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KURUKSHETRA INSTITUE OF TECHNOLOGY & MANAGEMENT, KURUKSHETRA

LABORATORY MANUAL

EXPERIMENT NO. : ECE-AV--EX-01 ISSUE NO. : 04 ISSUE DATE :

REV. NO. : REV. DATE : - PAGE: 01/12

DEPTT. ELECTRONICS &

COMMUNICATION DEPARTMENT

LABORATORY :- AUDIO VISUAL ELECTRONICS

( ECE-410E)

SEMESTER : VIII

Object:- Familiarization of PCBs and Mechanical Components of Tape recorder/ CDPlayer/ VCD Player/ Colour T.V.

Apparatus required: - Tape recorder demonstrator, VCD demonstrator, Colour TVDemonstrator Kit.

Introduction of tape recorder:

To store information for long time is called 'recording'. To record this information a'medium' is required which is audible or visual. The first attempt is made successfully torecordIreproduce the sound was 'phoI10graph' later called 'gramophone', where the sound

vibrations are grooved upon a wax plate rotating at constant velocity. After short time,first sound recording on steel wire accomplished by 'Vladimir Paulson'. He use a pianowire to record a short message using _an electromagnet. But the medium 'steel wire' hasmany draw backs, hence does not adopted. Later 'G. Pelumore invents a plastic basecoated with iron particles, for magnetic recording. This medium was very successful since,it can be stored easily and the recorded messages have good quality. Few year laterinvention of poly vinayl chloride ( P.V.C.) solve whole problem of base for magneticmaterial coating, which-can be stored in form of rolled spools and it is called 'TAPE'.Later the Dutch company, Philips introduce first compact version of tape called 'cassette'.The standards established by the company are unchanged till date. Tape recorders used infield of entertainment, education, communication and personal requirements. The

principle of tape recorder also adopted in visual recording such, as video cassette recorder.

To understand the process of recording or reproducing of sound in tape recorder, first weshould know few basics described in following texts.

1. Sound:

Sound is completely 'mechanical waves' created by vibrations, does not have any'electrical energy'. There is a 'medium' required to travel these sound waves such, as air,water, woodor metal. It can't travel in vacuum. Since it requires a 'medium' to travel, this

brings its power down. Therefore it can't travel far. Its maximum velocity 332 m/sec in airwhile, in other mediums it lost its velocity. Sound waves can be converted in electricalsignals and vice - versa, using 'transducers'. Audible signal frequency for human, lye

between 16 Hz to 20 KHz. Below it the waves called 'infrasonic' and above it 'ultrasonic'.senergy involve vibrations. When this energy hit any flexible medium it producesvibrations on its surface.

2. Energy conversion 'the transducers:'

Sound is in form of 'mechanical energy involve vibrations .When this energy hit anyflexible medium, it produces vibrations on its surface.These vibrations are specified inthree terms called 'amplitude', 'frequency' and 'rate of change of frequency.. Thetransducer should be able to fulfill these conditions. To convert sound energy in electricalenergy the transducer used called 'microphone' in brief 'MIC'. To record these convertedelectrical signals upon magnetic surface( tape) another tranducer used called recordhead.To regenerate the sound this recorded magnetic surface(tape) passed throughtranducer called playback head.Finally regenerated electrical singals converted back to

sound using another transducer called loudspeaker Therefore four tranducer involved in tape recorders system.


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3. Microphone:

This transducer converts sound waves into electrical signals. There are many types ofmicrophones available. In tape recorders system two types used, one is dynamicmicrophone and other is condenser microphone. To cover a speech dynamic types are the

best as you have seen in many programmed, where it is called the 'stage mic'. microphoneconverts the sound energy into electrical energy (signals) corresponding with amplitude.

The last. Transducer used in tape recorder is loudspeaker. Its working is similar tomicrophone but its size is larger than that and it produce sound energy converted formelectrical energy (signals). former, suspended in gap around the pole piece.

