Guitar Workshop: Meter Reader
Now that you’re troubleshooting and soldering like a pro, the final part of our electronics for beginners series looks at the hallowed multimeter, a near-essential piece of kit for the serious DIY dabbler…
Although a multimeter, or multitester, is capable of taking many different types of electronic measurement, only two are usually required by guitar repairers: resistance and DC voltage. AC voltage readings are certainly possible with a multimeter; mains checking an extension lead, for example, is possible with the 250V AC range. However, these measurements are potentially lethal and should you intend to use a multimeter for this purpose, do take time to read the instructions carefully – high voltage AC seldom takes prisoners, and when we say you can kill yourself by doing such work wrongly, we mean it… so be warned!
Now, back to the safe world of meter functions within the electric guitar itself. Of the two that concern us, DC resistance will prove the most useful, allowing us to measure the resistance of a component – a pickup coil or volume pot – and thereby check that it’s working properly. The DC resistance ‘ranges’ on a multimeter can also be used to make ‘zero ohm’ continuity checks of wire linkages and dry solder joints.
ZERO AND COUNTING
Analogue needle-type meters need to be adjusted or ‘zeroed’ before making any measurements. This is usually achieved by turning a small trimpot or adjuster wheel on the front or side of the meter. Every time you select a resistance range, hold the probes together and turn the ohm (Ω) adjuster so that the needle lines up with the zero on the scale. This adjustment zeros the meter and ensures accurate readings.
Continuity checking is very useful and very simple to do. Set your meter to a fairly sensitive range – Rx100, for example – and then zero the needle. When the probes are touched together, the needle will move from the left to the right. It needs to settle on 0, as previously explained; in this condition the meter is measuring the resistance between the two probes as 0Ω on the scale. If you were to join the probes via a short length of wire the meter would also read zero because the wire has little or no electrical resistance. This state of affairs only exists because the wire is short and the meter fairly insensitive. However, if the length of wire increases dramatically (as in the case of a pickup coil) then the resistance will increase and become measurable by the meter.
Good solder joints and connecting wires have no electrical resistance so they should return a 0Ω reading on the scale when tested by the meter; if the reading shows any resistance at all then you have found a fault, perhaps even the fault. Hurrah!
USING A MULTIMETER TO TEXT A GUITAR’S EARTH
In the January issue of G&B we looked at how a faulty string earth can upset a guitar’s screening and cause the dreaded hands-off buzz. As the string earth is merely a wire which joins the earth side of the pickup circuitry to the bridge, it should be possible to test its continuity with the meter from outside the instrument. Placing one probe on the strings and the other on the output socket nut does the trick, testing most, if not all, of the instrument’s earth plane in one hit. If the meter needle moves to 0, then the earth plane is 100 per cent; if the needle reads any resistance at all, you know that the earth side of the circuitry has one or more dodgy connections; if the needle refuses to move then you can be sure that there is a break in the earth circuit somewhere.Continuity checking can therefore detect three conditions: good connection, poor connection, and no connection at all. Handy, huh?
Although the measurement scales on an analogue needle-type multimeter look confusing, they needn’t be if you follow this simple rule. First, look for the ohms range on the meter’s face – usually the one on the outer edge of the scale – and see how the number 10 appears around the middle. If the needle were to point to that 10, and the range switch was set to Rx100, then the scale reading (10) is multiplied by 100; the actual measurement then equals 1000Ω or 1kΩ. So whatever you were measuring between the probes would have a reading of 1kΩ. On this scale a 6kΩ Stratocaster pickup would send the needle to settle somewhere between the 50 and the 100 further up the scale.
If the range switch is moved to Rx1k then the 10 becomes 10 x 1000 which equals 10,000Ω or 10kΩ. On this scale, that Strat pickup settles the needle down the scale between the 5 and the 10.
It is much easier to read the meter scale with the range set to Rx1KΩ than Rx100 as the scale is less cramped and therefore the divisions are easier to read.
If the range is changed again to Rx10K the 10 on the scale becomes 10 x 10,000, which equals 100,000Ω or 100kΩ. This setting would be useful for measuring the resistance of a volume or tone pot, both its total track resistance and the behaviour of the wiper. A 500kΩ pot would send the needle up the scale to settle on or near 50. The accuracy of each of these tests is dependant, of course, on how carefully you have zeroed the meter and your ability to read the scale.
In theory, digital meters remove the ‘tricky-to-read’ measurement scale and replace it with a numeric LCD read out, often with two or more decimal places. The only thing to remember is that a digital meter will take a little while to settle on a value, so make sure that you don’t read them too quickly. The range selector operates in the same way as with the needle-type meter, so a digital meter can be used for both continuity and resistance readings in exactly the same way as outlined for an analogue meter. Most digital meters will auto-zero and auto-range themselves, so read the manual first to find out just how clever your unit is!
The DC ranges on both types of meter measure voltage and current, which is useful when trouble-shooting active circuits in guitars and stompboxes. Some meters also offer great ‘battery test’ ranges which put a load of a few milliamps on the battery to simulate a circuit before they measure the voltage. These ranges can tell whether a battery is fit for either the gig or the bin.
That draws to a close our look at the basic techniques involved in guitar electronics. If we’ve whetted your appetite, there are various books – available in good guitar shops – dedicated to the subject from which you can learn a lot more. Donald Brosnac’s Guitar Electronics is now quite old but still a good reference guide; likewise Adrian Legg’s Customising Your Electric Guitar. Dan Erlewine’s Guitar Player Repair Guide is a tome of professional-aimed information that includes electronics, while Dave Burrluck’s The Player’s Guide To Guitar Maintenance is an excellent starting point. Knowledge is power – so keep learning, and work safely.
Tags: Home, Workshops