A digital multi-meter is a a useful piece of equipment for any guitar player. Even if you are not particularly technically inclined, there are a few simple tests that can save hours in set-up, tuning, and fault finding.
The tests described here are all low voltage, and safe even if you get things wrong. Devices connected to wall power contain high voltages and currents that can cause serious injury or death. Guitar tube amplifiers are especially dangerous, with voltages often exceeding 300V and currents of several amps. Due to the large capacitors used in tube amplifiers, lethal conditions can remain present even when the amplifier is turned off and the power cable removed. Testing of amplifiers and other high voltage equipment should be done by a qualified service person.
Let’s get started. Occasional instrument cable failures are pretty much guaranteed especially with large rigs, and finding the bad one can be frustrating. The continuity feature of a DMM is perfect for helping tracking down bad cables. First, connect the probes to the continuity/resistance inputs on the meter. Markings can be different so check the user manual for your meter if it’s not clear which these are. Select the continuity setting using the switches or buttons on your meter. To test that it’s working, touch the metal tips of the probes together. The meter should beep, indicating a good connection. To test the cable, touch one probe to the tip of one end of the cable, and the second probe to the tip at the other end. The DMM should beep indicating a connection. Hold the probes there for a few seconds and the beep should continue. Now do the same thing with the sleeve at each end, and check for the beep. If there is no beep, then you may have a broken connection in the cable. Next, touch the probes to the tip and sleeve at one end of the cable. There should be no beep. If there is continuity between the tip and sleeve, then you may have a short in the cable. If you think you might have an intermittent cable problem you can move the cable around at the jacks and then measure again.
If your amp won’t power on, you might have a blown fuse. The DMM continuity setting can be used to check this. Switch the amp off and unplug it from the wall. Unscrew the fuse holder and remove the fuse. On the continuity setting, touch the probes to each end of the fuse and the DMM should beep. A blown fuse will have no beep and you should replace it with the correct type of fuse. If you repeatedly blow fuses, check you are using the correct fuse type and rating. If these are correct, then you may have a fault on your amp, and you should get it checked out.
If you have a tube amp or effect unit with an incandescent indicator lamp, you can check the lamp in a similar way to a fuse. Hold one probe on the metal tip in the center of the base of the lamp, and the other probe on the metal ring around the side. The DMM should beep if the lamp is good.
The DC voltage setting can be used to test batteries. Connect the probes to the voltage inputs and set the DMM to read DC Volts. For 9V batteries, touch the probes to one each of the metal connectors on the top. A new, fully charged alkaline 9V battery should read around 9.5V. Anything below about 7 volts can be considered dead for most applications, so replace it or recharge it if it’s a rechargeable type.
For 1.5V batteries, touch the probes to the two metal ends. A new alkaline 1.5V battery should read about 1.6V. A rechargeable NiMH type will likely read a little less at full charge, maybe around 1.4V. Once the reading drops below about 1V, the battery can be pretty much considered dead.
The order of the probes is not critical when measuring battery voltage out of circuit. If you reverse the probes connecting the negative probe to the positive terminal and vice versa, the DMM should just indicate it on the display, normally with a – sign. The voltage reading will be the same.
Measuring batteries with no load is not very accurate and the reading will only be a guideline. If you want to measure the battery voltage more accurately, you can do so in circuit. Let’s try measuring with an effects pedal. Connect the battery to your effects pedal, and connect a guitar cable to the input so that you can turn the pedal on. It’s common for battery operated pedals to turn off when the input cable is removed to save the battery when the pedal is not is use. The pedal will need to be on to do this test. With the DMM on DC volts setting, touch the probes to the battery terminals where they connect to the battery snap. The DMM should now give a more accurate voltage reading with the battery under load.