The lower portion of it connected with yoke via flexible link called spider. When varying electricalcurrent given to its coil, the magnetic field generated across it interacts with the field established bypermanent magnet. It create vibrations which are transferred mechanically to the cone, hence insurrounding air in form of audible sound. Due to the applications in tape recorders such speakersused which produce good sound in mid frequency range.

4. Record I playback head:

Head is a transducer which converts electrical energy (signals) into magnetic energy.Conversely the head converts Ganging magnetic energy into varying electrical energy.Therefore it is an electromagnet. In tape recorders single unit 'RIP head' done both the jobs.Head constructed with laminated (alloy called alfinol) strips formed in ring shape, wherethe 'gap' at the ends is very small. This 'gap' usually kept 0.025 mm in mono heads and lessthan .02 mm in stereo heads. the high frequency response depends upon the 'gap'. Someheads use double C shaped core which has two 'gaps'. The rear one is but welded and front'gap' filled with silicone dioxide layer. These heads called 'super per molly sand glass head,

produce excellent frequency response.

5.Loudspeaker:

6 Magnetic tape (Cassette)

The magnetic tape is the content, of a tape-recorded which store the information. In modern systems the form of magnetic

tape is housed in a plastic case called 'cassette'. The base material of tape is P. V. C, polymer or cellulose acetate ribbon,

having thickness around O.05mm. Its width is 3.8mm, coated uniformly with ferric oxides fine particles, having thickness

about .012mm. The tape rolled over two toothed wheel$ called 'spool', housed in molded plastic housing. The tape travelupon two roller guide which make alignment in proper way.


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Tape transport mechanism

3.FlywheelIt is a casted wheel (alloy of aluminium and cast materials) balanced around its axle (shaft) made of steel. One end of this s

.

3.FlywheelIt is a casted wheel (alloy of aluminium and cast materials) balanced around its axle (shaft) made of steel. One end of this shaft

rotate upon plastic bush mounted upon support plate, and other end, has enough length to fit in the cassette opening.

The long portion of shaft is called 'capstan', which play major role in tape recorder. The flywheel maintains drive smooth

to reduce flutter.

To store (record) or to reproduce (play) information, a mechanical system required to move the magnetic tape. Since

the recorded or reproduced information will be quite clear and faithful, if the tape moves at constant speed. Also there

must -be arrangement to move tape faster if required. The mechanical assembly to perform this operation is called tape

transport mechanism'. There are main parts of these assemblies are follows

4.PinchrollerIt is a rubber mounted small wheel, which is forced by a tension spring towards capstan. It is attached with capstan

when playback or record mode used. In high speed rewind or fast - forward and pause modes it is kept away from

capstan. The tape lye between the capstan and pinch roller. When capstan moves the pinch roller also moves, due tomechanical contact. The direction of rotation oppose cause to pull tape at uniform speed. If pressure of pinch roller is

more or less, then !ape start slipping, hence sound quality will be degraded. To correct it a tension spring is used.

1 The drive motorIt is a machine, which converts electrical energy into kinetic energy lost modern tape recorders

used 'permanent magnet field armature controlled dc motor' in brief PM DC motor. these motors are rated for 6, 9 or

12V and 2 to 4 watt power. The motor has three segment pole commutator with a set of carbon - graphite brush. Most

motors govemed by an electronic circuit which control its speed to maintain constant RPM.

2. Rubber belt:The mechanical (kinetic) power generated by motor, transferred to the rest of mechanism (toflywheel) through a rubber belt. The

belt has many sizes and shapes, such in present system it is a flat belt.


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The RF stage:ignalAfter antenna, the front end of receiver is similar to radio, a superhetrodyned tuned stage called the

Tuner._A tuner has local oscillator, RF amplifier and mixer stages with set of coils which are tuned toreceive the desired signal, while rejecting the others. These may be mechanical version or electronicallytuned by mean of varactor diodes. The prime aim of tuner is to receive the transmitted (amplitudemodulated) signal from antenna, to amplify it and to create an intermediate frequency signal byheterodyning this signal with locally generate RF s.

