Radio Units

A theoretical wiring diagram is useful to the constructor only when it contains the necessary specifications for the parts. The type numbers of the tubes must be given, the number of turns for the coils must be indicated, and the values of resistances and condensers must be clearly labelled. For this reason it is imperative that you learn the meaning of the various radio units used.

As in all electrical work, the most used units are volts and amperes. Because of space limitations, how-ever, it is usually impossible to spell these words out. For this reason, the abbreviation "a" is used to indicate amperes and "v" for volts.

Since many radio measurements are extremely small, smaller voltage and amperage units are some-times used. These are the microvolt, the microampere, the millivolt, and the milliampere. Being derived from the ordinary metric system, micro means one one-millionth part and milli means one one-thousandth part. Thus, one microvolt indicates one one-millionth of a volt while one milliampere represents one one-thousandth of an ampere.

A good way to fix the electrical difference between volts and amperes in your mind is to understand the water analogy shown in Fig. 19. The amperage flowing in a circuit is the quantity of electricity flowing and can be likened to the amount of water passing through the pipes of a water system. Voltage, on the other hand, like the pressure of the water flowing, is the pressure of the electricity passing in a circuit.

Condensers are devices for storing electrical energy and their capacity for storing this energy is measured in farads. However, a condenser of one farad capacity would be a large affair so that a smaller unit, the microfarad is the most common capacity unit used in radio work. Like the microvolt or the microampere, the microfarad is one one-millionth of a farad. Abbreviated to save space, it is written mfd. or merely mf.

A still smaller unit for condenser capacity is the micromicrofarad or one one-millionth of one one-millionth of a farad. By division, it is easy to see that the

micromicrofarad is one one-millionth as large as the microfarad. When abbreviated, the micromicrofarad is written as mmf.

The fourth important electrical quantity is resistance—the ability of a circuit, a part, or a length of wire to resist the flow of current. The basic unit of resistance is the ohm generally abbreviated as the Greek letter omega (Q). Since large resistances often are used in radio circuits, a larger resistance unit also is used. It is

the megohm, representing one million ohms. In its abbreviated form, the megohm is written merely as meg.

Incidentally, a table drawn up like that in Fig. 20 and tacked under the tool rack of his bench will help the amateur in finding out quickly what the prefixes micro, mega, milli, kilo, and micromicro mean when encountered on a radio diagram or blueprint.

Another quantity that must be specified on a radio diagram if it is to be of any use is the number of turns of wire used on home-wound coils. The turns in most cases are specified as a number followed by the letter "t". The notation 17% t lettered next to a coil, for instance, indicates seventeen and one-half turns.

To be complete, the size and kind of wire also must be designated. In radio work, the gage can be assumed to be the Brown and Sharpe or American wire gage unless otherwise specified (see Appendix).

The wire used, in most cases, will be insulated and the type of insulation also should be specified either on the drawing or in the list of parts. Insulations vary from a coating of enamel to one or two wrappings of cotton or silk. In the specifications, they will be designated by the following abbreviations:

Single cotton covered....... S. C. C. or SCC

Double cotton covered...... D. C. C. or DCC

Single silk covered............ S. S. C. or SSC

Double silk covered........... D. S. C. or DSC

Single silk-cotton covered.. S. S. C. C. or SSCC

Double silk-cotton covered. D. S. C. C. or DSCC

Enameled......................... Enam.

Aids in Reading Diagrams

One of the best ways to understand the workings of a circuit is to draw a simple picture wiring diagram after carefully studying the circuit diagram and deciding just what parts are required. A picture wiring diagram, consisting of pictures of the actual parts connected with bold pencil lines to represent wires, is

easy to follow and will be of great assistance in placing the parts and arranging the panel and chassis. In making a picture wiring diagram it is possible often to eliminate wires and simplify the connection by making one wire serve where two wires are shown in the circuit diagram. A picture diagram of this type is shown in Fig. 22. It corresponds to the circuit diagram given in Fig. 21. As an exercise in blueprint reading, draw a picture diagram from Fig. 21 and then compare it with Fig. 22 as a check on your knowledge of symbols.

Arranging the Parts

The simplicity of a radio circuit and the ease with which it can be assembled and wired will depend to a great extent on the way in which the various parts are placed on the chassis and panel. For this reason, many desire to build a circuit bread-board fashion before attempting to mount it in its finished form.

Study the circuit and attempt to group the parts. A condenser and a resistance that are to be connected should be placed adjacent to one another. Short connecting leads are a desired feature in any circuit and especially in those designed to cover the short waves.

The same procedure should be followed in planning and laying out the panel. Controls mounted on the front panel should be placed at the right, at the left,

or in the center to match up with connecting parts. Phone or speaker terminals should be placed close to the audio end of the circuit while antenna and ground binding posts should be located near the coils and tuning condenser. If the circuit is to be battery operated, the battery terminals at the rear of the chassis should be as close as possible to their points of en-trance into the circuit.

Checking Your Circuit

Once you have constructed your circuit, your first important job will be to check it for errors. Of course, the simplest way to do this is to compare it, wire for wire, with the original diagram or blueprint. However, by this method it is possible often to completely miss a connection or pass over the same mistake repeatedly without noticing it. A better way is to put your original circuit diagram away and with a pencil and paper make your own drawing of the circuit as you have wired it. This drawing then can be compared with the original circuit diagram. If any connections have been omitted or made to the wrong parts, the differences in the diagrams will make it possible for you to locate them quickly.

Above all, do not be in a hurry to connect your power supply in order to try the set out. A few minutes spent in checking the circuit is time well spent if by so doing you can avoid burning out a tube or ruining a coil.