only really need to run 5mm or so around all the openings.
Once the cavities are lined, it’s time to link them together, and to think about ground continuity.
That tremolo, and the springs which hold it in place, and the plate which holds the springs – they will all need grounding too, otherwise there’s a risk the guitar might hum every time I ground the bridge with my hand. Normally, a ground wire from the pickup circuit runs through to the spring cavity at the rear of the guitar body, and is soldered directly onto the spring plate. This can be a bit tricky – especially if you ever have to disconnect it and reconnect another – so I intend to provide a central earth point in the rear cavity, from which all the different ground wires can radiate.
The picture above shows a thin, cloth coated wire, (covered by insulating tape), soldered between the two control cavities. Another wire runs back to the tremolo cavity, where it is fixed via a small ring and screw to the side of the cavity – creating a grounding point. A permanent wire can then be soldered onto the screw plate, and linked back to this grounding point. The ground side of the pickup jack is also connected to this path. The bridge and all the cavities are now part of the Farraday cage, and continuity has been achieved between them. Once the pickguard is screwed down, this too becomes part of the cage. Continuity can be checked and demonstrated – using a multi-meter. Once everything checks out, all I need to do when installing the pickup cicuitry, will be to connect the ground wire from the pickups to this continuous ground.