SYSTEM INTRODUCTION

Until recently, RADIO COMMUNICATIONS brought to mind either telegraphy (cw), voice (AM), or possibly radio teletypewriter (rtty) communications. Today, radio communications has become a highly sophisticated field of electronics. Even small Navy ships have the capability to "come up" on the commonly used ship-to-ship, ship-to-air, and ship-to-shore communications circuits. These circuit operations are accomplished through the use of compatible and flexible communications systems.

A communications system (as you will see later in this chapter) consists of two or more equipment sets (sets will be explained a little later). Communications systems follow the system subdivision shown in figure 1-1. Systems are arranged and interconnected to perform a circuit operation that cannot be performed by any single piece of equipment. Navy communications systems vary from the simple to the very complex, depending upon the circuit operations involved. Because a Navy ship must use every inch of available space, the communications equipment may be spread over several portions of the ship, for instance, receivers in one location, transmitters in another, and terminal equipment in another. The equipment must be installed in such a manner that it is flexible and can be used interchangeably with other installed communications equipment. Consequently, large numbers of sets which make up the shipboard communications system are installed and are capable of operating separately and simultaneously. Flexibility is provided through a complex arrangement of interconnections. These allow the physically separated equipment to be selectively switched (patched) by you into different circuit configurations.

Figure 1-1. - System subdivision.

As naval electronics has grown in capability and complexity, an orderly plan of equipment designations has been adopted. The largest designator, system, describes equipment that work together for a specific function. For example, the radar system of a ship includes every item of electronics equipment used in or with a radar on board that ship. The smallest designator, part, describes one single piece, such as a bolt or a resistor.

System

A SYSTEM is a combination of sets, units, assemblies, subassemblies, and parts joined together to perform a specific operational function or several functions. Examples are communications systems, radar systems, or navigation systems. You will find the majority of troubleshooting done in the Navy is system oriented. Figure 1-2 is a pictorial view of a typical communications system containing the components necessary for transmission and reception of voice, telegraphy, and teletypewriter signals. Figure 1-3 is a block diagram of the same communications system with the arrows showing the direction of signal flow.

Figure 1-2. - Communications system pictorial view.

Figure 1-3. - Communications system block diagram.

Set

A SET consists of a unit or units and the assemblies, subassemblies, and parts connected or associated together to perform a specific function. A good example of this is a radio receiving set or a radio transmitting set.

Figure 1-4 is a block diagram of a radio transmitting set. It consists of a radio-frequency amplifier unit (1), a radio transmitter unit (2), a power supply unit (3), and an antenna coupler group.

Figure 1-4. - Radio transmitting set.

Group

A GROUP is a collection of units, assemblies, subassemblies, and parts. It is a subdivision of a set or system, but it is not capable of performing a complete operational function. The coupler requires power and signals from the radiofrequency amplifier unit for operation. An example is the antenna coupler group in figure 1-4.

Unit

A UNIT is an assembly or any combination of parts, subassemblies, and assemblies mounted together. A unit is normally capable of independent operation in a variety of situations. An example of a unit might be a power supply.

Assembly

An ASSEMBLY is a number of parts or subassemblies, or any combination thereof, joined together to perform a specific function. Figure 1-5 shows a unit (2) with its six assemblies. The assembly (A6) contains six subassemblies.

Figure 1-5. - Unit and assembly.

Subassembly

A SUBASSEMBLY consists of two or more parts that form a portion of an assembly or a unit. It is replaceable as a whole, but some of its parts are individually replaceable.

The distinction between an assembly and a subassembly is not always exact; an assembly in one application may be a subassembly in another when it forms a portion of an assembly. Figure 1-6 shows a printed circuit board subassembly and some of the parts which may be mounted on it.

Figure 1-6. - Typical subassembly.

Part

A PART is one component or two or more components joined together. A part is not normally subject to disassembly without destruction. Resistors, capacitors, and transistors are examples of parts.

Reference Designations.

Reference designations consist of letters, numbers, or both and are used for identification purposes. Reference designations can be used in several different ways. One important way you will use them is as a cross-reference for locating supply stock numbers. When you know the reference designator, ordering the correct replacement for a failed component is easy. You will also use them frequently in corrective and preventive maintenance. Reference designators will help you to locate test points and adjustments; they will also help you to move back and forth between various technical manuals, schematics, tables, or other References.

Each set within a system is assigned an AN nomenclature. Each unit, assembly, subassembly, and part of a set has an assigned reference designation. Systems, sets, and groups have no reference designation. The unit is the highest level assigned a reference designator.

Each unit is assigned an identifying number. This number begins with the number 1 and runs consecutively for all units of a system or a set. Let's look back at the radio transmitting set AN/URT-xx with the unit numbers 1, 2, 3 on figure 1-4. You should note that these units may also have an AN nomenclature, such as T-xxx/URT. The T indicates the equipment is a transmitter. The xxx would be replaced by 3 digits that indicate the model number.

By examining the reference designator of a unit, you will be able to determine in which group, if any, the unit is contained. Let's look at a complete reference designator for a unit. A good example for us to break down is the reference designator 2A2A3C1 on figure 1-7.

Figure 1-7. - Reference designations.

The first indicator, 2, is numeric and refers to unit 2. The next indicator, A2, is alphanumeric and refers to assembly A2. The next indicator, A3, is also alphanumeric and refers to subassembly A3. The last indicator, C1, like the two previous, is alphanumeric and refers to the part C1. We have just located capacitor C1 on subassembly A3, which is on assembly A2, which is in unit 2 of the equipment.

Reference designations may be expanded or reduced to as many levels as required to identify a particular part. Let's look at a couple of examples on our figure. The designator 2J1 identifies jack J1, which is mounted directly on unit 2. The designator 2A4C3 identifies capacitor C3, which is on assembly A4 in unit 2.

Partial reference designations are used to save space on diagrams. For example, refer back to figure 1-6. Partial reference designations are placed near the parts on subassembly A15, and a note indicates the reference designation prefix is added. Capacitor C3 on subassembly A15 has the complete reference designation 2A7A15C3.

Q.7 A system is subdivided into what levels?
Q.8 In the example 1A6CR3, what is the assembly designator?