BASIC COLOUR TELEVISION SYSTEMS

 

BASIC COLOUR TELEVISION SYSTEMS

 

Basic Features:

          As discussed in the previous chapter, three additive primary colours Red, Green and Blue have been chosen for colour television. The outlines of a system utilizing these colours is shown in fig. The system shown here is a closed circuit system and three cameras have been used. Each of these cameras along with the lens, filters and the picture tube forms a chain. The light obtained from the scene is focused by a common lens and this is then given to the three cameras through the diachronic mirrors and the filters. Each of the filter absorbs all the colours except its own colour. Thus each camera receives the image of the object pertaining to its own colour. Each of the camera scans the Image and produces signals pertaining to its own colour. Three signals are thus obtained one from each of the cameras. Let us call these signals as red, green and blue signals respectively. These signals are amplified by the respective colour signal amplifiers and the amplified  signals are given to the tri-colour picture tube. The colour signal voltages modulate the intensity of the three electron beams pertaining to each colour. These beams stimulate the related phosphors on the picture tube screen. Thus the brightness or each phosphor dot at any instant will depend on the relative intensity   of the various colours at the spot of the picture being scanned at that instant.

 

Colour Picture Tubes :

          Colour picture tubes are used in Colour TV receivers for displaying the picture. Three types of colour picture tubes have been developed so far. The Shadow mask  or the 'Delta-gun', 'Precision in-line' (PIL) and the 'Trinitron'. All these picture tubes are tri-colour picture tubes and have three electron guns (or one gun giving three electron beams), one for each of the three primary colours-red, green and blue. The screen of these picture tubes is coated with three phosphor groups one for each of the colours. A shadow mask or aperture mask is placed close to the screen which permits electron beam from only one gun to strike !he phosphor coating pertaining to that colour. The three picture tubes operate on similar principles but differ in constructional details.

 

Shadow Mask Colour Picture Tube:

          The Shadow mask colour picture tube, also known as Deltagun picture tube, was the first colour picture tube ever developed. This picture tube' has three separate electron guns-one for each of the colours. This picture tube essentially consists of the Delta gun assembly, the phosphor dot screen, shadow mask and the glass envelope. Outlines of this picture tube are shown in fig. 5-1.

 

The Delta Gun Assembly:

                   The Gun assembly consists of three :

          Identical electron guns, whicl1 are placed at 1200 to each other and at an equal distance from the centre axis of the assembly. The guns are so arranged in the tube neck that the electron beam from one of these strikes only the red phosphor dots on the screen (after passing through the mask), the second beam only the green phosphor dots and the third beam only the blue phosphor dots. The guns are therefore referred to as the red, green and blue guns. The construction of each of these guns is similar to that of the gun used in monochrome picture tube and perform more or less the same functions.

 

Phosphor dot screen:

          One of the' major differences between the normal monochrome picture tube and tri-colour picture tube is in the screen. The screen of the monochrome picture tube has a uniform coating of a phosphor which produces white light when the electrons strike it. In the shadow mask colour picture tube the screen is coated with triangular groups of phosphor dots. One dot in each group glows red, the second green and the third blue when the electrons strike them.

 

Shadow mask:

          A shadow mask (or aperture mask) is placed- close to the screen. This is made from a thin sheet of nickel alloy which has an array of closely spaced round holes equal in number to the groups of phosphor dots. Each of the holes is accurately aligned with respect to one triangular group of the dots. The shadow mask allows the beam from only one of the guns to reach the selected dots during the scanning. Thus at any instant, only the bean from the appropriate gun reaches the proper individual colour dot of a group.

 

Glass envelope:

          The glass envelope of the colour picture tube is similar to that of the monochrome picture tube. Its front part forms the screen. The phosphor triad dots are deposited on its inside very, precisely and the shadow mask IS mounted close to the screen. The gun assembly etc are also mounted properly and the glass envelope is evacuated and sealed.

 

Operation of shadow mask colour picture tube:

          The operation of the shadow mask picture tube can be explained with the help of fig.  5-2 which shows the three guns, a section of the shadow mask and  he screen. As already stated the guns, the holes in the shadow mask and the phosphor dots on the screen are so aligned that the electron beam from each of the guns will reach only a phosphor dot of one particular colour. Thus the beam from the red gun will, strike only, the red phosphor dot, the beam from the green gun only the 'green phosphor dot and the beam from blue gun? only the blue phosphor dol. The three beams converge at the shadow mask and pass through the same hole. When deflected these advance from hole to hole. Because of the angular separation of PO° between the three guns, each guns sees only the phosphor dots of -the colour associated with it, the phosphor dots of other colours being blocked by the shadow mask.

 

          The signal of each colour (after extraction from the colour composite Video signal are applied to the cathode of the respective gun. These signals control intensity of each of the electron beams. The glow of each of the dots will therefore depend on the colour composition of the picture being transmitted at that instant. Thus three displays are produced simultaneously which correspond to the three signals. Since the number of phosphor dots is very large and these are grouped close to each other, when these are viewed from the normal viewing distance the eye combines the three colours and  normal colour picture is seen.

 

Disadvantages:

          The shadow mask colour picture tube has several disadvantages as given below:

1.    Convergence is difficult -and it requires considerable circuit complexity and critical servicing adjustment. In most of the shadow mask tubes four static convergence magnets and a dynamic convergence assembly are employed. In all as many as twelve pre-set controls are necessary to achieve proper vertical and horizontal convergence over the entire screen.

2.    The focus cannot be sharp over the entire screen because the focus and convergence planes cannot remain coincident for the three beams which emanate from guns located at 1200 with respect to each other around the tube axis.

3.    The electron transparency of the mask is very low. It permit only about 20% of the beam current to pass through it. Owing to these disadvantages the delta-gun picture tube is no longer used in the colour television receivers.

 

Precision-In-line (PIL) Picture Tube:

          The Precision-in-line (PIL) picture tube also has three separate guns. It primarily differs from the delta-gun picture tube in the arrangement of the guns. While the guns in a delta-gun picture tube are arranged at 1.200 with respect to each other, in PIL picture tubes the guns are arranged in a horizontal line very close to the central axis (fig. ). Moreover, these are aligned very precisely (that is why this colour picture tube is known as precision-in-line picture tube). The colour phosphors in these tubes are deposited on the screen in the form of vertical stripes in triads (i.e. red-green and blue). These are repeated along the entire  width of the screen. To obtain the required fineness the stripes are very fine and their number is the same as that of colour triads in a horizontal line in  a delta gun picture tube.

 

The Trinitron:

          The Trinitron colour picture tube was developed by Sony Corporation of Japan. This picture tubes similar to the precision-in-line picture tube except that in place of three separate guns only one gun, with three separate cathodes which are precision mounted in the horizontal plane, is used in this picture tube. Moreover, an aperture grille is used in this picture tube in place of the shadow mask. This increases the transparency of the grille. The principle of this picture, lube is shown in fig.  The constructional features, focus and convergence details are  shown in fig.

 

          The use of only one gun with three separate cathodes simplifies the construction. Moreover, the single gun can be placed centrally thus permitting better focusing and convergence.

 

Colour fundamentals:

          Colour is a sensation which results from the perception of the human eye to a certain range of the electromagnetic waves (frequencies ranging from about 1014to 1015Hz). There are three important aspects of the colour-brightness, hue and saturation. The brightness of a colour indicates the amount of light intensity perceived by the eye, hue its shade and saturation the contents of pure colour and white.

 

          Three basic colours Red, Green and Blue have been chosen for TV transmission. These are known as 'primary' colours. Additive mixing is used in colour television and all the colours and shades are obtained in colour television by mixing these colours and wl1itein different proportion~.

 

Luminance signal:

          In colour transmission the colour camera splits the image in three parts, each corresponding to one of the primary colours. Three separate signals are thus obtained which correspond to the three primary colours-Red, Green and Blue. The three colour signals are processed (matrixed) to obtain the luminance and chroma signals.

 

          The luminance signal, which corresponds to the brightness part of the picture and by itself can produce black and white picture, is also known as 'Y' signal. This signal is obtained by combining the three colour signa1s in the proportions given below

          Y= .59G + .3R + .11B.

 

          These proportions are chosen as the response of the eye differs for these colours.

 

Chroma Signal :

          The chroma signal is next obtained. This is done in two step. Firstly the R-Y and B-Y signals are obtained from the luminance and the respective colour signals. These two are then weighted to obtain the U and V signals which form the colour signals which are transmitted. The weighting factors are-:

V = .877 (R – Y) and

U = .493 (B – Y), the weighting is done to avoid over modulation.

 

          The process of combining the three colours to obtain Y, U and V signals is known as matrixing and is done by a section known as ‘matrix’.

 

Colour TV receivers:

          A colour TV receiver receives the colour   telecast signals and displays the video signals colour composite video  signals (CCVS) as colour picture on a colour picture tube and reproduces the sound. The main functions of a colour TV receiver are:

1. To select the signals of the channel and .to amplify these and convert these into IF signals.
2. To amplify the video and sound IF signals, to demodulate the video IF signals to obtain the composite video signals, to mix the two IF signals to obtain the inter-carrier sound IF signals and to pre-amplify these.
3. To separate the combined luminance and chroma signals from the video IF signals and to amplify these.
4. To separate' the luminance (Y) and chroma (i.e. U and V) signals from the combined video (CCVS) signals.
5. To regenerate sub-carrier signals with phase locking with the sub-carrier bursts.
6. To decode the chroma signals to obtain the modulated U and V signals and to demodulate these with the help of regenerated reference carrier.
7. To combine the Y, U and V signals (matrixing) to obtain the colour (i.e. R, G and B) signals.
8. To amplify these signals and to give these to the respective cathodes of the colour picture tube which reproduces these as colour picture with the help of horizontal and vertical sweeps.
9. To produce horizontal and vertical sweeps synchronized with the respective sync signals and to give these to the respective defection coil for producing the sweep.
10. To separate the sound IF signals, to amplify, limit and demodulate these, and to give these to the loudspeaker at sufficient amplitude.
11. To provide power at suitable d.c. voltages of the TV receiver. The working of a TV  receiver has been discussed.

 

WORKING OF A COLOUR TELEVISION RECEIVER

 

          A colour television receiver is designed for receiving colour telecast Signals and reproducing these as colour picture along with sound.

 

          As explained Part-I of this book, colour television signals are More complex than B &W signals and thus a colour television receiver. has to perform several additional functions in comparison to a black and white receiver. It has, therefore, several additional stages and its circuit is more complicated than that of a B & W receiver and may appear confusing at first.