Since the subject is related with CN receiver, which use an electronically tuning system we will describe itonly. The electronic tuner has input RF stage with tuned circuit and selection system between set ofchannels called band, namely VHF -1, VHF - 3 and UHF, abb as VL, VH & UH. The band selection ismade by selecting the appropriate set of tuned Circuits applying the control voltage\ (dc voltage) to pindiodes. The channel selection is done by a dc varying voltage Tuning voltage applied to varactor diodeswhich vary their capacitance hence the variation in local oscillator frequency. The two frequencies RF input

and local oscillator frequency is mixed in third stage where it generate two frequencies either sum or adifference. A tuned circuit having resonate frequency equal to difference frequency select the lowest onewhile the sum frequency is rejected. The output of this tuned circuit is called IF signal. The TUNERelectronic circuit works on +12Vdc supply applied to pin marked MS. The band selection is madeby applying +12Vdc to pin L (VHF 1 for ch 2 - 5), +12Vdc to H (VHF - 3 for ch 6 -12) or'+12VdctoU (UHF fork 23 -69 in simple and 21 - 83 in extended range tuners).ln modern tuners, the VHFrange is extended as S band range. Forchannel selection 933Vdc applied to varactor diode at pin T. To correct the amplification factorAGC voltage fed to RF stag- via pin AGC. Similarly for accurate tracking of oscillator AFT voltagefed to local oscillator via pin AFT The cover (CAN) is solidly grounded with ground track of PCB.


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Colour T.V:The TV Receiver:The transmitted TV signal which include the 'Picture (vision information) and sound (audio information) isreceived at TV receiving antenna. Since the telecasted signal is complex in nature it is not possible todemodulate it in simple manner just' as in radio receivers. A TV signal has information of picture elementsand audio information.- The picture signal has complex nature which includes the point to point detailcalled pixel,_the colour information, and synchronization signal which is required to reproduce the pictureas it was (seen) scanned at studio. Thus a TV receiver includes more signal processing stages than a radioreceiver. Already given in text books, let it should be detailed briefly stage by stage in following texts.

Circuit description:Tuner operate at +12Vdc applied at MS, through jumper L4S. The +12V obtained from horizontal section, and operates

this section. Transistors QA02, QA03, QA04 are used to select the required band of channels. To switch these

transistors control signals are generated by system controllC901. +33Vdc obtained from +1 00 Volt supply, used for

tuning purpose. Tuning is done

by varying voltages at TU, by IC901 and associated components around Q903.To kept proper tracking AFT voltage,

and for proper gain AGC voltage, obtained from IC7680.A sample of AFT voltage send to IC901, through Q910A, which

stores it in memory. The output signal is available at IF.


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Colour TV

The TV Receiver:

The transmitted TV signal which include the 'Picture (vision information) and sound (audio information) is received at TV

receiving antenna. Since the telecasted signal is complex in nature it is not possible to demodulate it in simple manner just' as in

radio receivers. A TV signal has information of picture elements and audio information.- The picture signal has complex nature

which includes the point to point detail called pixel,_the colour information, and synchronization signal which is required to

reproduce the picture as it was (seen) scanned at studio. Thus a TV receiver includes more signal processing stages than a radio

receiver. Already given in text books, let it should be detailed briefly stage by stage in following texts.

The RF stage:

After antenna, the front end of receiver is similar to radio, a superhetrodyned tuned stage called the Tuner._A tuner has local

oscillator, RF amplifier and mixer stages with set of coils which are tuned to receive the desired signal, while rejecting the others.These may be mechanical version or electronically tuned by mean of varactor diodes. The prime aim of tuner is to receive the

transmitted (amplitude modulated) signal from antenna, to amplify it and to create an intermediate frequency signal by heterodyning

this signal with locally generate RF signal.


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Circuit description:

Tuner operate at +12Vdc applied at MS, through jumper L4S. The +12V obtained from horizontal section, and operates

this section. Transistors QA02, QA03, QA04 are used to select the required band of channels. To switch these

transistors control signals are generated by system controllC901. +33Vdc obtained from +1 00 Volt supply, used for

tuning purpose. Tuning is done

by varying voltages at TU, by IC901 and associated components around Q903.To kept proper tracking AFT voltage,

and for proper gain AGC voltage, obtained from IC7680.A sample of AFT voltage send to IC901, through Q910A, whichstores it in memory. The output signal is available at IF.