Speakers are rated with an AC impedance which varies with frequency, and is difficult to measure, but you can easily measure the DC resistance with A DMM. Measuring the DC resistance is not particularly accurate as there is no frequency involved, but since we know that guitar speakers are commonly 4, 8 or 16 Ohm nominal, a DC resistance measurement is normally close enough to determine which type of speaker you have.
If the speaker is in a combo amplifier, switch it off and remove the power cable. Disconnect the cables from the speaker tabs, taking note which is which so you can reconnect them afterwards. Connect the probes and set the DMM to measure resistance (Ohms). Touch the probes one to each of the speaker tabs. The order of the probes is not important. The DC resistance will normally measure just below the nominal AC impedance rating of the speaker. For example, if the speaker measures, 7.5 Ohms DC resistance, it’s probably an 8 Ohm speaker. If it measures 3.8 ohms, then it is likely a 4 Ohm speaker, and so on.
If you need to know the total impedance of a multi speaker passive cabinet you can just connect a speaker cable to the input, touch the probes to the tip and sleeve of the other end of the cable, and measure the resistance. Make sure the cable is a good one, and use as short a cable as possible as you will be adding some resistance from the cable itself. Again the DC resistance measurement should be somewhere just below that of the impedance rating of the cab.
Pickups can be measured in a similar way to speakers. The pickups will need to be removed from the guitar, or at least the wires will need to be disconnected from the rest of the circuit first, otherwise you will be measuring the whole circuit including tone and volume controls which will make it difficult to understand what the results mean. Single coil pickups are straightforward as they normally have just two wires, ground and hot. Set the DMM to resistance, and touch the probes to the end of each wire. Compare the measured resistance to the manufacturers specification. If the resistance measurement is not stable and the values keep jumping around, the pickup may be damaged. Humbucking pickups will have more wires, so you will need to check which are the hot and ground wires from the manufacturers specifications. Then make the measurements the same way as with the single coils.
A specialty tube tester is required to fully test amplifier tubes, but there are a couple of basic tests you can do with just a meter. Turn off the amplifier and remove the power cable. If the amplifier has recently been switched on, give the tubes a good few minutes to cool down before handling. Carefully remove the tube to be tested being careful not to bend the pins or damage the tube. Look up the data sheet for the tube type and determine which pins are the heater. Set the DMM to the continuity setting. There should be continuity between the two heater pins, but not on any others. Test for continuity between all the pins and make sure the meter beeps, only when connected across the heater pins. Make sure to replace the tube in the correct socket.
To check the resistance value of a potentiometer, set the meter to the resistance setting, and touch the probes to the two outside pins of the potentiometer. Potentiometers will have a tolerance rating. A 500K Ohm potentiometer with a 10% tolerance could measure anywhere between 450K ohm and 550K Ohm for example, and still be in spec. To test the operation of the potentiometer, move one of the probes to the center pin and rotate the pot. The resistance should read the value of the pot when fully turned one direction, and something close to zero Ohms when fully turned the other.
Sometimes you may want to know what the power consumption of an effect pedal is, for example if you are specifying a pedal board power supply, or calculating expected battery life. We can do this by measuring the current flow. This is a little more involved than previous measurements as to measure current, the meter probes must be in series, rather than the parallel. Fortunately, there is an easy way to do this that works with many battery powered effects pedals that doesn’t involve cutting any wires or soldering any connections. Turn the battery snap so that just one of the connections is snapped to the battery. Connect an instrument cable to the pedal input so that the pedal can be turned on. Set the meter to measure current in milliamps. Note that on most DMM’s you will need to move the red probe to the mA current input. Make sure to do this correctly otherwise you may blow a fuse in the DMM, and be careful not to short anything out.
Touch one probe to the unconnected battery terminal, and the other to the unconnected pin on the battery snap. Turn on the pedal and current should flow through the meter to the pedal. The meter will read the current in mA. If the pedal has different modes, try them all to see if there is any difference in the current flow. It’s a good rule of thumb to specify a power supply capable of providing 2x the total current draw of your devices.