 

          The block-diagram of a colour TV receiver has been used in this chapter to explain its working. .

 

          Simplified block-diagram of a colour TV receiver is shown in fig. and a more detailed block diagram is shown in fig. The latter is based 011 more commonly used circuits.

 

Main Sections:

          A colour television receiver can be divided into the following main sections/stages:

1. Tuner
2. Video I F section.
3. Sound section
4. Video Amplifier
5. Chroma section.
6. Colour signal (R, G and B) output section.
7. Picture tube stage.
8.  Sweep section; and
9. Power supply.

 

          Out of these, sections 1, 2, 3, 8 and 9 are common to black and White TV receivers and perform more or less the same functions. The remaining sections are specific to colour TV receivers and handle the colour signals.

 

Tuner:

          The signals received at the aerial are given to, the tuner through the 'balun’. The tuner which consists of two stages-RF amplifier and the frequency changer, partly selects the signals of the desired channel, amplifies these and converts these into intermediate frequencies.

 

          The RF amplifier amplifies these signals with the addition of as little noise as possible, and gives these to the frequency changer. The gain of this stage is controlled for AGC.

          The frequency changer (also known as converter) changes frequency of the incoming channel signals into the intermediate frequencies. This stage performs two functions. Firstly it produces oscillations at a frequency which is higher than the picture carrier frequency by 38.9MHz (which is the video IF). Secondly it mixes the incoming signals and the oscillations. This results in the production of the video IF and sound IF (38.9MHz and 33.4MHz respectively) These intermediate frequencies are selected and given to the video IF section.

 

          The frequency of the oscillator is controlled by an arrangement known as automatic fine tuning (AFT). If the frequency of the oscillations drifts resulting in shift in the IF's this arrangement automatically corrects the oscillator frequency thus automatically correcting the fine tuning.

 

          VHF or VHF/UHF electronic tuners are commonly used in colour TV receivers.

 

Video IF Section :

          The video IF section is normally based on a single IC. It performs the following functions :

1. Suppressing the signals of adjacent channels and reducing the level of self-sound signals.
2. Amplification of Video IF and Sound IF signals (frequencies I 38.9 and 33.4 MHz respectively).
3. Detection of the Video IF signals to obtain the composite video signals (colour composite video signals-CCVS).
4. Mixing of the two IF signals to obtain the inter-carrier sound IF signals (frequency 5.5 MHz) and so often to pre-amplify these.
5. Producing AGC voltage and controlling the gain of Video IF amplifier for AGC.
6. Producing delayed AGC voltage for the tuner.
7. Generating AFT voltage for automatic fine tuning.

 

          The Video signals have a large bandwidth (of over5.0 MHz in colour TV receivers) and sound IF signals have also to be amplified. The Video IF section should therefore have adequate bandwidth and suitable frequency response. This can be achieved by using wave traps tuned to suitable frequencies at the input of the section and by using single and double tuned circuits for inter-stage coupling and as load in the Video IF amplifier. The de overall frequency response can be achieved by choosing proper Q of the coils, providing suitable coupling between double-tuned circuits and tuning different coils at slightly different frequencies (staggered tuning). In the modern colour TV receivers saw-filters are used along with tuned circuits to obtain the desired frequency response in the video IF section.

          The composite video signals (these are usually known as 'Colour Composite Video Signals, (CCVS in colour TV receivers) obtained from the Video IF section are given to the video amplifier and the filter carrier sound IF signals are gives to the sound section.

 

Sound section:

          The inter carrier sound if signals (frequency 5.5 MHz) are given to the sound section which usually contains two sub-sections-Inter. carrier sound IF and Sound output.

 

          The inter carrier sound If signals, which are frequency modulated are amplified, limited and detected by the sound IF sub-section. Pre-amplification may also be provided to the sound signals so obtained.  An IC like TDA 4420 or TA 7176 AP may be used in this subsection. The sound signals obtained from the sound IF sub-section are not of sufficient amplitude to drive the loudspeaker. The sound output sub-section amplifies these signals and gives sufficient output to drive the loudspeaker.

 

          This section may be based on transistors or an IC like TDA 1035 may be used. In some receivers the complete sound section may be based on a single IC capable of performing all the functions of sound section.

 

Video Amplifier :

          Composite video signals obtained from the Video IF section are given to the video amplifier which amplifies these signals. This section can be divided into two parts-buffer-amplifier and the final amplifier.

 

          The buffer-amplifier amplifies the composite video signals (CCVS) which consist of the signals, chroma signals, sync signals and the colour sub-carrier bursts. Since these signals have a bandwidth from d.c. to about 5.0 MHz this amplifier should have adequate bandwidth. This normally utilizes one or two transistors.

 

          The amplified signals obtained from the video buffer-amplifier are given to the following sections/stages:

 

i.             Chroma section.

ii.            Final video amplifier.

iii.           Sync separator.

 

          The final video amplifier amplifies the luminance signals. This normally consists of a low-pass filter (to remove the chroma signals and a multi-stage amplifier. The amplified luminance signals obtained from this section are given to, the chroma IC (for matrixing) and in some cases to the R, G and B output stages also.

 

Chroma Sections:

          Chroma section is the most important section in a colour TV receiver. It extracts the colol1rsignalsfrom the modulated and coded chroma signals and gives these to the R, G and B output stages. Block schematic diagram of the chroma section is given in Fig.

 

          Owing to the complex nature of the chroma signals the chroma section has to perform a large number of functions. The main functions performed by this section are:

1. Amplification of chroma signals (with colour killer).
2. Separating the quadrature modulated chroma signals into their constituents i.e. modulated U and V components.
3. Demodulating the modl1latedU and V signals with the help of regenerated sub-carrier.
4. Regenerating the sub-catrier (reference sub-carrier) to enable demodulation of the U and V modulated signals which are transmitted with suppressed carrier and giving the sub-carrier in correct phase to the U and V demodulators.
5. Obtaining the colour difference signals i.e. R- Y, B- Y, and G - Y signals from the U and V signals.
6. Obtaining the colour signals i.e. R, G and B signals from the colour difference signals and the luminance (i.e. Y) signals.

 

Chroma amplifier:

          This 'sub-section includes a band-pass filter to pass the chroma signals and a multi-stage amplifier. It amplifies the chroma signals (which consist of the quadrature modulated U and V signals). The gain of this sub-section can be controlled by the colour control.

 

          The amplified chroma signals obtained from the chroma amplifier are given to the delay-line decoder.

 

Colour signal output section:

          This section consists of three more or less identical stages, one each for the colours R, G and B. Each of these stages utilises a transistor in resistance coupled (directly coupled) circuit. These provide colour signals at suitable level for driving the colour picture tube. Their outputs are given to the respective cathodes of the colour picture tube.

 

 

Picture tube stage:

          The colour picture tube stage consists of the picture tube along with its accessories i.e. the deflection coil assembly and colour purity and centering magnets placed on its neck. The colour picture tube displays the R, G and B signals given to its cathodes in the form of the colour picture with the help of the two- sweeps given to the deflection coil assembly.

 

          Precision-in-line (PIL) picture tubes are commonly used in the television receivers manufactured in our country.

 

Sweep Section :

          The sweep section produces horizontal (line) and vertical (frame) sweep signals and gives these to the respective deflection coils, it also separates the sync signals from the composite video signals an arranges to keep both the sweep oscillators in synchronization With the camera sweeps with the help of the sync signals, The functions of various stages/sub-sections in the sweep section are described here briefly.

 

          It may be stated here that the functions of the sweep section in black and white TV receivers and colour TV receivers are similar. The differences in these arise due 'to' higher EHT voltage required in colour TV receivers, higher focusing voltage required (which are invariably derived from the line output stage) and larger power required for the sweeps,

 

          The sweep section can be divided into the following three parts:

1. Sync separator,
2. Vertical sweep-'section, and
3. Horizontal sweep section,

 

Sync separator:

          The picture tube converts the video signals (colour signals in colour TV receivers) into picture with the help of the horizontal and vertical sweeps. If the picture is to be reproduced correctly the frequency and phase of the sweeps should be in synchronization with the camera sweeps. Synchronizing signals are, therefore, transmitted from the TV  transmitter along with the video signals to maintain the synchronization of the horizontal and vertical sweep oscillators in the receiver.

 

          The sync-separator separates the sync signals from the composite Video signals divides these into the horizontal (line) and vertical (frame) sync signals and gives these to the respective oscillators.

 

Vertical Sweep Section :

          The vertical sweep section produces oscillations at, the vertical sweep rate. This further amplifies these oscillations and provides saw tooth  current to the vertical deflection coil. An IC like TDA 1870 can he used for producing vertical sweep.

 

Horizontal sweep section:

          Horizontal sweep section consists of the oscillator, arrangements for keeping it in synchronization with the sync signals, driver and the horizontal output stages.

 

Horizontal oscillator:

          Horizontal oscillator produces oscillations at the horizontal (line) sweep rate (15,625 Hz). The phase and frequency of these oscillations is maintained in synchronization with the help of horizontal sync signals. A discriminator circuit is used for comparing the phase and frequency of these oscillations with the sync signals and correcting these.

 

          An IC like TDA 1940 is used in some colour separating the sync signals. producing horizontal lions and maintaining these in synchronization.

 

Driver:

          The output voltage obtained from the oscillator is not sufficient to drive the horizontal output stage. A driver is therefore, used between the oscillator and output stage. This stage is normally based on a transistor like 2SC 206g in transformer coupled circuit.

 

Horizontal output stage:

          The, horizontal output stage drives current of saw-tooth wave-form through the horizontal deflection coil with the help of the drive obtained from the driver stage. The following voltages are also obtained from the line output stage:

1. DC voltage of about 25 KV for giving to the final anode of the colour-picture tube (EHT).
2. DC voltage of about 5 KV for giving to the focusing anode of the picture tube.
3. DC voltage 9f about 500 volts for giving to the accelerating anode (screen).
4. DC voltage of about 200 volts for giving to the collectors of the R, G and B output transistors.
5. DC voltages from 12 to 24 volts for operating the various stages using transistors and IC's.
6. AC voltage of about 6.3 volts for giving to the heater of the picture tube.

 

          A transistor like BU-205 or BU-208 is commonly used in this stage. An auto-transformer known as EHT transformer having several  windings is used with-this transistor to obtain the different d.c voltages as also to match the output impedance of the transistor with that of the horizontal deflection coil. Some safety arrangement for limiting the EHT voltage is also used in most of the colour TV receivers for preventing dangerous X-ray radiation from the picture tube.

