Figure 8-17.--Location of automatic degaussing coils.

Automatic Degaussing

The metal aperture mask often becomes

magnetized when the receiver is moved through the

earth's magnetic field, Metal parts in and around the

picture tube also become magnetized when a

man-made magnetic field collapses or expands in

close proximity to the receiver. If these stray fields are

allowed to exist, the electron beam from each gun will

strike the incorrect phosphor dots on the face of the

screen, causing color impurities. Unless the parts have

become permanently magnetized due to a prolonged

exposure to a strong magnetic field, the magnetism

can be canceled by the process of degaussing. During

degaussing, a controlled magnetic field is developed

by passing alternating current through coils of wire.

The magnetic fields thus produced not only cancel the

existing stray magnetic fields but also serve to prevent

Figure 8-18.--VDR/thermistor automatic degaussing circuit,

schematic diagram.

any future buildup.

In modem color receivers, the problem presented

by stray magnetism is somewhat alleviated by

When the on-off switch is initially closed, the

automatic degaussing coils. These coils are an integral

resistance of the thermistor is high and nearly all of

part of the color receiver and are usually activated each

the circuit current flows through the degaussing coils.

time the receiver is turned on. The coils, usually from

After the first few seconds, degaussing is complete

two to four in number, are evenly spaced around the

and it is necessary to remove the coils from the circuit.

magnetic shield of the receiver picture tube (fig. 8- 17).

Coil isolation is completed by using the combined

Of the many circuits designed to provide

properties of the VDR and the thermistor. After the

automatic degaussing, the voltage dependent resistor

first few seconds of operation, the resistance of the

thermistor begins to decrease due to the current

(VDR) thermistor circuit arrangement is the most

flowing through the circuit. Simultaneously, the VDR

popular method of obtaining trouble-free operation,

The schematic diagram of such an arrangement is

begins to increase in resistance because less voltage is

shown in figure 8-18. The circuit consists of a

now applied. The combined action of the two special

thermistor, a voltage dependent resistor, and the four

resistors results in the coils eventually being

degaussing coils.

electrically removed from the circuit.

TELEVISION ENTERTAINMENT

SYSTEM

The television entertainment system (circuit

15TV) receives VHF broadcast television signals and

amplifies and broadcasts the signals to various

locations throughout the ship. The system consists of

an omnidirectional antenna, a lightning arrestor box,

a signal amplifier, a signal splitter, line taps, TV

signal/duplex power receptacle outlet boxes,

impedance matching transformers, and standard

commercial television receivers. The system is

basically passive except for the amplifier, which

Figure 8-17.--Location of automatic degaussing coils.

requires 115-volt, 60-Hz, single-phase power.

8-19