Reception

characteristics are necessary to assure uniform

The Y signal goes to the matrix and the C signal goes

amplification of the high-frequency color subcarrier

to the Q and I demodulator. The 3.58-MHz oscillator

is synchronized by the color sync burst from the

sidebands that carry chrominance information.

transmitter. The demodulator separates the

The video amplifier stage, which in monochrome

chrominance C signal into the individual I and Q

receivers usually consists of one stage of amplifica-

signals. The matrix circuit forms the original red,

tion, usually has three stages of amplification.

green, and blue signals.

Additional stages are necessary because the

The color picture tube is specially made for color

luminance signal is used to drive the cathodes of all

reproduction. Three electron guns, one for each color,

three electron guns in the color cathode-ray tube

are used. The tube's screen consists of small, closely

(CRT) as compared to the single gun in a monochrome

spaced phosphor dots of red, green, and blue. The dots

CRT.

are arranged so that a red, green, and blue dot form a

A video delay line is usually located between the

small triangle. The shadow mask provides a centering

video output stage and the CRT simultaneously. The

hole in the middle of the triangle of dots. The

fixed delay (fig. 8-14) is necessary because the

convergence electrode causes the three separate

chrominance signals pass through additional stages

electron beams to meet and cross at the hole in the

before being applied to the control grids of the CRT.

shadow mask.

Were it not for the delay of luminance information, the

Each electron gun is electrostatically focused by a

two signals would not arrive at the same time. A

common grid voltage. In other words, each gun has its

distorted video presentation would be the result.

own electrode, but all three are connected together

The use of an aperture mask type of picture

requiring only one grid voltage. The three electron

tube makes the brightness of a color receiver

beams scan the screen controlled by the deflection

yoke mounted externally around the neck of the tube.

characteristically low. Therefore, higher voltage is

As the three beams scan the phosphor screen in the

necessary to maintain adequate brightness. The output

standard scanning pattern, the dot trios are lighted

voltage of the high-voltage supply is nominally 20 to

according to the video input signals.

25 kV as compared with 15 to 18 kV for monochrome

receivers.

The purifying coil produces a magnetic field

within the tube, which aligns the electron beams

All three electron guns must be sharply focused

parallel to the neck of the tube. Rotating the purifying

onto the screen to obtain good monochrome and color

coil adjusts the electron beams so they strike their

reproduction. The focus rectifier in color receivers

respective color dots without striking the neighboring

provides a variable focus voltage (4 to 5 kV) that is

dots. When this adjustment is made for the red dots,

applied to the electrostatic focus elements of the CRT.

the other two electron beams are aligned as well.

Another factor that requires design techniques much

different than monochrome is the load of the

The high-voltage anode is a metallic ring around

high-voltage rectifier must be held fairly constant.

the tube. The field neutralizing coil aids color purity

Otherwise, severe blooming or shrinking of picture

at the outer edges of the picture tube. A metal shield,

called a mu-metal shield, is placed around the bell of

size will occur during the reception of signals with a

the tube to prevent stray magnetic fields from

varying brightness level. The voltage regulator circuit

affecting the electron beams.

provides a fairly constant anode voltage regardless of

the brightness level of incoming signals,

The color TV monitor/receiver has many

adjustments in addition to those of a black-and-white

The color demodulator section is the "heart" of the

unit. Consult the manufacturer's maintenance

color television receiver. In this section, the 3.58-MHz

handbook before you make adjustments. A color

subcarrier sidebands are demodulated to produce

television receiver, as shown in figure 8-14, contains

color information signals. The color information

many circuits that are different from the circuits used

signals are then applied to a matrix. In the matrix,

in monochrome (black-and-white) receivers. The

color difference signals are produced by matrixing

differences are outlined in the following paragraphs.

proportionate amounts of the demodulated signals.

The color difference signals are amplified and applied

The tuner and amplifier stage in color receivers

to the control grids of the CRT in the proper

are designed to pass a wider band of frequencies than

proportions to reproduce the televised scene.

conventional monochrome receivers. Wideband

8-16