 

Power supply:

          The power supply in colour TV receivers normally provides d.c. voltage of about 120 to 160 volts. As the performance of colour TV receivers depends largely on the supply voltage, the supply voltage has to be stabilized to maintain it very close to the nominal  voltage.

 

          Two types of voltage regulated supplies are normally used in colour TV receivers-Transistor regulated power supplies and 'Switch- ed Mode power supplies.' In the first transistors are used as series regulators for regulating the output voltage. In the second, transistors are used as switches in a complicated circuit to regulate the output voltage. Owing to the higher efficiency of the 'switched mode power supplies' these are finding increasing use in colour TV receivers.

OPERATING CONTROLS IN COLOUR TELEVISION RECEIVERS

 

          A number of controls are provided in a colour television receiver  for operating it.  These enable obtaining a clear and crisp picture  with proper brillance, contrast and colour  on the desired channel  as also the sound at the desired level.  The functions of the operating controls provided  in ordinary  colour television  receivers (with manual controls).

 

          The following operating controls are provided in colour television receivers:

1. On off switch
2. a) Channel selector or

b) Programme selector

3. Fine tuning  control
4. Brightness (brilliance) control
5. Contrast control
6. Colour control
7. Volume control
8. Tone control
9. Vertical hold

 

          Besides the above controls, AFT (automatic fine tuning) on off switch may also  be provided on some receivers. All of these except the colour control and AFT switch are common to monochrome and colour television.  The functions are of these  controls have been discussed  here briefly.

1. On – Off Switch :

Mains supply  to the receiver  can be connected or disconnected with this switch. Thus the receiver can be switched on off with it.  A push button switch (push on push off), or a piano key type switch is generally used for this purpose. In some receivers the switch may be a part of the volume control.

2. Channel Selector or Programme Selector :

All the colour receivers marketed in our country are multi channel receivers. A channeling the programmes  on the desired  channel.

In the first – a rotary switch just like the one used in most of the black and white receivers is used for selecting the desired channel. This may have eleven positions (usually for channels from 2 to 12 for VHF). This switch   makes necessary changes in the tuned circuits of the RF amplifier and the oscillator.

In the second type push button programme selectors are provided  for selecting  the programme on the desired  channel.  In these the tuned circuits connected with each of these can be pre set of receive   signals from any one of the desired  channels. Six to eight  buttons  are normally provided which enable  pre selection of an equal number of channels.

3. Fine Tuning Control :

Fine tuning control is provided to enable the viewer to tune the receiver exactly to the desired channel.  Adjusting of the fine tuning control makes changes in the oscillator frequency permitting accurate tuning.

In the colour receivers using channel selector the fine tuning control is normally  provided at the center of the channel selector knob.

Separate fine tuning control is not provided in receivers having programme selector  push buttons. Fine tuning  in these receivers can be achieved by carefully adjusting  the tuning of the tuned circuits connected  with the selected push button.

It may be added here that tuning is more critical in colour TV receivers than in monochrome  receivers and any mistuning  in colour TV receivers may result in loss of colour. 

4. Brightness Control :

Brightness control or brilliance control enables the control of the average  brightness of the picture.   If the setting of this control is retarted  the picture brightness  will be less resulting in  too dark  a picture  and if it is too much advanced the picture will be too bright thus lacking in details.  The brightness control (along with the contrast control) should be adjusted to obtain a picture with desired brightness.

5. Contrast Control:

The level of luminance signals given to the picture tube cathodes  depends   on the setting  of this control.  If this control is advanced the picture will be dark and if it is retarded the   picture will be dull (having  insufficient contrast).

For obtaining  the best possible (black and white)  picture,  the brillance  and contrast  controls  are adjusted  simultaneously  (with colour control set to minimum),  till a picture  having correct brillance  and contrast  is obtained.

6. Colour Control :

The level  of colour signals given to the cathodes  of the colour picture  tube depends on the setting of this control. Thus   the colour intensity  of the picture can be adjusted with it.

This control  should be adjusted (after adjusting the brillance and  contrast controls for the best black and white picture)  to obtain the desired  amount of colour.  The skin  colour  of any artist  appearing   on the screen  can be taken  as a suitable  reference  for adjusting  the amount of colour  in the picture.

7. Volume Control :

The level of the sound signals can be adjusted with this control.

8. Tone Control :

The tone of the sound can be adjusted  with this control.  This control should be adjusted  to give the most pleasing tonal quality.

9. Vertical Hold :

                For correct picture reproduction it is essential that the frequencies, and phases of the line and frame oscillators be correct and these should remain in synchronization with the sync signals obtained from the transmitter. If the frequency of any oscillator shifts beyond certain limits (known as pull-in range) these will not be synchronized by the sync signals.

          The free running frequency of the vertical oscillator can be adjusted by the vertical hold (V hold) control. If the picture rolls up or down (vertical rolling), it indicates that the vertical oscillator is oscillating at a frequency beyond the pull-in range. This can be adjusted, to obtain a steady picture with the vertical hold control. In many of the receivers vertical hold control is not provided (fully automatic receivers).

 

Proper Adjustment of Operating Controls of a Colour TV :

          The first principle in adjusting the operating controls in a colour TV receiver is to tune in the best black and wl1itepicture. The colour control should then be adjusted.

          The following procedure is recommended for operating a colour TV receiver:

1. Switch on the receiver and let it warm up till the raster comes. In many a colour TV receivers quick-start colour picture tubes are, used which require just 5 to 10 seconds for warming up.
2. Select the desired channel with the channel selector or programme selector.
3. Adjust the volume control and tone control for the desired' sound level and most pleasing tone.
4. Turn the colour control completely anti-clock wise (to the minimum colour position). Turn the brilliance and contrast controls in turn to obtain a picture of proper brightness and details.
5. Now tune in colour. As discussed, the colour intensity of the picture depends on the setting of the colour control. Carefully adjust the colour control to obtain the desired amount of colour in the picture.

 

          The skin tone of a person appearing on the screen provides a good reference point for adjusting the colour. If at any time during a colour telecast, there is no colour in the picture even when the colour control is advanced, check the channel tuning (or the fine tuning) and adjust it carefully. It should be remembered here that correct tuning is essential for receiving colour.

 

POWER SUPPLY

          Colour television receivers utilize integrated circuits (IC's) and transistors in various stages/sections. Power is required at suitable d.c. voltages for their operation. Supply at suitable voltages is also required for the operation of the picture tube.

 

          The normal power requirements in a colour television receiver are:

1.    DC voltage of about 110 to 120 volts for the operation of some of the high level stages such as horizontal (line) driver and output stages, vertical output stage and in some cases the sound output stage.

2.    DC voltage of about 200 volts for some high level stages including the R, G and B output stages. .

3.    DC voltages from 12 to 24 volts for -the operation of stages/ sections utilizing IC's and transistors.

4.    Suitable voltage for heating the filament of the picture tube and for supplying to its various electrodes. These include)

i)             Heater supply (6.3 V or 12 V a.c. or d.c.).

ii)            DC voltage of about 500 volts for the screen.

iii)           DC voltage of about 5000 volts for the focusing anode.

iv)          High voltage d.c. (EHT) for the final anode of the picture tube, This is around 25 KV for the PIL picture tubes.

 

          Out of these, the first (i.e. 110 to 120 V d.c. supply) is provided by the main power supply, the third (i.e. 12 to 24 volt d.c. supply) may also be obtain some receivers from the main power supply. The remaining supplies. are usually obtained from the line output stage.

 

Requirements :

          In colour receivers the supply voltage should not vary from the nominal voltage as this can have the following effects:

1. If the supply voltage increases the EHT voltage may become excessive which can cause dangerous X-ray radiation.
2. Changes in the supply voltage can also adversely affect the operation of some other stages.

 

          As such voltage regulated supplies which provide almost constant output voltage irrespective of load variations and variations in the mains voltage, are used in colour television receivers. Two types of supplies are commonly used in colour television receiver. Transistor regulated power supplies and Switched mode power supply (SMPS). Both the types have two sections-rectifier-filter section and the regulator section. .

 

          Out of these the Transistor regulated supplies have been discussed in. this chapter and the Switched mode power supplies have been discussed in the next chapter.

 

Transistor Regulated Power Supplies:

          In transistor regulated power supplies, transistors are used for the regulation of the d.c. supply obtained from the rectifier-filter section. The basic arrangement of a transistor voltage regulator circuit is shown in fig.6-1.

 

          As shown here, it consists of series regulator, voltage comparator and error amplifier.

 

          The unregulated d.c. voltage obtained from the rectifier-filter is given at the input of this circuit and this is regulated by the series regulator with the help of the voltage comparator and the error amplifier. For this the output voltage is compared by the comparator  with a reference voltage and if these differ an error voltage is  produced by the voltage comparator. This error voltage is amplified by the error amplifier and is used to control the resistance of the series regulator in such a way that it counteracts changes in the output voltage. Thus the output voltage remains almost constant.

         

          Simplified circuit diagram of series transistor regulator is shown in fig. This circuit is based on two transistors of which Q1 is the series regulator and Q2 performs the functions of voltage comparator and error amplifier. The unregulated supply is given at the input of this circuit and regulated supply is obtained at its output.

 

Defects :

          Defects it can occur in the transistor regulated power supplies due to Defects in the rectifier-filter circuit or due to defects in the regulator Circuit and these should be checked separately.

         

          Defects in the rectifier-filter circuit can occur due to defects in the  transformer auto-transformer, rectifiers or the filter condenser. These can lead to no Voltage, low voltage, blowing of the fuse or hum in picture or sound (these defects are common to all power supplies and need not be discussed here).

         

          Defects in the regulator circuit can occur due to failure of transistors, Defective voltage adjusting potentiometer (R15 it fig.), or Failure of  some other component. These can result in the following:

1. No output If no output voltage is obtained from the power  supply and the rectifier filter section is working normally (this is assumed for the following defects also), the defect can be due to:
1. Blown fuse F2
2. Open series regulator transistor Q1
3. Open current limiting resistance R8
4. Defect in the voltage comparator and error amplifier circuit.
2. Blowing of fuse: If fuse F1 or F2 is found blown and it blows again when it is replaced the defect can be due to :
1. Series regulator Q1 short
2. Excessive current being drawn from the power supply this is  not possible in the circuit of fig. 6-3, but can occur if current limiting arrangement is not provided in the supply).
3. Defect in the voltage comparator and error amplifier circuit.
3. Failure of regulation-If the voltage regulator does not function the output voltage will vary with the variations in the input voltage and load current. This can occur due to:
1. Shorting of series regulator transistor QI (this in blowing off of fuse).
2. Defect in the voltage comparator and error (including defective transistor Q2 or Q3).

