Modern electronic test equipment affords the technician and engineer accuracy and convenience of use far beyond what was ever possible with volt-ohm meters and other analog devices of not- so-long ago. Though VOM’s still have their place on the test or repair bench, the introduction of digital technology into these devices has improved reliability and dependability while maintaining a price easily affordable to the average consumer. There are a couple misconceptions concerning test equipment shared by some of those starting out in electronics, either as a student or as a hobbyist.

The most common misconception is the need or requirement for accuracy and precision (which translates directly into $$). These are two very different qualities! As an example, if you fired five shots at a target, and the bullets all passed through the same hole made by the first bullet, you would have great precision. But if the hole was not in the bullseye, you would lack accuracy. If you had five holes in the bullseye, you would have accuracy, but with much less precision. Were the one hole from five shots in the bullseye, you would have both accuracy and precision. So how much accuracy and precision do you need in a piece of test equipment? This, of course, depends entirely upon what your practical needs are.

Most consumer electronics are built mainly from “off the shelf” components having 5% to 10% tolerance ratings. Though this may sound pretty “loose”, it is every bit practical. Part of the criteria for a good circuit design is being insensitive to the “tolerance stack” possible when using such parts. This is called a “reproducible” or repeatable design, and is a highly desirable quality.

A 100W resistor with a 5% tolerance could measure anywhere from 95W to 105W and be within its rating. It should therefore stand to reason that two different, though identical electronic devices might show a range of different voltage or other measured value at a given place in its circuitry. There goes your need for high accuracy and precision! If a point in the circuitry of 1000 identical amplifiers always measures nominally from 12.5 to 14 volts while functioning as it should, it makes no difference if it is displayed on the meter accurate and precise to three decimal places! There’s no real need for a highly precise measurement in this instance. But if you need to tune the output of an oscillator to precisely 72.0125 MHz, you are going to need accuracy and precision in your measuring equipment to get reliable adjustments.

Another common misconception is that the right test instrument will tell you what is wrong with a defective piece of equipment. It seldom, if ever, happens that way! Test instruments allow you to identify symptoms you must recognize and translate into a possible cause of a problem. This process is called “Troubleshooting”, and is a cultivated skill based upon knowledge of theory and experience. In a nutshell, it is rarely possible to repair a device having no or insufficient knowledge of how it works.

Volt-Ohm Meters Volt-Ohm-Meters (VOM) Volt-Ohm meters can often measure current in small amounts, usually below 200 mA, but otherwise are for voltage and resistance measurements only. VOMs can offer excellent accuracy, but are very inconvenient to use as compared to a DMM. VOMs use an analog needle movement which must be “zeroed” each time the resistance range setting is changed. Another inconvenience is the scale graduation. A VOM’s accuracy is measured at “full range deflection” (FSD) or when it is deflected all the way to the right of the scale. If a voltage measurement is being taken and it reads 27.5, you will need to determine what each graduation is worth in the particular range setting being used. If the range scale is 30 volts, the reading is taken directly. If the range is 100 volts, then each graduation on the scale is worth 100 ¸30 or 3.33 volts. The voltage reading would then be 3.33 X 27.5, or 91.7 volts. Much more complicated than reading it directly from and LCD display!

So why would anyone want a VOM today? Two main reasons: 1) A VOM will “load” a device more heavily than a DMM, and this can be useful when checking a diode or transistor. The VOM may therefore identify a leaky or weak device when a DMM indicates it as being good. 2) A DMM updates the LCD display about 12 times a second. This makes it difficult to see a slow oscillation or slow voltage change in a circuit. A meter movement can rise and fall with the change, but the LCD display may just appear unstable and jumping around. A simple rise/fall trend may not be at all obvious. A VOM can also provide you with a “second opinion” on a measurement, such as when measuring DC voltages that may have AC currents riding on them.

Digital Multimeters (DMM) The digital multimeter is the most widely used test instrument today. They can range in price from under $20.00 to hundreds of dollars. The DMM is referred to as a “multimeter” because it measures (or meters) multiple qualities such as resistance, voltage, current, and often others. The accuracy and precision of nearly all DMMs is more than satisfactory for a great majority of repair and troubleshooting needs. A better or more expensive meter will often have more “ranges” of adjustment for things like voltage and resistance. This requires more “knob turning” by the operator in its daily use, but the tradeoff is a bit better accuracy. There are also “auto-ranging” meters that automatically select the proper range when the leads are first connected to the device. True RMS meters will measure AC voltage and current at much higher frequencies than a standard DMM which are mostly intended to measure AC at 60 Hz.