The RF IF section:1. The tuner used to receive, amplify input RF signals, heterodyne them with high frequency locallygenerated signal to produce constant frequency signal PSIF (picture intermediate frequency), in shortwritten as I. F.2. It is required to preamplifier this I. F signal (T161) due to insertion loss Of SAW filter.3. The SAW filter (21 01) used to trap the adjacent frequencies in I.F. signal. It may be treated as bandpass filter.4. L 151, used to match picture and sound signals, within pass band.5. IC1 01 T A7680 incor orate followin functions.


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2. The photo diode array: The electrical signal (information) is generated by photo diodes at reflectedbead from CD surface. The photo diode has four diode namely A to 0, where A,C and B,D are grouped andcontain information of data and focus signals, where DC voltage generated by tracking signals. It is shown,that due to cylindrical lens mounted upon photo diodes, which produce ellipse if light is not centred, calledthe astigmatism effect, the reflected beam is centered by Focus and tracking arrangement for proper outputfrom photo diode pair, _The signal output has two antiphase signals, from which amplitude detection(A_C,&


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4.Loading System in CD

player(front londing):It is asingle unit , composed of plastic frame,disc holder tray,(tray

drive motor) loading motor,and gear assembly, drive belt and limit switch.The optical

unit with motors mounted upon chassis,is suspended in this frame.At the top side a free

run magnetized drum in form of iron disc,is placed in centre of drum holder.When PEN

button or command feed to the system,the loading motor servo feed dc voltage to this

loading motor.Mechanical power of this motor transferred by belt, to gear assembly

which move tray outside by a rack molded inside tray. At same time a cam system release

the chassis of optical unit, to release the CD.

1. Optical pick up assembly: Recorded digital signals (fig 3a), at CD surface is scanned (read) by alaser system composed of, laser diode (AIGaAs type), optical lens etc as shown. The diode laser beamsemit mono chromatic light of 780'nrn wavelength, 0.12mW, is differentiated by diffraction grating mounted atlaser Diode assembly. The differentiated beam is reflected by glass plate coated with polarizing< material. A

set of collimating lens convert tl1 isaifferentiated beam in parallel beam. The incident beam is focused up to1.5/lmil with objective lens upon CD surface. To focus it electromagnetic focus system used.In focus system, the objective lens mount is made of plastic, has a coil is wound around it, which is excitedwith current. Two permanent magnets fixed aside, produce uniform magnetic field around this coil. Whenexcited by dc current, the generated magnetic lines cut the field of magnetic field, and coil displacement isobtained which is suspended upon a pair of leaf springs. the movement of lens is up - down depends uponpolarity of excitation current. The current to this coil is supplied by focus servo system. The movement ofobjective lens is 4m from 'its rest position. There is another pair coils wound, axially upon objective lensmount. These called tracking coils. Excited by dC,-, current it pulls objective lens either side from centre. It'scurrent controlled by tracking servo system. The movement of objective lens is 0.1 mm in each directionfrom its rest position.

The mechanical system of a CD player can be divided in 4 parts, which are described in following texts.

Apparatus required: - VCD demonstrator kit.

Objective:- Study of Mechanical system of VCD Player.

Mechanical system in CD player:

3. The carriage' system: Carriage system is optical pickup assembly, displacement procedure. The

optical unit fitted upon a slide system having worm and gear system drive by a small. PMDC motor called


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5. The disc rotatingsystem: The CD rotate at constant linear velocity by another motor called the 'discdrive motor' or 'spindle motor'. This motor ac voltage controlled by disc motor servo to maintain its speedaccording to data flow in half filled memory. This motor has a long 20mm, spindle, holding a round ironplate to hold CD, called 'disc holder' fitted in optical unit chassis. When chassis: lifted up the CD, holdbetween the disc holder and free run magnetized drum. The proper adjustments of disc holder ensure theCO rotate without any obstruction.