 

          If the operation of the regulator. circuit is suspected, it can be checked by giving mains supply to the receiver through a variac (a normal step voltage-stabilizer can also be used), For checking the operation connect a multi meter (set to proper d.c. voltage range) at the output of the power supply (TP2 in fig. 6-3) and apply 220 volt a.c. and note the output voltage (which should be close to the nominal output voltage of the power supply). Now vary the input. Voltage from about 180 volts to 250 volts. If the regulator circuit is operating normally the output voltage. should remain practically the same when the voltage is varied from 180 to 250 volts.

 

          If the regulator circuit is not working or it is not working satisfactorily, the defect can be located by checking the voltages at various points in the circuit followed by testing or replacing the suspected components. The normal voltages at important points in the power supply circuit shown in fig.

 

          Note: If the series transistor is found defective, before replacing it the resistance shunting the transistor (R2 in fig. if used) should be checked and it should also be ascertained that the power supply is not overloaded.

POWER SUPPLIES

 

Switching Mode Power Supplies :

          Switching mode power supplies (SMPS) in which transistors are used as rapidly opening and closing switches to regulate the supply, are finding increasing place in colour television receivers.

 

          In the transistor regulated power supplies discussed in the previous chapter, transistors are used as series regulators. The efficiency of these regulator circuits is low particularly when the regulation range is large. On the other hand, in the switching mode power supplies transistors are used as switching elements for regulating the d.c. voltage. Since transistors are very efficient as switches the efficiency of switching mode power supplies is high. Besides, the switching mode power supplies are smaller in size, lighter and dissipate less power than the series regulated power supplies. Their use is therefore rapidly increasing in colour television receivers.

 

          Just like the series regulated power supplies the switching mode  power supplies also have two sections the rectifier filter and the voltage regulator. Before going into the details of the switching mode power supplies the basic principles of switching regulators which form the basis of SMPS have been discussed here.

Switching Regulators:

          Switching regulator basically consists of a switching element which operates as a rapidly opening and closing switch. This switch chops the input d.c. at a high frequency thus .converting it into square waves. These can be stepped up (or stepped down) by a transformer and rectified and filtered.

 

          Regulation the d.c. output is achieved by a control circuit which senses the output voltage and if it tends to vary adjusts the duty cycle (the on and off periods of the switch) in such a way so as to counteract the changes in the output voltage. Before going into details of the switching regulators basic principle of switching converters which form the basis of the switching regulators is briefly described here.

 

Switching Mode Power Supplies:

          Several types of switch mode power supplies have been developed so far. Besides the main regulated output voltages a low voltage output may also be obtained from these. The block schematic of a typical switching mode power supply is given in fig. This supply operates off the line (i.e. no mains transformer is used). This supply provides two output voltages-a main supply voltage (usually around 110 volts) and another (auxiliary) voltage supply (typically 12 to 20 volts). It also provides a low voltage supply for operating the control circuit. Brief description of this supply follows.

          The mains voltage is rectified and filtered by rectifiers and condensers in the rectifier filter section. This unregulated d.c. voltage obtained from the rectifier-filter section is chopped (switched on and off) lit II high frequency by the switching transistor and the control circuit. This chopped DC is applied to the primary of the trans former T-I and the voltage at its secondary is rectified by diodes and smoothed, to give the required output. For regulating, a part of the  voltage is fed back to a sensing amplifier which compares this voltage against a reference and if these differ, develops a control voltage which is given to the control circuit. The control circuit in turn adjusts the duty cycle of the switching transistor via the driver to keep the output voltage constant.

 

          A second supply is also obtained through a separate secondary winding on the transformer by rectifying and filtering it and regulating it by a series regulator. A small transformer has been used for obtaining a low voltage supply for supplying the control circuit.

 

Circuits:

          The circuits used in switching mode power supplies in different. models of colour Tv receivers vary widely. Besides the normal features described above these include facilities like soft-start, over current and over voltage protection.

 

          Most of these utilize one (or sometimes two transistors) as switching elements. In some of these the sensing, control and ramp generator (and protection) circuits may be based on transistors while in others an IC may be used for these functions. To familiarize the readers with the practical SMPS circuits the power supply circuit used in the ITT (European kit) receiver has been described here.

 

Switched Mode Power Supply Circuit (ITT colour TV receiver):

          The circuit diagram of the SMPS used in ITT colour TV receivers is shown in fig.

 

          This power supply provides two output voltages at +115 volts and +20 volts. A brief description of the operation of this circuit is given below

 

          The mains voltage is rectified by a bridge rectifier comprising of diodes D1 to D4. Condenser C7 is the filter capacitor. The resistor R3 has been used for surge limiting.

 

          The rectified DC voltage is chopped by driving the switching transistor Q4 on and off. The output voltage is stepped down to the required values by the transform. These are rectified by the diodes D9 and DIO and are filter by the condensers CIS and ("7 respectively. The main output voltage (+ 115 volts which is regulated) is sensed and fed to the base of the transistor Q1 (pulse width modulator). This in turn' controls the 'on' time of the switching transistor Q4 through a current sink Q3. Transistor Q2 is used for short circuit protection.

 

          Starting pulse of about 6 volts at the line sweep frequency is provided to control the chopping frequency. This can be provided from the EHT transformer of the receiver by giving a couple of turns on one of the limbs of the core of the EHT transformer.

 

          The main output voltage ( + 115 volts) can be adjusted by the preset         R 7.

 

Defects:

          The circuits of the usual switched mode power supplies are more complex and some of the components in these operate on high voltages. These supplies are therefore more prone to defects.

 

          Defects in power supplies can result in no output, no regulation or blowing off of the fuse. The defects can develop in the rectifier filter section or in the regulator section.

 

          The defects in the rectifier filter section are common to all other types of power supplies and need not be discussed here. The defects in the regulator section occur usually due to failure of some transistor transistors, some diode or an IC.

 

Troubleshooting:

          In case of a defect in the switched mode power supply the step should be to ascertain if the defect is in the rectifier filter section in the regulator section. This can be checked easily by removing the fuse or disconnecting the output of the rectifier filter, to isolate and checking it by normal techniques.

 

          If the defect is located in the regulator section, careful checking of the circuit will be needed as this circuit is usually complex and consists of several transistors and other components. The first step, in this case should be to check the voltages at various points in the circuit. If any of the voltages are found to be abnormal, the relevant circuit components in9Juding the transistors should be checked to locate the defect.

 

VIDEO IF SECTION

 

          Video IF section follows the tuner. Video IF and Sound IF signals obtained from the tuner are given at the input of this section.

 

Functions:

          The functions performed by the Video IF section in a colour receiver are almost the same as those performed in a black and white receiver. These are-amplification of Video IF and sound IF signals (38.9 and 33.4 MHz respectively), detection of video IF signals to obtain the composite video signals (more commonly known as colour composite video signals 'CCVS' in colour television), mixing of the two IF signals to obtain the intercarrier sound IF signals (5.5 MHz),

automatic gain control of the video IF amplifier with noise cancellation, generating delayed AGC for the tuner and generating AFT (automatic fine tuning) voltage for the tuner. Thus the only additional function performed by this section as compared to a black and white receiver is generation of automatic fine tuning voltage for the tuner. This ensures correct tuning of the tuner and eliminates the Deed for frequent adjustment of the fine tuning control.

 

          Another difference between the Video IF sections of monochrome and colour TV receivers is in their bandwidth. Since the frequency1 of the sub-carrier used for the chroma signals (in colour TV) is 4.43 MHz and the side-bands extend up to about 5.0 MHz, the pass-band of the Video IF amplifier section in the colour TV receivers has to be wide enough to accommodate these. Thus the response curve of the video IF section in colour TV revivers differs from that of the black and white TV receivers. The video IF response curves of black and white arid colour TV receivers are shown in Fig. 8-1.

 

Circuit:

          Integrated circuits which can perform almost all the functions of video IF section are now available-BEL CA3068 and TA 7607AP being two examples. The circuit diagram of video IF section\using an IC TA7607AP is shown in Fig.  A transistor QI (2SC94OC) has been used in this circuit for the pre-amplification of the video IF and sound IF signals. The IC performs the remaining functions. Thus the functions performed by this IC are:

 

1. Amplification of the video IF and sound IF signals (with automatic gain control).
2. Detection of the video IF signals to obtain the video signals and also to mix the two IF signals for obtaining the inter-carrier sound IF signals     (5.5 MHz).
3. Pre-amplification of video signals and pre-amplification of inter-carrier sound IF signals.
4. Automatic gain control of the video IF amplifier with noise cancellation.
5. Generating delayed AGC voltage for the RF amplifier in the tuner.
6. Amplification and limiting of video IF signals for automatic fine tuning (AFT).
7. Generating AFT voltage for the tuner.

 

Automatic Fine Tuning:

          The frequency changer (in the tuner) converts the channel video carrier frequency into video IF (38.9 MHz). For this the incoming carrier signals are mixed with the oscillations Produced by the local oscillator.

 

          The frequency of the oscillations produced by the local oscillator is set to be' higher than the video carrier frequency of the selected channel by the video IF (i.e. 38.9 MHz). However, if the oscillator frequency drifts-say due to changes in temperature or due to fluctuations in the mains voltage, the IF produced will differ from the exact value causing the signals to fall on incorrect position on the IF response curve. This may result in poor picture or ever complete loss of colour if the drift is large.

 

          Automatic fine tuning (AFT) is provided in colour TV receivers to maintain the oscillator frequency at its correct value. It controls the local oscillator frequency and if it drifts from the correct value corrects it thus maintaining the IF at its correct value. For this the video, intermediate frequency (IF) as obtained from the video IF amplifier is checked by a discriminator circuit (AFT det) and if this differs from the correct value, the discriminator circuit produces a d.c. voltage the polarity and amplitude of which depend on the difference. This d.c. is given to the varactor diode forming part of the local oscillator tuned circuit (in the tuner). Since the effective junction capacity in a varactor diode depends on the d.c. voltage applied to it, the d.c. voltage so applied changes the junction capacity thus correcting the oscillator frequency.

 

          The AFT circuit shown in Fig. 8-2consists of the limiter-amplifier and AFT detector (both forming part of the IC) along with the tuned transformer (T3) which is tuned to 38.9 MHz, and the AFT balance circuit. If the IF obtained differs from its nominal value this circuit produces a d.c. voltage. As explained above this voltage is applied to the varactor diode in the oscillator tuned circuit to correct its frequency.

