Published On: Wed, Feb 5th, 2014

Guitar Workshop – Electric Learning Land

Your guitar doesn’t work. But what’s wrong – the jack, the pots, the switch, the pickups, or something else? Before you pack your guitar away to visit the professionals, read our basic guide to troubleshooting electric guitar wiring

The most important aspect of successful electrical repair work is an efficient diagnostic approach that enables you to find out exactly why a circuit is malfunctioning. Diagnosing electrical problems is a tricky process combining a little hardcore knowledge with intuitive judgement and a disciplined process of elimination; of course, accumulating knowledge sharpens up your intuitive judgement and reduces the time spent eliminating possible problem areas.

Before attempting guitar electro-surgery, however, it’s necessary to understand a few basic principles. The most important principle is that electricity – or, more to the point, electrons – flow in circles; if a circle is broken, the electrons can’t flow and the circuit won’t work.

The wiring system within a guitar is made up of many circles, most of which are straightforward and easy enough to spot and check. Troubleshooting electrical problems always begins with ‘check the circles’ testing because just one broken circle somewhere in the circuit can silence the entire system; find it, repair it and the job’s pretty much done.

Spot the Circle
Let’s consider a single-coil Strat-type pickup connected to a volume control, a tone control and then an output socket (see Fig 1). The pickup circle starts at the top of the pickup coil at the ‘live’ or ‘hot’ lead or wire, which then connects to the top tag of the volume pot; this lead carries the all-important output from the pickup.

The bottom tag of the volume control is then connected to the bottom connection of the pickup coil to complete the circle; this lead is called the earth or, to use its full name, the earth return. In action, the electrons flow out from the pickup’s live lead, down through the volume pot’s resistive track and back to the pickup again via the earth return. In the diagram you can see that the bottom of both the pickup coil and the volume pot are also connected to the body or casing of the volume pot, which becomes the main earth connection for the circuit.

This is an example of what we call a primary circle. It’s primary because the resistive track in the volume pot is wired across the pickup at all times. Fig 1 also shows another primary circle, the one used around the tone control; again, notice that the circle is never broken.

Another type of circle, a secondary circle, is used to pick the output off the middle or ‘wiper’ tag of the volume control and send both it and the earth connection via the guitar lead to the amplifier, where the circle is completed. Secondary circles are interesting because they are not true circles until something makes them so – the position of a switch, for example, or ultimately the guitar amplifier itself.

Screen shot 2014-02-05 at 11.59.51

Primary and secondary circles may be different, yet both rely on a good earth return in order to work. It’s because of this that they are prone to breakdown, and for the same reason – poor conductivity.

In primary circles, high resistance solder connections (aka ‘dry joints’) introduce this poor conductivity into a circuit, and nine times out 10 they can be cured by a simple jab from a hot soldering iron. Resoldering a dodgy dry solder joint restores its electrical resistance to the electron flow to nearly zero ohms and, more importantly, fixes it at that point; the joint then presents the same near zero resistance each time it is used, and so becomes reliable.

Secondary circles can also suffer from dry joint syndrome, but they’re more likely to be troubled by ‘dry contact’ syndrome – a condition where the contacts within a switch, a pot or an output socket become tarnished. At its worst the tarnish is actually a thin film of very high resistance oxidation which prevents the electrons getting through. It’s possible to clean contacts to remove the oxidation, but as oxidation eats into the surface of the contacts and takes away any hope of true reliability, the best cure is often to bin the offending part and go for replacement.

At best, however, the electrons are halted by a thin film of crud shed by you, the player. This can usually be removed with appropriate spray cleaners, but be warned: crud is an acidic sweat-based compound, very partial to electrical contacts!

Screen shot 2014-02-05 at 12.02.50

Spot the Difference
Components suffering from heavy crud build-up will work properly most of the time, but every now and then they’ll need to be hit extra hard or given a vicious waggle to make them operate; conversely, components suffering from oxidation become very sensitive and usually only work if they are left untouched by the player. Both problems are very common and relatively easy to spot.

Hunting the Fault
Freshly armed with your understanding of circles and dodgy joints you are now ready to troubleshoot just about any guitar. First, plug the offending instrument, with its volume and tone controls turned up full, into an amplifier. Select the neck pickup position on the selector switch, and hit the strings. If you hear nothing from the amp, flip the switch to the bridge pickup and hit the strings again. If you still hear nothing, jiggle the lead in the output socket and listen for a crackling ‘make-or-break’ noise from the amp.

If you hear a sudden rush of sound, then you’ve found your major problem – the jack socket contacts and/or its solder connections are breaking the output’s secondary circle. Equally, you may find that the output socket works fine and jiggling the pickup selector switch restores the sound. Whichever way, the nature of the problem is the same: poor solder connections on the component and/or tarnished contacts within it are interrupting or breaking the circuit circle.