6. The control and display: The operating controls called PSC, (playback control) accessible either byremote control or by selected function of keys. The signal (command) from this board is in form of serialdata designated as IR, goes to microprocessor in MPEG decoder. The system (data time) information,status, track is displayed upon - CE panel. The signals from microprocessor to display designated as 01.The control panel and display work on 5Vdc supply, where remote on 3V (two dry cells).

7. The power supply: The power to operate the CD player, obtained from stepped down ac (10 -0 10V),

rectified, filtered, regulated to obtain by dc, for servo system and 5 Vdc for rest of the system. There are

two identical, 3.3Vdc transistor regulated supply used for memory built in MPEG card.

Result:- The mechanical parts of VCD player is studied.


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Objective: - Study of Sound Processing of VCD Player.

The test points (TP) and signal observation:

Apparatus required: - VCD demonstrator kit.

To study of CD player signal observations, the block diagram shown is helpful. The laser diode is excited bya signal LOON at TP7, and from reflected light, data read by photo diode array is available as twoantiphase signals A+C, 8+0, respectively at TP8 and TP11. The obtained signals are amplified equally, andmplitude and phase compared inside IC 1, to generate focus correction voltage as, FCS. After this process,dc level of this signal compared and tracking voltage generated as, TCS. These are dc voltages and not

given for test purpose 'since its observation time is, 2-4sec not for CRO purpose.


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..


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s

Objective: To study TUNER stage functions of colour TV.

Other apparatus: CRG (wideband is better), DMM and if available a sweep/marker generatorTheory:- An electronic attenuate controlled by IC901, bring attenuated sound signal at TP46, where it goesfor IC601 for further amplification and to drive loudspeaker. The AM detected output is again send to a phase

discriminator which has another tuned circuit L 152, C 17 4, 175 and 176 which are the parts connected

across the video detector coil L 151 and IC pin 16 and 19.The phase discriminator output which has a peakvoltage output vis the input signal strength is obtained at pin 13 of TA 7680. This de voltage is send to the

tuner oscillator to track and lock the local oscillator frequency for maximum strong signal. It is similar to a

phase lock. Loop circuit which automatically tracks the VCO for match the correct phases hence the accurate

The amplified signal is then routed to IC2, to decode (demodulate) 14bit symbol into 8bit word. This processcalled 'eight to fourteen bit demodulation' and generated signals are EFM decoded signals at TP9 and TP10 respectively. It is an important signal for further process, since it contains data bytes, control bits andsynchronization bits. A part of it send to clock signal generation circuit inside IC4 , for synchronization oftiming signals, to produce status signals DT for display. Other part of it after integration sends inside IC2.

Result:- The sound processing unit of VCD PIayer is studied.


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input - output frequency tracking is possible. The AM detected signal does also used to generate AGC

voltage controlled by R 151 AGC preset, for internal VIF and RF.

The IF stage description:

The heterodyned signal from the tuner I.F. is passed through a transistor amplifier Q 161, which has a voltage

gain within 12 dB approx. The combination of R165 and L 162, with current feedback gives a wideband

operation to boost the signals within 7 MHz bandwidth. The necessity of this stage is due to introduction of

SAW filter ahead which has insertion loss of -10 dB. The SAW filter (surface acoustic wave filter) is a multiple

ceramic resonator which is designed to attenuate and pass the required frequencies signals thus we obtain a

required pass band signal within the bandwidth of 33.4 - 38.9 MHz. The output of SAW filter is send to the

video I. F. amplifier stage of T A 7680 (pin 7 & 8) which ~ differential input with a tuned circuit across it. Thefunction of this tuned circuit (L 102, called PSF match coil and R101) is to match the picture and sound

signal. The gain of video I.F. amplifier is nearly60dB.The AM, detected signal called CCVS (composite colour

video signal, with 5.5 MHz sound intercarrier signal), is passed through a low pass filter LOS, R202 and

available at TP43.The sound take off is performed by ceramic resonator Z601 coupled with TP43 through

C602A After passing Z601, sound intercarrier available at TP42, the FM detector input. L651 serve FM

detector coil and demodulated sound (AF signals) available at TP44 after amplification.