Defects:

          Defects in the video IF section 'can lead to the following faults:

1. No picture, no sound raster normal with no snow.
2. Weak picture (usually with no colour), sound may be weak or normal.
3. No sound-picture normal.
4. Overloading.
5. No colour, black and. white reception normal.
6. Automatic fine-tuning inoperative.

SOUND SECTION

 

          The functions of the sound section in a colour TV receiver are same as in a black and white TV receiver. These are-to amplify and to limit the inter carrier sound IF signals (5.5 MHz), to detect these to obtain the sound signals, amplification of the sound signals and to provide output at proper level to drive the loudspeaker.

 

Circuits:

          One of the alternative circuit arrangements can be used in the sound section.

1. Using two separate IC's-one in the sound IF sub-section (for amplifying, limiting and detecting the sound IF signals and to pre-amplify the sound signals).and the other for providing sound output.
2. Using an IC in the sound section which can perform all the functions of the sound section.
3. Using an IC in the sound IF section and transistors for providing the sound output.

 

          Either of these arrangements can give satisfactory results. The circuit of a typical sound section of a colour TV receiver using the think management is shown in Fig.  This circuit utilizes an IC TA-7176AP for the sound IF sub-section and two transistors Q1 and Q2 (both 2SC-2073) in single ended push-pull circuit in the output stage.

 

Defects:

          Defects in the sound section can result in either of the following faults:

(i)           No sound.

(ii)          Low sound

(iii)         Distorted sound.

 

Trouble shooting:

          Defects in the sound section can be located by signal substitution testing followed by voltage checking and testing/replacing of suspected components. The normal d.c. voltages at the terminals of the IC TA 7176APand the transistors.

 

1. No Sound
1. In case of no sound* first check. if there is any hum or buzz from loudspeaker. If it is not there check the loudspeaker by connecting a good one across its terminals or by checking its continuity. If the loudspeaker is found to be normal check voltages at the terminals of the output transistors (or output IC). If any of these voltages are abnormal check the relevant components including the transistors (or IC).
2. If there is slight hum or mush from, the loudspeaker, check the faulty stage by signal substitution testing. Once the defective stage is known check it by testing voltages in it, If any of the voltage is found to be abnormal check the connected circuit components followed by testing/replacing of transistors or IC.
3. If the sound section is found to be working normally and even then there is no sound (i.e. on giving a signal of inter-carrier sound IF signal at the input of sound section normal sound is heard), the defect may be in the Video IF section. In this case check the voltages at the pins of the Video IF IC and if any voltages are found to be abnormal try replacing the IC. .

 

2. Low Sound or Distorted Sound

          If the sound is low or it is distorted (raster and picture being normal), the first step should again be to check the loudspeaker by connecting a good one in its place.

 

          If this does not help locate the defective stage by signal substitution, testing. Once the defective stage is known it should be checked by measuring voltages in it .and if any voltage/voltages are found to be abnormal the components in the suspected circuit should be checked/replaced.

 

          If the voltages are normal in the sound output and the sound IF I sub-sections and still the sound is low and or tinny check the coupling electrolytic condensers (C14 and Cl7 in Fig.) by connecting a condenser or the same value in turn across each of the condenser. If this increases the sound the circuit condenser is faulty and it should be replaced. If this does not help the decoupling condenser CIS should be checked.

 

          If the sound is found to be normal and still the sound is low, the defect may be in the Video IF section. In this case the relevant circuit of the Video IF section should be checked. This should be I011owedby checking the voltages at the pins of the Video IF IC.

 

 

 

VIDEO AMPLIFIER

 

          The composite video signals obtained from the video IF section consist of the luminance signal, chroma signals, sync signals and the colour bursts. These are given to the video amplifier. The function of the video amplifier is to amplify these signals and to give these to the various sections.

 

          The video amplifier consists of two sub-sections-buffer amplifier and the luminance (video) amplifier. Out of these the buffer-amplifier amplifies the video signals (CCVS) which have a bandwidth from DC to over 5.0 MHz. The buffer amplifier is designed to provide almost uniform gain over this bandwidth. The amplified signals obtained from the buffer amplifier are given to the various sections as given below:

 

1. The chroma signals are given to the chroma amplifier(ACC amplifier) in the chroma IC.
2. The luminance (Y) signals are given to the luminance (Y) signal amplifier sub-section of the video amplifier.
3. The composite video signals are given to the sync separator in the sweep section.
4. The composite video signals are also given chroma section at the required points.

          The luminance signal amplifier (video amplifier) amplifies the luminance (Y) signals. The amplified luminance signals are given either at suitable point in the chroma IC or to the R, G and B output stages.

 

Buffer Amplifier:

          The buffer amplifier usually consists of a sound IF trap and one or two amplifier stages utilizing transistors. This stage provides isolation between the buffer amplifier (or pre-amplifier) in the video IF IC and the various stages to which it gives the signals.

 

Video Amplifier:

          The luminance signal amplifier which is usually known as video amplifier, amplifies the luminance signals. This amplifier includes a low-pass filter circuit (chroma trap to filter out the chroma signals) and two or three amplifier stages using transistors. The amplified video (luminance) signals obtained from this sub-section are given either to a suitable stage in the IC or to the R, G and B output stages. A delay line, for providing suitable delay to I these signals is also provided in this circuit at a suitable point (the function of this delay line has been explained under circuits)

 

Pedestal Clamping:

          The video signals contain the information regarding the point to point brightness of the picture being transmitted (obtained by scanning the picture) as also the information regarding the average brightness of the picture. The average brightness information consists of low frequency and d.c. components and if these have to be amplified use of d.c. amplifiers will be necessary in the video amplifier.

 

Video amplifier circuit used in Korean ctv receiver Model 'Samsung’:

          The video amplifier circuit used in the Korean colour TV receiver Model 'Samsung' provides an example of the second type of video amplifier. This circuit is shown in fig. 10-2 and its simplified block diagram is shown in fig.

 

          This circuit is based on six transistors. Two of these have been used in the buffer amplifier, one as pedestal clamp and the rest three in the luminance signal (video) amplifier. A brief description of this circuit follows.

 

          Composite video signals (CCVS) obtained from the video IF IC are given at the base of the transistor Q1 through a 5.5 MHz wave-trap and resistance R2. This transistor has been used in directly coupled circuit and amplifies the composite video signals. The amplified signals obtained at its collector are given to the base of the transistor Q2, which has been used in emitter-follower circuit. The amplified signals obtained at the emitter of this transistor are given to the following stages/sections:

1. To the ACC (automatic colour controlled chroma amplifier) in the chroma IC.
2. To the sync separator in the sweep section.
3. To the main video (luminance signal) amplifier through the circuit formed by the resistors R6, R7 and R9 and condenser C3.

 

Controls :

          The following controls have been provided in this video amplifier:

1. Contrast control (operators controls),
2. Brightness control (operator's control), and
3. Sub-brightness control (servicing control).

          Their functions have been discussed here briefly.

 

Contrast Control:

          The contrast control circuit which primarily consists 9f R33, R34 and Cl2 has been connected between the base of transistor Q4 and ground. The level of signals given to the base of transistor Q4 will depend on the setting of the contrast control. It will increase as the contrast control is moved down and vice-versa (the shunting effect of Cl2 will increase as the contrast control is moved up reducing the level of signals given to the base of Q5).

Brightness Control and Sub-brightness Control:

          The brightness control and sub-brightness control' circuit consist of the resistors. R2J, R22, R29, R30 and R31. These two controls are effectively in series and the base bias of the transistor Q5 depends on their setting. Their operation is explained below:

 

          The fourth video amplifier stage using Q5 is d.c. coupled to the final video amplifier stage using Q6. The final video amplifier is also directly coupled to the emitters of the colour amplifier stages. Thus the d.c. voltages at the collector of the R,O and B amplifier transistors (which are again directly coupled to the respective cathodes of the picture tube) will depend on the base voltage of the transistor Q5. Thus the brightness of the picture can be varied by changing the base bias of the transistor Q5 by either the brightness control or the sub-brightness control.

 

Defects:

          Defects can occur in the video amplifier due to defects in the buffer amplifier or in the main video amplifier. These can result in the following faults:

1. No picture-raster and sound normal.
2. No raster-sound normal.
3. Very dull colour picture.

 

Troubleshooting:

          First step hi locating the defects in the video amplifier is to find out if the defect is in the main video amplifier or in the buffer amplifier. This can be ascertained from the symptoms and by watching the effect of adjusting the brightness and contrast controls.

 

          The defect can then be located in the suspected stage/section by checking the voltages at the terminals of the transistors followed by checking/replacing suspected components. The normal d.c. voltages at the terminals of the transistors in the video amplifier circuits described in this chapter are given .in tables 10-1 and 10-2.

 

Note:

          Directly coupled circuitry is used in the video amplifier. In this reference it should be remembered that in directly coupled circuits the effect of failure of a transistor in any stage or a defect in any stage will affect the d.c. voltages in other stages also. The should be kept in mind while analyzing the d.c. voltages measured in the suspected stages/sections.

 

 

1. No picture - raster and sound normal

          If the raster and sound are normal but there is no picture the defect is likely to be in the video amplifier or in the chroma section depending on the circuit used in the video amplifier.

 

          Let us first consider the circuit shown in fig. In this case this defect can be caused due to defect in the video amplifier. The following procedure is recommended in this case:

 

1. Adjust the brightness control and see -if the brightness of the screen can be changed with this control. If it can be changed, the defect is likely to be in the buffer amplifier or pedestal clamp other, wise in the main video amplifier.
2. If the buffer amplifier appears to be defective, check the voltages at the terminals of the transistors Q1 and Q2. If anyone of these is found to be abnormal check/replace the relevant components including the transistors.
3. If the voltages are normal in the buffer amplifier check the voltages in the pedestal clamping circuit.
4. If the defect appears to be in the main video amplifier, check the voltages at the terminals of the transistors Q4 to Q6. If the voltage is abnormal at anyone or more points check the entire circuit (as already stated this sub;-section uses direct coupling and in the case a defect in any stage can affect the d.c. voltages in other stages also).
5. If the voltages are normal in the video amplifier the defect may be in the colour signal output stages (R, G and B 01;ltput stages). In this case first check the 180 V supply which gives supply to these amplifier stages. If necessary this should be followed by usual testing procedures.

