SC's 1 ,3 ,5 Sw Position hum reducing?

[petemoore]

I have a Washburn with strat style [5 way sw with 3 SC's].
 The 2 and 4 positions of the switch when used are quite quiet.
 the 1, 3, and 5 positions are equally noisy, [with boost not too bad] and with Fuzz it renders them functionally unusable...
  Using a screwdriver to touch the magnets, I've determined 1 3 and 5 Sw postitions are lone single coil wirings, hence I conclude this is the essence of the noise problem.
  Would shielding the pickup cavities [each has a separate body cutout] do the trick you think?...I've done this before [lotta messin'] and gotten nominal results..on a Les Paul..not too sure I was adressing the actual problem on that one though...
 Any suggestions are welcome !!!
 Boiled down question: should I try just the pickup cavities...or should I shield around all the wires in the guitar?

[Mark Hammer]

Positions 2 and 4 involve a pair of pickups of approximately equal coil turns.  In recent decades, the middle pickup of a trio set has generally been produced to be wound opposite and have the opposite magnetic polarity of the other two.  What this means is that when you combine the middle pickup with one of the other two, you actually have a hum-bucker configuration, even though visually it involves two separate pickups.

Decent shielding of wires is usually a good thing, but shielding the cavities will not stop the individual coils from picking up hum from EMI sources around them.  The only reason you don't hear it in positions 2 and 4 is because the hum that IS picked up is cancelled out by the hum-bucking effect of the two pickups.  Where you have a single coil being selected, it will be hard to stop that coil from doing what it does so well: namely acting as an antenna for the perpetual and omnipresent "broadcast" of 60hz hum (and its "sister station" at 120hz).

One solution that some apply is to use a "dummy coil".  This is an antiphase coil of equal coil properties but lacking any magnets.  So, the hum picked up by each of the coils cancels out when combined but only the one coil senses the strings.  Some of the "noiseless single-coils" out there actually use something like that, by stacking one coil on top of another and having only the upper coil employ magnetic polepieces to sense the strings.

Another strategy is to restrict the sensing zone so that the string-to-other-crap ratio is optimized.  This is a bit like what the old Fender Jaguar and modern day Lace pickups do.  Here, the magnetic field is extended up the sides of the pickup.  Where the sensing zone of the coil would normally be like an omnidirectional mic, picking up anything in that hemifield extending from the tops to the bottoms of the polepieces, moving the bottom "end" of the polepieces up the sides of the pickup focusses the sensing zone between the tops of the polepieces and the raised metal edge just adjacent to it.  Such pickups are not hum-cancelling in any sense, but they can have drastically reduced hum susceptibility when well-designed, without changing the single-coil properties much.