6 IF

6 IFPhotograph of Electronic tuner used in CTV receiver

U: for UHF band selection BU, UB, UH

or U.

T: For tuning input voltage VT, TU or

T.

H: for VHF-3 band selection BH, HB, VH


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6 IF


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Result: - The tuner stage funct:on~olour TV a

Objective: Study of VIF section of CTV.

Other apparatus: CRO (wideband is better), DMM.

Theory:

Functional detail of TA 7680.1. It is associated with electronic sound atlenuator section. Applyingdc voltage between 0 - 6 volt controlsthe sound amplification level hence the sound output.

2. It is associated with audio preamplifier and serves as bias with frequency compensation. The network

connected with this pin determines the frequency response. There are 2.5 - 3 volt dc appears at this pin.

3. It is the audio signal output pin. The signal obtained at this pin is send to power amplifier. The dc voltage

at this pin is nearly 7 volts depend upon R 605 the emitter pull down resistor.

4. It is gmund pin connected with substrate.

5. Trds pin associated with video amplifier section. The R - C network connected with this pin bias pin. The

eic potential is about 3 volt.

6 & 9 These pills are termination associated with differential input of video IF amplifier. The high frequency

gain compensation is made externally by C 102 connected between these pins. The dc voltage level is

nearly 4.5 volt at both pins.

7& 8. These pins are input of video differential amplifier to accept the input IF signa! from SAW filter. The dc

voltage level is nearly 4.8 volt.

1C. it is the part of RF AGC delay section. It is connected with wiper arm or AGC set control R151.

. The dc voltage adjusted to this input is set between 4.5 to 5 volt depends upon the tuner. Capacitor C 103

bypasses the H.F noise. .

11. It is RF delayed AGC output pin. A varying dc voltage between 3 - 8 volt obtained here depends upon

the Tuner and dc level of pin 10. Capacitor C 104 filters the high frequency noise.

12. It is -ve in put, hence groun ded.

13. It is AFT voltage output termination and de correction voltage obtained here is send to tuner to track the

oscillator. The dc voltage present here depends upon the input signal and discriminator output varies

between 5 - 9 volt.

14. It is AFT amplifier bypass pin grounded by C 172 (4 - 6V).

15. Emitter follower output termination for composite video signal output. The pull down resistor is R202,

having dc voltage nearly 3 - 4 volt depends upon strength of video signal.

16 & 19.These are phase discriminator input pins of AFT, connected with AFT tank circuit L 152. The dc

voltage is 4.2 volt.17 & 18. These are video detector tuned circuit input pins connected with L 151. Part of this signal is

coupled to AFT coil through C 175, 176 to generate the phase shift signal required in phase discrimination.

DC voltage about 7.4 V.

1. Unplug the input ANT. Connect DMM, -ve lead with ground and measure voltage at jumper L4B_(RF

section),measure dc voltage and note as MB ( 11- 1

2V).

2. Connect DMM, with TP48 (RF section). Note the dc voltage here. If it is zero (in order of mV), the VL band is not

selected.

3. Select AUTOTUNE function by MENU and note the screen show function, where TP48 voltage is +11 V approx.

4. Now connect DMM with TPSO (RF section) and note it is OV, which is switch over to +11 V as soon as VH selected

by auto tune function.5. When UHF function selected by auto tune, connect DMM with TPS1 (RF section), and note the dc volt variation

progressive in +ve voltage direction as tuning advance.

6. Now connect the ANT with input and again select auto tune function.

7. Connect DMM with TP39 (system control section) and note the voltage variation (0.5 - 4V) as tuning progress.

8. After complete the auto tune function, if signal received connect CRO at TP47 (RF section) and observe the signal

there (see wave shape).