 

          In the circuit of the video amplifier corresponds to fig. 10-1 the following procedure is recommended:

1. Adjust the setting of the brightness control. If the picture brightness can be varied with this control the defect is likely to be in the video amplifier, other wise in the chroma section.
2. If the defect appears to be in the video amplifier check the voltages at the terminals of the transistors used in it. This should be followed by other fault finding steps.
3. If the defect appears to be in the chroma section refer to Chapter-12.

 

2. No raster sound normal:

          This defect can be due to a defect in the video amplifier only when the circuit of fig. 10-2is used.

 

          If there is no raster and the defect appears to be in the video amplifier (i.e. the picture tube filament is glowing, the EHT is normal and the voltages at the pins of the picture tube are normal-except at the cathodes), the defect can be in the final video amplifier stages. In this case check voltages at the terminals of the transistors Q4 to Q6 and if any of the voltage are found to be abnormal check/replace the suspected components.

 

3. Very dull picture:

          This indicates that only the chroma signals are reaching the picture tube cathodes and no video signals are reaching: Again this defect is possible if the video amplifier circuit corresponding to fig. 10-2 is used.

 

          In this case check the final video, amplifier stages thoroughly including the coupling condensers following the usual methods.

CHROMA SECTION-I

 

          Chroma section is one of the most important section in colour television receivers. This section, which performs a large number of functions, extracts the colour signals from the composite video signals and gives these to the R, G and B output stages at suitable level.

 

          Two slightly different systems are used-in the Chroma section. In one of these the output of this section consists of the colour difference signals, i.e. R-Y, G-"Y and B-Y signals. In the other system matrixing is done a step further and the output of this section consists of the R, G and B signals directly.

 

          As discussed earlier, the composite video signals in colour television (colour composite video signals) consist of the following:

1. Luminance signal (i.e. Y signal).
2. Chroma signal which consists of the quadrature modulated U and V signals.
3. Line and frame sync signals.
4. Colour sub-carrier bursts which consist of about 10 cycles of colour sub-carrier (4.43 MHz) which are transmitted during the back-porch of the line sync signals.

 

CHROMA SECTION – II

 

          It will 'be evident from the discussions in the previous chapter, that the main function of the chroma section is to obtain back the colour signals (Le. R, G and B signals and in some cases R- Y. G- Y and B- Y signals) from the coded chroma signals. This involves a large number of functions. The circuits of chroma section are designed to, perform 'all of these functions.

 

Circuits:

          Integrated circuits which can perform almost all of the functions of the chroma section have been developed and the circuits of the chroma section in modern colour television receivers are based on such an IC.

 

          The circuit diagram of chroma section of the colour television receiver model 'Samsung' (PAL version) is shown in fig. This circuit is based on  an IC TA7193P/KA2151. This IC includes a large number of stages sub-sections. These are given below along with their functions.

1. ACC Amp-Chroma amplifier with automatic colour control.
2. Chroma Burst Amp-Chroma signal amplifier, burst signal separator and amplifier.
3. Gain Control Amp-Chroma amplifier with colour control.
4. Burst Gate--Shapes the H sync pulses for operating the burst signal separator in 'Chroma burst Amp' (the term burst gate is used in some places for the stage which separates the burst, signals).
5. ACC Det- Detects the burst signals to obtain control voltage for automatic colour control.
6. Killer Ident Det-It detects the presence of the sub-carrier bursts and if these are not present biases the chroma amplifier to cut-off.
7. Killer Amp-Amplifies the colour killer signals.
8. Killer Ident Level Compa-It clips the level of ident signals and maintains these at an almost constant level.
9. 4.43 MHz Osc-It produces oscillations at the colour sub carrier frequency.
10. APC Det-Automatic phase control detector. It compares the phases of the colour signal bursts and the regenerated subcarrier and if these differ generates a controlling voltage for correcting the phase of the regenerated sub-carrier.
11. Phase Cont-Circuit for correcting the phase (and frequency) pf the regenerated sub-carrier with the help of the control voltage obtained from APC Det.
12. Ident Amp-It amplifies the identification signal for phase reversed V lines.
13. Flip-Flpp - Bistable multivibrator for controlling the PAL switch.
14. PAL SW-PAL switch for reversing the phase of the subcarrier signals given to the V demodulator on phase reversed lines.
15. B- Y-Demod - Demodulates the B- Y (or U) signals.
16. R-Y Demod - Demodulates the R-Y (or V) signals.
17. G- Y Matrix - Obtains the G- Y signals from the B- Y and R- Y signals by cOIrlktning these in suitable proportions.
18. Power regulator Regulates the supply and gives it to the various stages in the IC.

 

          The operation of this circuit has been described here briefly.

 

Chroma amplifier :

          The chroma amplifier sub-section consists of the following stages of the IC-ACC amplifier, a part of the Chroma burs amplifier and the Gain control amplifier. This sub-section amplifies the chroma signals with automatic colour control.

 

          Composite video signals (CCVS) obtained from the video buffer amplifier are given to pin 15 of the IC (ACC Amp) through the resistor RI, a high-pass filter formed by the condensers C1 and C2 an (inductor L1, and sound IF trap T2. The high-pass filter and IF trap remove the luminance signals and intercarrier sound IF signals respectively from the composite signals thus separating the chroma signal. Only chroma signals, therefore, reach pin 15 of the IC.

 

          The chroma signals are amplified by the ACC amplifier, Chroma Signal amplifier stage of the chroma burst amplifier and the Gain cont. .amplifier stages. The amplified chroma signals which appear at pin 19 of the IC, are given to the base of the delay line (DL) amplifier 1ransistor,Ql through a resistor R9 and condenser C9.

 

          The gain of the ACC amp (automatic colour controlled amplifier) is controlled by the automatic colour control voltage developed by the ACC Det. (automatic colour control detector).

 

          The gain of the Gain cont. amplifier can be controlled with the .colour control potentiometer R8 (the amount of colour in the picture ,can be controlled by this potentiometer). This amplifier is also controlled by the colour killer which is operated by the presence of the sub-carrier bursts. If the bursts are not received the colour killer blocks the amplifier thus preventing appearance of coloured snow on the screen of the picture tube. Simplified circuit of chroma section used in Korean colour television model 'Samsung' bowing the chroma .amplifier and associated circuit is-given in fig.

 

Trouble Shooting:

          Defects in the chroma section can be located by the normal techniques of checking voltages at the pins of the IC and the transistor, replacing/testing of the IC or transistor and other methods discussed under specified effects. The normal voltages at the pins of the IC and' the terminals of the transistor.

 

          If the voltage is found to be abnormal. at any point it indicates some defect. The usual procedure of checking/replacing the suspected components including the IC and transistor should be followed to locate the defect followed by checking the relevant adjustments etc. Step-by-step procedure for locating these defects have been discussed in the following paragraphs.

 

1. No colour-black and white picture normal:

          This defect can be caused by one or more6f the following faults:

1. Defective IC
2. Defective delay line (DL) amplifier transistor QI
3. Defect in colour control circuit
4. H sync pulses not reaching pin 13 (burst gate) of the IC
5. Defect in the circuit between pin II and 17 of the IC
6. Defect in the oscillator circuit-
1. Oscillator not working
2. Oscillator producing oscillations at a wrong frequency.

 

          The following procedure is recommended in this case:

1. Check voltages at the pins of the IC. If voltage is found to be abnormal at any point check --the relevant circuit. If the external circuit is normal replace the IC.
2. If the voltages at the pins' of the IC are normal check the voltages at the terminals of transistor QI. If any voltage is found to be abnormal check the connected circuit and if suspected replace the transistor.
3. If the voltages are normal even at the terminals of the transistor, disable the colour killer circuit (this can be easily done by .connecting a 10K ohm resistor between pin 2Lof the IC TA 7193 P and ground). If normal colour appears, it indicates that the defect is either due to H sync pulses not reaching pin 13 of the IC or -due to defect in the circuit between pin 17 and II of the IC. In this case check the presence of sync signals at pin 13, 17 and 11 of the IC if a CRO is available. If CRO is not available check the circuit between the sync separator output in the H sweep section 'to pin 13 of the IC and the circuit between pin 17 to 11 of the IC.
4. If disabling of the colour killer circuit also does not help check the oscillator circuit and its frequency (this will require a frequency counter and if it is lot available try adjusting the oscillator, frequency with the potentiometer RI7).
5. If this also does not help check the delay line decoder circuit.

 

2. Specific tinted colour :

          This defect can be caused due to the following faults:

1. Defect in the Matrix sub-section of the IC.
2. Wrong adjustment of the delay phase adjust (PAL Matrix adjustment).
3. Defect in the R, G and B output stages. In this case the following procedure is recommended

 

i.             Check voltages at the pins I, 23 and 24 of the IC. If any of these voltage is abnormal disconnect the wiring between that pin and the base of the respective colour amplifier and again check the voltage at that pin. If the voltage now becomes normal the defect is likely to be in the respective colour output stage which should be looked into. * However, if the voltage is still abnormal replace the IC.

ii.            If the voltages are abnormal try adjusting the delay phase adjustment (PAL matrix adjustment). This can be adjusted in the field till almost normal colours are received. **

iii.           If the defect is not located by the above tests, it may be due to some fault in the colour signal output stages or due to- wrong adjustment in these stages. In this case reference should be made to Chapter 13.

3. No raster (sound noisy or weak):

          This defect can be caused by the chroma section only if the IC is drawing very heavy current from the supply (12 volt supply). The supply output voltage will be low resulting in this defect.

         

          To ascertain the defect disconnect supply to pin 22 of the IC (by cutting the PCB at some suitable point). If this. normalizes the supply voltage of the IC is defective and it should be replaced.

 

COLOUR SIGNAL OUTPUT STAGES

 

          The colour signals (i.e. R, G and B signals) obtained from the chroma section are given to three separate amplifier stages. Each of these amplifier stages, which are almost identical, amplifies the respective colour signals and gives these to the respective cathode of the colour picture tube.

 

          The picture tube reproduces these signals in the form of colour picture with the help of the horizontal and vertical sweeps.

 

          The circuits of these amplifier stages are normally located on a PCB which 'is fixed at the base of the picture tube.

 

Colour Signal Output Stages :

          Three separate amplifier stages, each of which normally utilizes a transistor is used for amplifying the three colour signals (i.e. R, G and B signals). These are also known as Red, Green and Blue signal amplifiers or Red, Green and Blue output stages. These stages utilize direct coupling and their outputs, which contain the colour signals at the proper level, are given to the respective cathodes of the picture tube.

