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Guitar Workshop

Screening a Stratocaster

Nothing sounds quite like a Fender 'Strat', but unfortunately along with their pleasing, distinctive tones lurks the less desirable presence of background hum. In the early fifties when these guitars were revolutionary in their concept, I'm sure the unwanted hum was quite acceptable; bearing in mind that the amps of that era generated plenty of their own extraneous noise, thereby disguising the guitar's Achilles heel. Today the high output sophisticated PA rigs, coupled with multiple effects and lighting consoles leave no place to hide. But don't panic, there is a way to greatly improve this situation without the trauma of changing the guitar's character.

Those inquisitive enough to have peered beneath the scratchplate will probably have noticed that all the internal wiring is unscreened, as are the routed recesses; with the exception of an alloy shim mounted behind and echoing the shape of the scratchplate on earlier Strats, and a foil groundplate mounted on the control area of the scratchplate on most other models. In combination with the single coil pickup, there lies the problem.

Figure 1. Screening a Stratocaster pickup: a) brass shim wound round the coil; b) earth connection; c) tape over the coil wires to prevent damage by the shim; d) screening braid held in place by heat-shrink sleeving (e).

The first rule when you are going to carry out any work on your instrument is to find a well cushioned and uncluttered working surface; also make sure that the lighting is adequate and that you don't cast a shadow over the work area. Before starting work, have a mental run through of the procedures and tools that will be necessary.

Once the guitar has been de-strung, the first task is to remove the scratchplate; this is achieved by undoing the small Phillips screws on its perimeter (using the correct screwdriver of course). The pickups and electronics are mounted on the scratchplate, and are therefore withdrawn at the same time; although three wires, two to the jack and the other to the tremelo spring anchor will prevent complete separation until they have been unsoldered from the chassis (two black) and from the centre tag (white) of the volume potentiometer. A 65W soldering iron should be used for the chassis connections, with a 25W instrument for the centre tag. Remember that hot solder can easily damage the guitar's finish, so cover any surface likely to be affected before picking up your iron.

Once the scratchplate has been safely removed, unscrew the two screws securing the jack plate and withdraw it complete with wires. By this time you should be gazing down on a near naked body and neck. Although the most effective way to screen it would be a laminated copper and brass lining it shouldn't take more than a second glance to realise that this option is totally impracticable. Even the use of tin foil rarely achieves an acceptable result, so thank goodness for modern technology in the form of carbon conductive paint which can be used to coat all the routed cavities on the top of the body.

Whilst the carbon is drying (usually at least two hours) the task of removing and screening the pickups can be undertaken.

Unsolder the pickup lead from the volume pot chassis and switch, taking care to mark or colour code the connections for reference when reconnecting. Separate the pickups from the scratchplate by unscrewing the retaining screws; watch out for the springs, which are easily lost, although older Strats used rubber tube. Once the pickups are free remove the covers (take care, the coils are delicate) and using a ⅜" masking tape, wrap the coil twice. The next stage is to wrap the coil with brass shim which should be cut to a ⅜" width with a ½" overlap on one circuit of the coil; tin the ends before wrapping so that minimal heat will be required to solder the shim once in place. Remember, too much heat will damage the pickup, so have a dummy run first to make sure that everything is to hand and that you are familiar with the procedures required.

Finally, solder a thin strand of wire from the shim to the pickup eyelet housing the black lead and replace the covers; (depending on the circumference of the pickup it will probably now be a tight fit, so use your brain not brawn) place the pickup in a safe place until required for re-assembly. NB: care should be taken when handling the brass shim, as the edges are as sharp as razor blades.

For those Strats with no ground plate on the scratchplate it will be necessary to make one. The pots and switch will need to be removed, then using the brass shim cut out an area large enough to cover all the circuit area. Using a contact adhesive (Thixofix etc) lightly coat both the shim and scratchplate (the surface liquid from the top of the tin is best as it spreads thinly) and when touch dry bring both surfaces together; use a rag, not your fingers when rubbing down the edges as the shim could quite easily slice their ends off. Insert a sharp blade from the underside and rotate it in the pot and switch holes to cut away the unwanted brass, before replacing the circuit.

Figure 2. Wiring of the controls. The short wires to the tone controls need not be screened, since they are inside the control cavity.

There are two methods of screening the internal wiring; one is to replace all the wires with screened cable, the other is to sheath the existing wire in braid. The latter is my preference; to achieve this strip the braid from some screened cable, taking care to keep the centre cavity open (if the braid is compressed it is more manageable). One by one unsolder the pot end of the circuit wires and slip over the appropriate length of braid, stopping a short distance from the soldered end, but allowing enough overlap to form an earth tag at the pot end (the braid need only be connected one end to provide a screen). In order to secure the free end of braid and prevent it from shorting to the 'hot' terminal, slide a short length of heat-shrink sleeving over the join and apply heat (gas lighters are fine, but matches blacken the surface). Resolder the 'hot' wire and solder the braid tag to ground before proceeding with the next wire. Once all the wiring, including the pickup leads and jack socket leads have been screened accordingly, the pickups can be refitted to the scratchplate and re-wired in, following the colour codes already marked.

One last step before reassembling the scratchplate to the body is to ground the carbon painted surfaces; the easiest way is to screw a solder tag into the side of the switch cavity (making sure that there will be a clearance between it and the switch) and connect it to the circuit's ground using an 8" length of insulated cable.

Providing that all the connections have been made correctly, and that any exposed screening braid in the close proximity of 'hot' wires has been sheathed by heat-shrink, once re-assembled your Strat should be a sight for sore ears.

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Featuring Ike Isaacs

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Electronics & Music Maker - Copyright: Music Maker Publications (UK), Future Publishing.


Electronics & Music Maker - Feb 1982

Scanned by: Stewart Lawler

Feature by Peter Cook

Previous article in this issue:

> Featuring Ike Isaacs

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> Making Notes

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