9. If sweep/marker generator available connect it with antenna and select auto tune function connect CRO with TP47,

and as soon as signal obtained at screen, select mode FINETUNE. Adjust fine tune to obtain the resOonse curve.


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20. It is positive supply pin connected with +12 volt dc with appropriate filtration network.

21. It is sound IF amplifier input pin having dc voltage level 4 volts.

22 & 24. These are sound discriminator tuned circuit pins connected with FM discriminator coil L 651. The

dc voltage level is 4.5 V.

23. It is part ofFM demodulator output circuit termination. The dc level at this pin is nearly 5 volt

Procedure


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1. Tune CTV receiver for clear signal received through antenna.2, Connect CRO at TP43, w.r.t ground, to observe the CCVS signal (see fig 6).3. Examine the waveform to identify sync, burst and vision signals. Since this signal has all the informationrequired to produce vision and sound, it is called composite colour video signal with sound intercarrier,4. Now connect CRO with TP42. The signal observed (fig 8) is called sound intercarrier signal. Its frequency is5.5 MHz.5. Select other programmer, for no signal. Check the voltages at TP45 and TP39. Note the difference betweentwo measurements.

Objective: Study of sound section of CTV.

Other apparatus: CRO (wideband is better), DMM.

1. It is associated with ele.ctronic sound attenuator section. Applying dc voltage between 0 - 6 volt controlsthe sound amplification level hence the sound output.2. It is associated with audio preamplifier and serves as bias with frequency compensation. The networkconnected with this pin determines the frequency response. There are 2.5 - 3 volt dc appears at this pin.3. It is the audio signal output pin. The signal obtained at this pin is send to power amplifier. The dc voltageat this pin is nearly 7 volts depend upon R 605 the emitter pull down resistor.4. It is ground pin connected with substrate.

5. This pin associated with video ampJifier section. The R - C network connected with this pin bias pin. Thedc potential is about 3 volt.6 & 9. These pills are termination associated with differential input of video IF amplifier. The high frequencygain compensation is made extemally by C 102 connected between these pins. The dc voltage level isnearly 4.5 volt at both pins.7& 8. These pins are input of video differential amplifier to accept the input IF signal from SAW filter. The dcvoltage level is nearly 4.8 volt.10. It is the part of RF AGC delay section. It is connected with wiper arm of AGC set control R151. The dcvoltage adjusted to this input is set between 4.5 to 5 volt depends upon the tuner. Capacitor C 103bypasses the H.F noise.11. It is RF delayed AGC output pin. A varying dc voltage between 3 - 8 volt obtained here depends uponthe Tuner and dc level of pin 10. Capacitor C 104 filters the high frequency noise. 12. It is -ve input, hencegrounded.13. It is AFT voltage output termination and dc correction voltage obtained here is send to tuner to track theoscillator. The dc voltage present here depends upon the input signal and discriminator output variesbetween 5 - 9 volt.14. It is AFT amplifier bypass pin grounded by C 172 (4 - 6V).15. Emitter follower output termination for composite video signal output. The pull down resistor is R202,having dc voltage nearly 3 - 4 volt depends upon strength of video signal.16 & 19.These are phase discriminator input pins of AFT, connected with AFT tank circuit L 152. The dcvoltage is 4.2 volt.17 & 18. These are video detector tuned circuit input pins connected with L 151. Part of this signal iscoupled to AFT coil through C 175, 176 to generate the phase shift signal required in phase discrimination.DC voltage about 7.4 V.20. It is positive supply pin connected with +12 volt dc with appropriate filtration network.

21. It is sound IF amplifier input pin having dc voltage level 4 volts.22 & 24. These are sound discriminator tuned circuit pins connected with FM discriminator coil L 651. Thedc voltage level is 4.5 V.23. It is part ofFM demodulator output circuit tennination. The dc level at this pin is nearly 5 volt


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1. Connect CRO at TP44 to observe the demodulated and amplified sound signal.2. Connect CRO with TP46, to observe the attenuator controlled sound signal. Vary the volume set up.-andobserve this signal amplitude increaseIdecrease with volume setting.3. Measure dc voltage (about + 7V) at TP45 (AGC).4. Measure dc voltage (about 2 - 3V) at TP39 (AFT).5. Select other programmer, for no signal. Check the voltages at TP45 and TP39. Note the difference

between two measurements.