 

          As discussed earlier two slightly different systems are used for matrixing. In one of these which is simpler the matrix circuit only combines the B- Y and R- Y signals (obtained after demodulating and deweighting the transmitted chroma signals) producing the G- Y signal.

 

          In these receivers the output of the chroma section consists of the colour difference signals. i.e. R-Y, G-Y and B-Y signals. These are given. to the respective colour signal output stages along with the luminance (Y) signal, the final combining to obtain the colour signals being performed in the colour signal amplifier stages (Fig.).

 

          In the other system matrixing is carried out a step further and the colour difference signals (i.e. R- Y, G- Y and B--Y signals) are combined with the luminance (i.e. Y) signal to obtain the colour signals directly. Thus in receivers based on this system the output of the chroma section consists of the colour signals (i.e. R, G and B signals).  which are amplified by the colour signal output stages and given to respective cathodes of the picture tube (Fig.).

 

Circuit:

          The circuit diagram of colour signal amplifiers using colour difference signal (i.e. R-- Y, B- Y and G- Y signals) drive is shown in Fig. As shown here three transistors (QI to Q3) have been used in this circuit for amplifying the R, G and B signals respectively. These transistors have been used in directly coupled circuits and their outputs are given to the respective cathodes of the picture tube.

 

          The colour difference signals, i.e. R-Y, B-Y and Q.-y signals are given to the bases of these transistors and the luminance.(i.e. Y) signal is given to the emitters. Thus the final matrixing to obtain the colour signals from the colour difference signals and the luminance signal is done by these stages.

 

          To enable the adjustment of the drives of three amplifier stages (for obtaining proper white level adjustment) the luminance (Y) signal is given to the emitters of two of the transistors through preset potentiometers (drive-adjust) and to enable adjustment of the collector voltages of the three transistors (for providing correct d.c. potentials to the three cathodes of the picture tube) the emitters of all the –three transistors are given d.c. voltages from 12 volt supply through pre-set potentiometers (these are for grey-scale adjustment).

 

          For full modulation of picture – tube cathodes, drives. of about 100I volts (peak to peak) are required. To be able to get drive voltages of this amplitude, collector of these transistors are given a supply of about 150.to 200 volts. .

 

          The circuit diagram of colour signal amplifier section using colour signal (i.e. R, G and B signal) drive is shown in Fig. This circuit differs from the circuit shown in Fig. 13-3 in the following respects:

1. No luminance signal is given to the emitters in this circuit as direct colour drive is given to the bases of the transistors.
2. Pre-set potentiometers are provided in the emitters of two of the amplifier transistors for adjusting the drives (for white level adjustment). These potentiometers are given in the negative feedback circuit. The feedback will therefore, depend on the setting of these potentiometers. Thus the feedback given to these stages can be adjusted with these potentiometers permitting adjustment of their gains (and drive).

 

          DC voltages are given to the emitters of the three transistors from 12 volt supply (just. as in the circuits using colour difference drive) through separate pre-set type potentiometers (for grey scale adjustment).

 

          The difference between the two circuits can be 'easily noted from. fig. in which simplified circuit of colour amplifier stages of both the types are shown.

 

Defects:

          Defects in the colour signal output stages can lead to the following faults:

 

1. Abnormal colour reproduction - This may be due to absence of
2. Very dull colour picture -This will normally be due to complete lack of Y signal. This defect can occur due to a fault in colour. amplifier stages only in receivers using colour difference drive.
3. Improper white balance - In this case the reproduction of whites will not be correct and there will be some tint of colour on whites
4. Wrong grey scale tracking - In this case the' reproduction of grey will not be correct and there may be some tint of colours.

 

Trouble Shooting:

          Defects in colour signal amplifier section can generally occur due to failure of transistors or defective pre-sets. The defects in this section can be located by checking the input and output signals with 1\CRO it available. If a CRO is not available these can be located by normal techniques of checking voltages at the terminals of transistors followed. by testing of suspected components, replacement of transistors and by attempting adjustment of pre-sets.

 

          The normal d.c. voltages at the terminals of the transistors in the Korean receiver models Samsung and Gold Star and Crown receiver Model CT-701 are given below.

PICTURE TUBE STAGE

 

          The picture tube stage consists of the colour picture tube, the deflection coil assembly, the colour purity and convergence magnets and the associated circuit for supplying proper voltages to the picture tube. The picture tube reproduces the R, G and B signals given to its cathodes in the form of picture with the help of horizontal and vertical sweeps given to its deflection coil assembly.

 

Picture Tube:

          As stated earlier. Precision-In-Line (PIL) Colour picture tubes are used in the colour television receivers being manufactured in our country. These picture tubes have three separate guns which are precision aligned in a line. Each of these guns has the normal electrodes of an ordinary picture tube i.e. cathode, grid, screen and focusing anode besides the final anode (ultor) and a common heater.

 

          All of these electrodes (except the final anode) are brought at the base of the picture tube. The pin connections arid electrode system of a PIL picture tube are shown in Fig.

 

          The amplified colour signals (i.e. R. G and B signals) obtained from the colour signal output stages are given to the respective cathodes of the picture tube through current limiting resistors and suitable d.c. voltages are applied to its electrodes.

 

DC voltages :

          Suitable d.c. voltages are applied to- the electrodes (and heaters) of the picture tube for its proper operation. These are given below:

 

1. Heater :

The heater is usually given an a.c. volts. It heats the cathode which emits the electrons.

2. Cathodes :

The cathodes are maintained at a d.c. voltage of about 120 to 130 Volts. This voltage is applied from the collectors of the respective colour  amplifier transistors along with the signals (direct coupling).

3. Grid:

This is normally grounded.

4. Screen:

A positive voltage of about 500 to 600 volts is required at the screen. The supply to the screen is given through a potentiometer enabling adjustment of the voltage given to the screen.

5. Focusing Anode:

A positive voltage of 5.0 KV is required at the focusing anode. This voltage is given through a focusing pack consisting of a few resist on and a potentiometer forming a voltage divider. The voltage given to the focusing anode can be adjusted with the potentiometer. Best over-all focusing can be obtained by adjusting the voltage given to the. focusing anode.

6. Final Anode : 

High d.c. voltage (EHT) is given to the final anode through a well insulated cable. The EHT voltage is about 25  KV in PIL picture tube.

 

Deflection Assembly:

          The deflection assembly, which is placed on the neck of the picture tube, consists of the deflection yoke and convergence magnet assembly. The deflection yoke consists of the horizontal and vertical coils. The convergence magnet assembly consists of six magnets, two for colour purity adjustment (purity magnets), and two four-pole and two six pole magnets for correction of convergence errors. A typical deflection assembly is shown in fig.

 

Circuit:

          The circuit diagram of picture tube stage used in Korean Colour receiver Model Gold Star is shown in fig. As shown here the circuit of the picture tube stage consists-of the colour picture tube, arrangements for providing proper voltages to its heater and other electrodes, circuit for giving colour output signals to its cathode and spark gaps to protect the colour signal amplifier transistors.

 

          As shown in Fig. the beater voltage is given to the heater pins through a current limiting resistance R4 from a part of the winding of the line output transformer.

 

Defects:

          Defects can occur in the picture tube stage due to defects in the circuits supplying voltages to its various electrodes, defective picture tube, defects in the deflection coil assembly, wrong adjustment of the yoke or wrong adjustment of convergence or purity magnets or due to magnetization of the picture tube. These can lead to the following faults:

1. No raster (sound normal).
2. Very low brilliance.
3. Poor focus.
4. Only a vertical1ine traced on the screen.
5. Only a horizontal line is traced on the screen.
6. Coloured spots.

 

Trouble Shooting:

          Defects can be located in the" picture "tube stage by the normal techniques of checking voltages at different pins of the picture tube checking EHT and testing/replacing suspected components. If felt necessary, this should be followed by adjustment of the deflection coil assembly and adjustment of colour purity and convergence magnets.

 

          The step-by-step procedures of locating faults in ,the picture tube stage are discussed here symptom wise.

 

1. No Raster:

Absence of raster can be due to screen voltage being very low or absent, no EHT or very low EHT, no supply to the heater of the picture tube, open heaters or the picture tube being defective otherwise.

 

          For locating the defect first attempt adjusting the brilliance and contrast controls. If these do not help proceed as given below:

1. Check that the filament of the picture tube is glowing. If it is not glowing, check voltage at the filament pins. If there is no voltage check the heater circuit. If normal voltage is available check the continuity of the heater.
2. If the heater is glowing check the screen voltage. If there is no voltage or the voltage is low try adjusting it with the screen voltage control (R6 in Fig. 14-3). If this does not help check the screen voltage supply circuit (particularly R5, preset R6 and R9).
3. If the heater is glowing and the screen voltage is normal check EHT. If the EHT is low or absent the defect is likely to be in the line output transformer and rectifier assembly.
4. If the filament is not glowing and there is no screen voltage, no focusing voltage and no EHT the defect is likely to be in the line output stage including the transistor and output transformer and rectifier pack.
5. If the filament is glowing but the screen voltage, focusing voltage and EHT all are low this can be due to shorting of some turns -of the line deflection coil or due to defect in the line output transformer or due to defect in the line oscillator or the line output stage. To ascertain the defect-open one end of the line deflection coil. If the voltages become normal on opening one end of the line deflection coil, it is short, otherwise the defect is in line output section.
6. If the filament is glowing normally, the screen grid, and focusing anode voltages and the EHT are normal check the voltages at the cathodes of the picture tube. If all the three cathodes are having high voltage the defect is likely to be in the R, G and B I amplifier stages.

 

2. Very Low Brightness:

          If the brightness on the screen is less than normal and it cannot be corrected by- adjusting the brightness and contrast controls, the effect may be due to low' screen voltage or due to low EHT (or both). In this case check the screen and cathode voltages at the picture tube base and -the EI-IT. If any of these are abnormal check the relevant circuit.

 

          If these are normal, try adjusting the sub-brightness control (if provided). If this also does not help the defect may be due to low emission from the cathodes of the picture tube.

 

3. Poor Focus :

          The focusing in colour picture tubes is more critical than in the black and white picture tubes. If the focus is poor check the voltage at the focusing anode. If it Js somewhat normal try adjusting this voltage with the potentiometer in the focusing pack. If this does not help the defect may be in the potentiometer in the focusing pack.

         

          If there is no voltage at the focusing anode but EHT is normal check the focus pack and the focusing potentiometer.

 

4. Only a bright vertical line is traced on the screen :

          This indicates failure of horizontal (line) sweep. Since light in there on the screen EHT will be there indicating that the line oscillator and the line output stages are working.