A DMM in the $50.00 range will commonly have high quality test leads, a rubber boot to protect from drop damage, and large LCD displays, and should provide years of trouble-free service. Other features such as diode checkers, continuity testers, beta-testing for transistors, and inductance/capacitance measuring capability are also common DMM features.

Signal/Function/Audio Generator Available in audio and radio frequencies, these devices produce test waveforms, and often more than one type. They are used most commonly for testing with an oscilloscope as described above. Common waveforms include sinusoidal, triangle, sawtooth, and squarewave. The different waveforms are provided because some problems may be more readily observed in one type waveform than in another. Generators are available with analog and LCD displays, various output levels and connectors.

A sweep function generator starts at a preset frequency, and sweeps up through a second preset frequency, at a given sweep rate. This can be used to show frequency response in real time, and response anomalies.

Oscilloscope The oscilloscope is used to view an AC waveform in “real time” or as it actually happens in the circuit. This can be extremely useful (or absolutely required) when making some types of repairs or designs. For example, the audio output of an amplifier can be viewed on an oscilloscope to see how or if it is distorting its signal. This is done by putting a known waveform into the amp, usually a sinewave, and checking to see if the output signal matches the input signal in shape. A “clipped” waveform (not symmetrical), one that doesn’t “zero-cross” smoothly, or that has “noise” or another frequency “riding” the fundamental, all would indicate a problem with the amp. In a non-functioning device, another repair scheme is to input a signal and then trace the signal to see where it stops or in what stage the distortion first appears. More expensive scopes can have “dual traces” to allow you to view two waveforms for comparison, high frequency capability (100 MHz or more), sample/download features, etc.

AC/DC Clamp Meters These devices clamp around a single conductor and are able to measure the current flowing through it. Measuring high current can be dangerous with a regular DMM as all of the current needs to flow through the meter to be able to measure it. This requires breaking into the circuit and placing the meter in series. This can be very inconvenient! The clamp meter measures current accurately by induction. When measuring AC, clamping the meter around a 2-conductor wire will result in cancellation and no reading. These meters will only read current through a single conductor.

Fox and Hound The Fox and Hound Wire Tracing System is a unique device allowing you to trace the path of a wire. The “Fox” injects a three-tone signal into the wire, the “Hound” will pick up the signal inductively from up to 12” away through wood, drywall, or any non-metallic material. The Hound detects the tone, which gets audibly louder the closer the pickup is to the conductor carrying the signal. This also allows you to find the specific conductor when many are present or bundled together such as in telephone or computer networking hubs.

Frequency Counters A frequency counter simply measures the frequency of a simple waveform. These are used to measure IF oscillators in radio and TV equipment, or any application where is a frequency measurement is required.

Computer Monitor Tester The computer monitor tester plugs directly into the data plug on a standard computer monitor without the inconvenience of having to connect a computer to it. It produces several test patterns and will work with virtually any monitor from MDA to SVGA.

Circuit Testers These are simple devices which usually test for the presence of voltage or continuity, indicating the presence of such with an audible beep or glowing lamp. They are simpler to use than VOMs or DMMs, especially when only the presence of voltage or continuity is being tested for.

LCR Meter LCR meters measure L (a coil’s inductance) C (a capacitor’s capacitance), and R (resistance) across a broader range of values (and generally with greater accuracy) than a standard DMM. A DMM may have a low-range resistance of 200 ohm, which would measure any resistance up to 200. Such a meter would not measure a .22-ohm power resistor with much accuracy. An LCR meter may have a 20-ohm range, which could provide much better accuracy a such low R values.

The Woofer Tester The Woofer Tester is used to test a single woofer, to check or verify its Thiele/Small parameters. It comes with software that must be loaded on an IBM compatible computer, it is a DOS-based program. The computer is connected to an interface which in turn connects to the woofer. The tester will figure all salient Thiele/Small parameters of the specific woofer to check for accuracy, or for building an optimal cabinet for the specific speaker tested. In the past, equipment costing literally thousands of dollars was required to obtain the same information provided by The Woofer Tester. It is a unique product of which we are the sole source.