Cable Cable installations are just as important as the other component installations. The cable used should be recommended by the manufacturer and in compliance with the NEC(c). For the best results in sound, a two-conductor shielded cable should be used.

In complex systems where the input lines are run in close proximity to the speaker lines for long distances, currents in the speaker lines may be picked up by the input lines. When these stray currents are fed back to the amplifier, cross talk and hum can be heard, or the amplifier may oscillate. Because of this, balanced line connections are recommended when long input and speaker lines are run close together. A balanced line is achieved by ungrounding the common terminal, leaving the outputs floating. Any current that develops on one side of the line and is offset by an equal and opposite current on the other side is called a BALANCED LINE. This reduces the possibility of creating stray currents in nearby input tines.

If hum is encountered with a balanced line, it may be necessary to run a shielded two-conductor cable to the speakers and ground the cable at the amplifier.

Figure 6-5.- Phasing speakers facing in the same and opposite direction.

MAINTENANCE AND REPAIR The best designed and built equipment occasionally develops faults. There are many factors that cause faults: moving equipment, atmospheric conditions, and the age of the equipment, just to name a few. A preventive maintenance schedule should be developed that requires a set routine of periodic tests, checks, and inspections to head off trouble before it develops.

When repairing a PA. system, you should always follow the manufacturers' recommendations and guidelines. Replacing faulty parts with the exact replacement parts is always the correct procedure.

Trouble in a public address system is often caused by nothing more than a loose connection or a break in the cable shield. Check for simple faults of this type before you begin a lengthy test of the system.

The identification and location of serious troubles in a system may require the use of signal-tracing equipment, such as an audio-signal generator, a meter, or an oscilloscope. When you test the electrical circuit, the most important point to remember is that you need to pinpoint the location of the trouble. A careful study of the circuit diagram is essential.

Some of the problems that cause defects in a P. A. system are poor solder connections and loose mechanical connections. When checking solder connections, make certain that both metals are absolutely clean and that the completed soldering job is firm and durable. Faulty soldering in a P. A. system can cause defects that are difficult to identify and locate. Too much solder can cause shorts in microphone connections that may not be visible.

Mechanical connections are easy to check; just ensure that all connections with a mechanical connector are tight. This type of connector will be found in the rear of the amplifier or in the console and speakers.

INTEROFFICE COMMUNICATION SYSTEMS An interoffice communication system is used to transmit orders and information among offices that are only a short distance apart. Frequently, such offices are in the same building. When an interoffice communication system is used, you are responsible for the installation and maintenance of the system.

CONFIGURATIONS An intercom system consists of two basic configurations: the all-master system and the single-master multiple remote system. With the all-master system, any station can call any other station or several stations can be connected together for a conference. With the single-master multiple remote system, the single-master station can selectively call any remote station, and any remote can call the master station.

COMPONENTS Basically, an intercom system consists of one or more stations, a junction box, one or more remote speaker units, and the wire necessary to make the connections.

The basic parts of a master station consist of a speaker-microphone, a selector switch panel, a combination volume control, ON/ OFF switch, a pilot light, and a listen-talk switch, all of which are mounted in a cabinet. The basic parts of a remote speaker unit consist of a speaker-microphone, a push switch for signaling the master, and a terminal board for interconnection to the master station.

MASTER AND REMOTE STATION INSTALLATION You can accomplish the installation of an intercom system easily if you follow the manufacturers' instructions and the NEC(c) guidelines. Any combination of master stations and remote stations up to the capacity of the master station can be used. Where it is not necessary for remote stations to communicate among themselves, you should usually install only one master station.

Install the master station within reach of a 120-volt, 60-hertz ac power outlet. The master station and the remote stations should be installed on the desk or in the working spaces of the personnel who will use them. If some of the units are to be installed outdoors, take the necessary precautions to protect them from adverse weather conditions. The size of cable to be used in making connections between units is governed by the length of wire and the type of system you install. The maximum wire resistance permissible will be stated in the operating instructions of the manufacturer's literature. Component and cable installation will depend on the type of system to be installed. After the cable is installed, check the resistance with an ohmmeter. Make certain that the maximum permissible resistance is not exceeded and that there are no opens, grounds, or shorts.

NOTE: Always follow the installation instructions that come with each system.

MAINTENANCE OF INTEROFFICE COMMUNICATIONS SYSTEMS In general there are four basic steps in intercom maintenance: inspect, tighten, clean, and adjust. Inspection is always of primary importance. The components in an intercom system are readily accessible, and, for the most part, can be replaced when faulty. With the solid-state devices of today, all maintenance programs are basically the same. One of the first and the most important factors you must take is to consult the manufacturers' recommendations and guidelines when performing maintenance on any intercom system.

Common troubles within an intercom system are normally nothing more than loose connections or breaks in the cable. If a component should need replacement, be sure to replace it with the manufacturer's suggested component.