 

          The defect can thus be only due to open line deflection coil or open circuit in the associated PCB or wiring etc.

 

5. Only a bright horizontal line is traced on the screen:

          This indicates failure of vertical sweep and can be due to open or short vertical (frame) deflection coil; open circuit in the PCB, broken connecting leads or due to defect in the vertical sweep section.

 

6. Coloured Spots:

          These can be caused due to magnetization of the picture tube or due to wrong adjustment of the colour purity and/or due to wrong adjustment of convergence magnets.

 

          If the picture tube is suspected to be magnetized demagnetize it using a demagnetizer (for details please refer to Chapter 16). If the defect appears to be due to wrong adjust1nent of colour purity or convergence magnets these should be adjusted for details please refer to Chapter 16).

SWEEP SECTION

 

          The functions performed by the sweep section in a colour television receiver are almost the same as in a black and white receiver. This section consists of three main parts:

1. Sync separator
2. Vertical sweep generator, and
3. Horizontal sweep generator.

 

          The functions/of each of these have been discussed in the following paras:

Sync Separator:

          The sync separator separates the sync signals from the composite video signals and further separates the combined sync signals into line and frame horizontal and vertical sync signals which are given to the respective sweep oscillators for synchronizing their' phase and frequency.

 

Vertical Sweep Generator:

          The main functions of-the vertical (frame) sweep generator are to produce oscillations at the vertical sweep rate (50 Hz) and to provide current of saw-tooth waveform to the vertical deflection coil with the help of the oscillations. Flyback pulses for suppressing the vertical retrace lines are also obtained from this sub-section.

 

Horizontal Sweep Generator:

          The main function of the horizontal (line) sweep generator is to provide current of sawtooth waveform at horizontal sweep frequency (15,625 Hz) to the horizontal deflection coil. Several other functions are also performed by this section.

          The horizontal sweep generator can be divided into two parts horizontal (line) oscillator and horizontal (line) output. The functions performed by these sub-sections are given below :

 

Horizontal (line) oscillator:

          The horizontal oscillator which consists of the oscillator, discriminator (for synchronizing the oscillator frequency and phase), pre-amplifier and driver, performs the following functions:

1. Producing oscillations at the horizontal sweep frequency and to synchronies the phase and frequency of these oscillations with the line sync signals.
2. To amplify the sweep signals produced by the oscillator.

 

Horizontal (line) output :

          The horizontal (line) output sub-section consists of two stages-driver and output. Out of these the driver stage amplifies the horizontal sweep signals obtained from the oscillator and drives the output stage. The output stage, with the help of signals obtained from the driver stage, drives current of saw-tooth waveform through the horizontal deflection coil producing horizontal sweep. The following a.c. and d.c. voltages and positive and negative flyback pulses are also obtained from this stage.

1. AC voltage of 6.3 volts for heating the filament of the picture tube.
2. DC voltage at about 12 volts for supplying the IC's and some low level transistorized stages.
3. DC voltage at about 200 volts for the high level stages such as R, G and B amplifiers:
4. DC voltage of about 500 volts for supplying to the screen (G2) of the picture tube.
5. DC voltage of about -5.0KV for giving to the focusing anode of the picture tube.
6. A high voltage d.c, at about 25 KV (EHT) for giving to the final anode of the picture tube.
7. Positive flyback pulses for synchronizing the line sweep oscillator and negative flyback pulses for suppressing the horizontal retrace lines on the picture tube screen.

          A special feature of the line sweep section of some of the colour television receivers is an arrangement to prevent the EHT becoming excessively high, which may result in dangerous X-ray radiations from the picture tube. This arrangement basically consists of some system which compares a part of the line output signal with a reference voltage and if the level of the line output signals exceeds a pre-determined level, this arrangement either blocks the line driver thus switching off the line sweep (fail-safe) or reduces the supply voltage thus reducing the EHT.

 

Circuits:

          Two different circuit arrangements are normally used in the sweep sections of colour TV receivers. In the first-the functions of sync separation, producing vertical oscillations, producing horizontal oscillations, their synchronizing and pre-amplification are combined. An IC like TA 7609 P being used for these functions. In this arrangement separate driver and output stages are used for both vertical and horizontal sweeps.

 

          In the second arrangement the vertical and the horizontal sweep sections are separate. In these an IC like TDA 1870is used for producing the vertical sweep. Another IC like TDA 1940 is used for sync separation, for producing line frequency oscillations, their phase locking and pre-amplification. This is followed by-line driver and output stages (similar' arrangement is also used very widely in B & W TV receivers).

 

          The circuit diagram of complete sweep section used in a colour TV receiver (Korean TV receiver Model Samsung) is shown in fig. and its simplified block schematic diagram is shown in, fig. 1-5-2.The IC TA 7609 P used in this section combines the functions of sync separator, vertical oscillator, horizontal oscillator discriminator and  driver. Three transistors have been used in the vertical driver and one output stages. Two transistors one as driver and one as output have been used in the horizontal output section. Brief description of this circuit follows:

 

          Composite video signals obtained from the buffer video amplifier are given to terminal J6 of the IC through a circuit formed by the condensers CI and C2, resistor R1 and the diode D1. The sync signals are separated from these by the sync separation section in the IC. The horizontal sync signals are given internal) Y to the phase detector (for synchronizing the line oscillator). The sync signals also appear at terminal 14 of the IC. The vertical sync signals are separated from these signals by the integrating circuit formed by resistors R4 to R6 .and condensers C4 to C6. The vertical sync signals so obtained are given to pin 12 of the IC for synchronizing the vertical oscillator.

 

          The vertical sweep generator part of the IC, which consists of the vertical oscillator, ramp generator and the driver produces oscillations at the vertical sweep rate, converts them into suitable waveform and amplifies these. These appear as terminal 7 of the IC from where these are given to the base of the vertical driver transistor Q1 (2SC- 2229). This transistor drive output transistors Q2 and Q3 (2SC2073 and 2SA940 respectively) which operate in complementary symmetry push-pull circuit the output is given to the vertical deflection coil through the condenser C13.

 

          The free running frequency of the vertical oscillator is controlled by the potentiometer R30 (V hold). The supply voltage given to the vertical oscillator (term. 10) can be adjusted with this potentiometer thus controlling the oscillator frequency. The picture height is controlled by the potentiometer R II (Height control). This controls the negative feedback given to the ramp generator (term. 9 of the IC) thus permitting control of the amplitude of oscillations.

 

          The horizontal (line) oscillator produces oscillations at double the horizontal frequency. The oscillator frequency is compared with the sync signals by the phase detector and maintained in synchronization.

 

          The frequency produced by the oscillator is divided to half by the flip-flop thus obtaining the desired frequency signals. These are given to the horizontal driver in the IC. The output of the driver appears at term. 4 of the lC from where it is given to the driver transistor Q5 through L2 and R53. This transistor which operates in transformer coupled circuit drives the horizontal output stage. The horizontal (line) output stage which operates in normal circuit provides the line sweep signals which are given from terminal 10 of the line output transformer to the line deflection coil through the primary winding of the side DPC transformer.

 

          Suitable voltage for heating the picture tube filament, various d.c. supplies and positive and negative flyback pulses are also obtained from this stage as given below:

1. Suitable voltage for supplying to the heater of the picture tube is obtained from terminal 4 of the horizontal output transformer. This is given to the heater of the picture tube through a series resistor (this resistor is part of picture tube circuit and is not shown here).
2. DC voltage of 16.5 volts for supply to IC's and some low level transistorized stages is obtained by rectifying the pulses obtained at the terminal 2 of the horizontal output transformer, by diode D14. Condenser C3J has been used for smoothing it.
3.  DC voltage of about 200 volts for supplying to the high level stages (RGB amplifiers) is obtained by rectifying the pulses obtained at term 9 of the horizontal output transformer by the diode D 15. Condenser C35 has been used for smoothing it.
4. DC voltage of about 500 to 600 volts is obtained by the rectification of flyback pulses obtained at the collector of the output transistor by the damper diode D17. Condenser C 44 has been used to smooth it. This voltage is supplied to the screen (G2) of the picture tube.
5. DC voltage of about 5KV is obtained by rectifying the pulses obtained over a part of the secondary winding of the horizontal output transformer. This is supplied to the focusing anode of the picture tube through the focus pack.
6. The EHT voltage (25KV) is obtained from the secondary winding, four diodes being used over the sectionalized winding for rectification.

 

Defects:

          Defects in the sweep section can lead to a large number of faults. The defects likely in the three main parts have been discussed here separately.

 

Sync Separator:

          As already discussed, sync separator separates the sync signals from the composite video signals and then separates the sync signals into vertical and horizontal sync signals. A defect in the sync separator can cause the following defects:

1. Vertical instability :

The picture will roll up or down.

2. Horizontal instability:

The picture may be torn apart or odd slanting lines may appear instead of the picture.

3. Both horizontal and vertical instability:

The picture may be torn apart and may also be rolling up or down.

 

Vertical sweep generator :

1. Absence of vertical sweep:

Only a bright horizontal line is traced on the screen.

2. Vertical Rolling:

The picture is not stable and it moves up or down at a slow or rapid rate.

3. Reduced Height:

The picture does not cover the full height or the screen.

4. Vertical non – litearity:

The picture distribution is not union form in the vertical direction. The picture is cramped at the bottom or top.

 

Horizontal sweep generator:

          Defects in the horizontal sweep generator can lead to one or more of the following defects.

1. Dead receiver:

The receiver does not operate-there is no sound, no raster and no light on screen.

2. Horizontal instability:

The picture is unstable in the horizontal direction-the picture is slanted or some odd lines appear on the screen instead of the picture.

3. No light on the screen-sound normal-

There is no raster and no light on the screen but sound is normal.

4. Horizontal nonlinearity:

The picture distribution is not uniform in the horizontal direction.

5. Wrong phasing of the pictureL

Horizontal blanking pulse is visible on the screen.

 

Trouble Shooting:

          Defects in the sweep section can occur due to defects in either of the sub-sections and the first step is to locate the defective sub section on the basis of the fault symptoms as given above.

 

          The next step (considering circuit given in :fig. IS-I) is to check voltages at various pins of the IC. If any of the voltage is found to be abnormal, the relevant circuit components. should be checked followed by replacing the IC (if the components test OK).

          If the voltages at pins of the IC are normal voltages should be checked on the terminals of the transistors in the suspected sub-section.

         

          Normal d.c. voltages at the terminals of the IC and the terminals of the transistors arc given in tables 15-1 and 15-2.