Figure 7-26.--Control rack.

To check a power-supply choke, use the voltmeter

to read the output voltage of the choke. If the reading is

excessively high, the choke has an internal short within

the choke. If there is no output voltage, the choke has an

open winding or a winding shorted to the case or core.

TRANSISTORS.-- To check a suspected transis-

tor, use an in or out of circuit transistor test set, or sub-

stitute the suspected transistor with a known good transistor.

SWITCHES.-- An ohmmeter can be used to make

continuity checks between switch terminals, or a

voltmeter can be used to measure the voltages at the

switch terminals.

INDICATOR LAMPS.-- To check a suspected

indicator lamp, you must replace it with a new lamp or

one known to be good.

CENTRAL AMPLIFIER SYSTEM

(TWO-WAY)

The two-way central amplifier announcing system

provides two-way transmission of orders and informa-

tion using central amplifiers. One example of a two-way

central amplifier announcing system is the AN/SIA-

120. The AN/SIA-120 comprises the circuit 29MC

(sonar control and information amounting system) and

provides two-way communications between the sonar

control room and various remote stations. The sonar

control room transmits to loudspeakers at the remote

stations, and the remote stations transmit to the

loudspeakers in the sonar control room.

The AN/SIA-120 operates on 115-volt, 60-cycle,

single-phase ship's service power. The major

Figure 7-26.--Control rack.

components of the system are the control rack and the

additional sound equipment used with the system.

The control rack has input facilities for one portable

microphone and five microphone control stations. The

CONTROL RACK

control rack also has the facilities for connecting five

The control rack for the AN/SIA-120 (fig. 7-26) is

individual loudspeaker groups to the outputs of the

a bulkhead-mounted enclosure containing two identical

amplifiers. In addition, the rack has a circuit with an

amplifier channels, A and B. Each channel uses one

amplifier priority control feature, which will connect an

self-contained modular audio-frequency amplifier

alarm signal provided from an external source to the

capable of 20-watt output power. The amplifiers are

inputs of the amplifiers while inhibiting operation from

located in two compartments in the upper half of the

the microphone control stations.

rack The bottom half of the rack is a control panel. All

the system's switching, testing controls, and indicators

Audio-Frequency Amplifiers

are located on the front of the control panel. The system

The two amplifiers located in the upper half of the

control relays, relay power supply, and various other

control rack are used for complete amplification of the

components are mounted on a bracket behind the control

microphone outputs. The inputs to the amplifiers are

panel. Terminal boards, which are used for making

parallel connected. The output transistors are physically

connections to the ship's wiring, are located on the lower

locatcd on the front of the panel. This design allows the

rear wall of the rack and are accessible by opening the

use of the whole front panel as a heat sink, providing

control panel.

7-25