Figure 7-34.--Public address set, lecturn type.

sound for entertainment or dissemination of

S2. Press-to-talk switch S1 supplies dc power to the

information. The set consists of an illuminated reading

amplifier only while the switch is held closed.

counter with a removable unidirectional dynamic

The current drain is very small when S1 is closed

microphone, a transistorized amplifier and controls, an

and no signal is applied to the microphone. The current

extended range loudspeaker, a battery meter, and jacks

is maximum when the loudest signal is being amplified,

for microphones, record player, tape recorder, and hvo

as the collector current of the output stage varies with

external speakers. Power to operate the set is provided

the strength of the amplified signal.

either by the self-contained dry battery, or an external

115-volt ac supply.

Microphone and Loudspeaker Assemblies

As an IC Electrician, you may be assigned the

The Mk 1 magnetic microphone has an impedance

responsibility for setting up and checking out public

of approximately 150 ohms. The microphone output is

address sets. To allow time for any minor adjustments

applied to transistor Q1 through volume control R1 and

or repairs that maybe required, always check the set or

capacitor C1. A selected portion of the sound radiated to

system well in advance of the time it is to be used.

the rear by the loudspeaker horn acts on the back of the

microphone diaphragm. This sound is phased so as to

SUMMARY

reduce acoustic feedback

In this chapter, we have discussed how amplified

Loudspeaker LS1 is a moving-coil, permanent-

voice systems are used to amplify and transmit both

magnet type. Amplifier output signals actuate the voice

verbal announcements and alarm signals on board ship.

coil and diaphragm, and the resulting sound waves are

We have identified the various announcing systems by

amplified and directed by the loudspeaker horn.

their circuit designations.

Maintenance

We have explained how the various types of

amplified voice systems operate and how to maintain

Preventive maintenance consists of replacing

the systems. Also, we have described the additional

batteries, routine cleaning, and inspections. When the

sound equipment used with the systems and the

batteries are replaced, inspect the battery contact springs

procedures for maintaining this equipment.

and clean them if necessary. If the springs are badly

corroded, they should be replaced. Keep the external

power cable free of dirt and corrosion. Clean the spring

clips with sandpaper and apply a thin coat of petrolatum

to reduce corrosion. Inspect the connector and clean it

if necessary.

Periodically check the microphone housing. Keep

the opening to the microphone free of dust, dirt, oil,

grease, salt crystals, or other foreign matter. Salt crystals

left by the evaporation of salt water and spray should be

dissolved and rinsed away with fresh water, and then the

parts should be dried with a soft cloth.

Check the inside of the pistol-grip handle

occasionally. Remove the handle cover and inspect the

switch contacts. Clean if necessary.

The manufacturer's technical manual contains de-

tailed instructions for troubleshooting and repairing the

set. All components are designed for easy replacement.

PUBLIC ADDRESS SET, LECTERN TYPE

Modern Navy ships are provided with the

lectern-type public address set. This set (fig, 7-34) is a

portable self-contained unit capable of reproducing

Figure 7-34.--Public address set, lecturn type.

7-35