How to Get the Most Out of Your VOM -- Contents and Intro

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1. The basic meter in the VOM

The principle of the d'Arsonval type movement. Sizes and shapes of basic meters. Forces of attraction and repulsion between magnetic poles. Homogeneity of the magnetic field. Why we use external resistances. Accuracy of the meter. Calibration accuracy. Why and how we change meter scales. Nonlinear meter scales. Protecting the meter. Determining the rating of a meter. Balancing the meter. Moving counterweights. Selecting a meter.

2. Multirange meters

How to measure range. How to determine internal resistance. Making multirange current meters. Changing the meter range. Making low-resistance shunts. Establishing standards. Making voltmeters and multirange voltmeters. Making series resistors. Switching arrangements. Making ac voltmeters. Meter rectifiers. Finding the rms value. Using series resistors for ac and dc. Ac vs dc scales. Calibration of ac voltmeters. Ac calibration methods.

3. Ohmmeters and volt-ohmmeters

Series and shunt type ohmmeter circuits. Potentiometer type ohm meter. Calibrating ohmmeters. Building your own instrument. Selecting the ranges required. Making meter scales. Photographic equipment. Applying decals. To buy or to build? High-accuracy ohmmeter. Increasing accuracy. Full-scale adjustment. Zero adjustment. High R test. Pin-jack VOM. Voltage and current circuits. The Ayrton shunt circuit. Ohms network. Galvanometer. Calibrating the galvanometer.

4. Kit type VOM's

Soldering. Rosin-core solder. Wiring. Choosing the kit. Meter sensitivity. Types of kits. Reverse-polarity switch. Using external multi pliers. Output-measuring circuit. How to read decibel scales. What a logarithm is. The JND ("just noticeable difference"). Ratios. Point of reference. Exponentials. Relationships between power and voltage ratios and db's. Relationships between ac voltage and db scales. Pictorial wiring diagrams. Tools.

5. Commercial VOM's

Variety of models, ranges and sensitivities. Full-wave bridge rectifier. Clamp-on ammeter adapter. Advantage of larger instruments. Switch doubles meter sensitivity. Thermal-compensating circuit. Germanium diodes protect meter. Separate test-lead jack for high-voltage and high-current ranges. A 100,000-ohms-per-volt meter. "Meter-open" position. Selecting an instrument. Imported instruments. Operation of the VOM.

6. Extending ranges and making accessories

Measuring higher voltages. The high voltage probe for dc. The high-voltage probe for ac. Measuring lower voltages. Adding cur rent ranges. Measuring higher resistances. Building the extender. Measuring lower resistance. Using ampere scales for measuring low resistance. Measuring capacitance. Using the VOM to measure capacitance. The Morecroft method for electrolytics. Measuring inductance. Measuring rf voltages. Measuring power. Ac power.

7. Making practical measurements

Measuring with an "insensitive" meter. Testing batteries. Using the VOM to check storage batteries. Checking B-batteries. Checking hearing-aid batteries. Checking transformers. Finding turns ratios and voltage ratings. 400-cycle transformers. Checking output trans formers. Checking rectifiers and diodes. Identifying and checking transistors. The mystery component. Tube filaments. Appliance repair. Expanded scale for the VOM. Detecting small changes in direct current and ac voltage.

8. The VOM as a basic meter

The VOM as a null detector. Wheatstone bridge. Null detection using a grid-dip oscillator. Grid-dip adapter. Lecher wire null detector. Vacuum-tube voltmeter adapter. Amplification for the VOM. Transistor current amplification. Field-strength measurements. Calibrating the field-strength meter. Transistor tester using the VOM. Checking vacuum tubes with the VOM. Tube-emission test. Transconductance test. Amplification test.

9. The VOM as a service instrument

The VOM in ac-dc receiver servicing. Checking series filaments. Checking B-plus voltage. Checking the oscillator. Signal tracing and alignment. Checking avc. Signal interference. Locating microphonic tubes. Using the VOM in FM and TV. Adjusting the linearity coil. Audio amplifier in a TV set. Portable radios. Transistor receivers. Batteries. Testing power transistors. The VOM in hi-fi. Tape recorders. Printed-circuit problems. Transmitters.