Volume and tone controls can also silence an otherwise functioning primary circuit circle if a resistive track on a pot has become so filthy that the wiper is unable to pick up the signal. Spotting this problem is simplicity itself: if the amp is filled with gritty sandpaper-type noises as you turn the pot back and forth,that’s the tell-tale sign that the track and wiper are dirty and need to be cleaned or the part replaced.

The diagnostic nature of electrical repairs has a real Sherlock Holmes feel and it can be a very satisfying process, particularly if you’re successful!

External ExaminationNext, armed with a pencil, let’s troubleshoot a Strat with a noisy pickup selector switch. Start with the same test procedure as outlined before: plug the instrument into an amp via a lead and turn all the guitar controls up full. Waggle the switch back and forth and listen to the noises from the amplifier. A switch that is working properly will create a mild ‘click’ from the speaker as it moves between the settings (this level of switch noise is acceptable, as the switch is making and breaking the signal path). A switch with worn or dirty contacts will sound and feel altogether different – moving it between the settings makes the amp crackle and roar, and the switch may feel stiff and difficult to move.

We already knew the Strat had a dodgy switch, so before removing the scratchplate to fix it, check if there is anything else at all wrong with the guitar. Start with the pickups, and be sure to note the results of your tests on paper as you go along.

Testing the Pickups
Select the neck pickup and turn the amplifier up a little. Next, hold a small steel screwdriver by its blade and touch it onto each pole of that pickup (you may want to lay the guitar down flat). The screwdriver will be drawn by the magnetism and should make a solid ‘clunk’ sound from the speaker as it makes contact with the pole. If that is all you hear, then the pole has passed the test.

Before moving on to clunk-test the next pole, make sure that the screwdriver is NOT being earthed out by touching a string. With the screwdriver correctly in place on the pole touch the blade and listen out for a loud hum from the amp. If the speaker still remains silent then the pole is functioning correctly, if it hums crazily then you have discovered a fault which could be within the pickup, its wiring connections or both, and this fault could also be contributing to the noisy switch. Move the selector switch to the middle pickup and test its poles in the same way, and finish up by clunk-testing the bridge pickup.

Next, hit the strings and turn the volume pot down and up to check that it works properly without adding any crackles to the sound. Copy the test with the two tone controls (just remember to select the correct pickups before you check the pots). When checking tone controls, also listen out for how the tone changes as you turn the pot up and down. If it behaves more like a volume control than a tone control then there is fault either with the pot, its capacitor, the wiring around the component, or all three.

The Verdict
We’ve carried out our external examination of the troublesome Strat. What do we know? Well, the selector switch has had it, the pickups seem okay, but the first tone control (the one for the neck pickup) is noisy and doesn’t sound quite right. Of course, you would have jotted all this down on your checklist. So now it’s either time for some DIY or if you don’t feel happy about that, a trip to your repair person.

Under The Hood
If you’re going DIY, the first step is to access the offending controls. Slacken and remove the strings and with a small cross-head screwdriver remove the scratchplate screws; place them on a tin lid or similar for safe keeping. Pull the scratchplate upwards and out from the body and turn it over to see underneath. Watch out for the output socket leads and the bridge earth wire, which will still be attached to the guitar body.

The photo below shows a typical Strat scratchplate; you can see how the three pickup ‘live’ leads go straight to their respective terminals on the switch, whereas the three pickup earth leads, and the bridge earth lead, are soldered together onto the back of the volume pot. This all-together-in-one-place earthing technique is a very reliable way to make a main earth return point for both the audio signal and the earth plane screening within the guitar. Some Strats also hardwire the casing of both tone pots and the chassis of the five-way switch to the casing of the volume pot, a technique which ensures that all the exposed metal parts of these components are properly earthed.

Screen shot 2014-02-05 at 12.04.49

Unfortunately, older Strats and many copies omit these additional earth wires and instead provide the metal parts with an earth connection via a metal foil screen stuck to the rear of the scratchplate. The electrical connection is made by squashing the foil onto the underside of the pot when the fixing nut is tightened. The squashed foil approach is all well and good when the instrument is new and clean but, unfortunately, oxidation and dirt can build up between the foil and the pots which increases the electrical resistance of their connection and upsets the earth return for the tone controls.

Back to our theoretical patient. The switch? It’s possible that cleaning might quieten it, but the best solution is to bin it and replace it with a new one. Dirt and crud has also built up between the foil and the first tone control, which is fortunately cleanable and so repairable. If this were a real guitar I would hard-wire the tone controls and the switch to the casing of the volume pot and thereby avoid this particular problem happening again.



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