10. Miscellaneous applications

Measuring L and C. Measuring inductance by the parallel-resonance method. Measuring inductance by the series-resonance method. Using a nomograph. Using the capacitance scales to measure inductance. Phase-angle determination. Time measurements with the VOM. The VOM in the garage. Checking distributor timing. Regulator adjustment. The VOM in the laboratory. Measuring the concentration of a solution. Level indicator. Measuring temperature.


THERE are many possessions in life we see so frequently that we take them for granted. Like the modern automobile, we even manage to use them relatively efficiently, at least up to a point.

The volt-ohm-milliammeter or VOM is one of these items. Many technicians and engineers learn how to use it for a limited number of measurements, without really understanding its versatility. Here then we want to tell you what the volt-ohm-meter consists of, how and why it works, what uses can be made of it, how to adapt it for other purposes not usually known, how to care for the instrument, how to check it for accuracy, how to use it in the many ways it can be used in servicing and constructing electronic equipment of all kinds.

It is absolutely no exaggeration to say that the VOM is the instrument most frequently used by professional service technicians, engineers and maintenance men. From radio and TV repair to such fields as automotive electrical repair, the advantages of the VOM are almost automatically realized and used without much conscious thought. It is for many the handiest instrument avail able, the first they will turn to in their work.

Vom's come in many sizes, shapes, ranges and price classes, and we will have something to tell you about the selection of the proper instrument for your purposes. But they all have similar characteristics in that they are applicable to many measurements (because of their many ranges) and they need no power source, in contrast to their more expensive cousins, the vacuum-

tube voltmeters. Incidentally, following trade practice, the VOM (in this book) is variously referred to as an ohmmeter, multitester, multimeter and volt-ohm-milliammeter.

The fact that a VOM needs no power source is one of its greatest advantages. For example, the marine radio technician, who must service small-craft radio equipment is not likely to find a 117-volt power source for instruments; thus whatever he carries he must be able to use without power. The mere fact that the instrument must be carried around a lot, requiring ruggedness and light weight, again recommends the VOM.

Then, too, the VOM can be relatively inexpensive, and is the first instrument to be purchased or built by the ham, experimenter and hobbyist. It can, for them, form the basis of many other instruments, and we will tell you something of how this is done. Laboratories, rather than purchase many single-range instruments, often use many VOM's, when their accuracy is sufficient. In this way they save a great deal of money since they have "many" instruments available in one case.

Besides the commercial units, there are many types of VOM kits, and we will take a look at those. In addition there are a number of ways of building your own VOM with basic instruments. The ranges of the VOM can be extended, to read higher voltages, higher resistances and, in many cases, lower voltages without necessarily making a vtvm out of the instrument. We will discuss the various ways of doing all this.

The VOM of course has some limitations. There is, for example a finite, and not always very high, input resistance to the instrument, so that it may present a load to the circuit to be measured.

This and the effects you may have to consider will be discussed in some detail. The ac ranges of the instrument may have some limitation as to frequency, so that the unit may become less accurate at higher frequencies, and may not be very useful at very high frequencies.

Because it is a rugged and multipurpose instrument, some compromise has had to be made with accuracy and, although the VOM is certainly sufficiently accurate for almost all practical purposes, it must not be regarded as a laboratory standard, unless it is especially calibrated for this. There is, too, the disadvantage that generally, as the higher resistances are measured, the scale becomes more crowded, more difficult to read. This can be avoided in expensive bridge type instruments only. But there is a saving grace. You can use the VOM to build your own resistance bridge.

Several books could be written on specific applications of the vom; we have limited this one to the most useful and general practical applications, hoping that when you have finished reading it, you will have gained new skills and a new respect for this jack-of all trades in the electronics and electrical world.

Also see: Guide to VOMs and VTVMs

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