Procedure


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Objective :- Study of chroma section of CTV.

Others apparatus: is CRO (wide band is better) DMM

The luminance and chrominance section:

Processing the CCVS, refer block diagram and circuit diagram, we start from the signal after passing through sound

trap. The luminance signal (Y): The representation of full colour picture upon screen is not possible without the

luminance or black and white images. The Y +C signal is takeoff from the sound trap (Z201, L201 ).directly and passed

through a buffer amplifier at pin 39 Of iC501. the output of this amplifier (pin40) is fed to luminous delay line (W201 ),

through an attenuators control which is called 'contrast control'. As shown in block diagram the luminous and chroma

signals are separately routed, where the .chroma section has longer path than luminous. To made equilibrium between

both, a lumped parameter L - C delay line (W201) inserted between the final luminous amplifier (pin3) and buffer (pin42).

The time delay produced by luminous delay is equal to one H line or64 uSee. The signal from delay line is than boosted

to required level to drive the video amplifier transistors. The de voltage level of Y output amplifier decide the brightnessof picture, therefore brightness control is connected with this circuit. An adder circuit (0202, 203,204) is inserted between

the video amplifiers and Y amplifier to add the vertical and horizontal blanking signals with Y signal. This blanking added

Y signal is fed to the emitter of video output transistors while collectors of them is connected with cathodes of CRT Since

these are NPN transistors therefore the Y amplifier output has ve going polarity signals. Thus higher the Y signal

amplitude higher will be intensity of the CRT. The blanking pulse- are positive going thus it cut - off the cathodes during

blanking periods. The chrominance section (C). The CCVS signal taken from

sound trap is inverted, amplified (pin40) and passed through a narrow band pass filter (C501, L501 ,C502) having comer

frequencies between +O.5Mhz, while the centre frequency is 4.43 MHz colour sub carrier frequency. The band pass

filter extracts the chrominance information. This signal is raised by first amplifier called 'chroma Band pass amplifier'.


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Obejective: To study the electronic portion of tape recorder.

Apparatus Required: tape recorder kit,CRO,Connecting probes.

Theory:The electronics circuitry:

In this section, the function details of tape recording/playback and transportSystem is studied.The signals either obtained from microphone for recording,orfrom playhead to listen has very low intensities.Therefore amplification is requiredto perform the satisfactory response.basically the systemis shown in followingblock diagram.

The output is sending to burst amplifier also called the burst gate. This stage is also a tuned voltage amplifier (L552,

C512; C513) with biased near cut - off level. The burst gate has one other input taken from the Horizontal output stage

as keying pulses. This pulse is required to confirm that burst gate open during peak fly back period only since the colour

sub carrier as burst signal is send in this period as in fig. The output of burst gate is send to ACC (automatic colour

control) detector circuit (via C511) which feedback part of this signal back to band pass amplifier. The working of ACC.

detector stage is similar to AGC circuit which controls the I.F amplifier gain. The signals from ACC detector output is

send through 'killer identity detector' stage. This stage incorporated in all colour televisions. The aim of this stage is to

kept off the chroma gain control amplifier when no burst received in received signal. It is a threshold detector stage

where it compares the threshold level with the input signal. The presence of burst in signal ensures that the level is

completed thus killer detector bias the chroma gain control amplifier; otherwise it will kill the gain. Failing of this stage

cause noise signal called 'colour snow or confetti'. Other effect of this stage failing is to appear small colour patchesupon white part of picture. The output of this stage is fed to chroma gain control amplifier and PAL switch. Because of

suppressed carrier AM modulation used in colour modulators, in every colour TV receiver a crystal controlled (X501)

colour sub carrier generator is used to generate the local sub carrier. The 4.43 MHz signal is generated in oscillator

section which has dc voltage controlled phase reference. To obtain correct tint or hue, of colour picture the phase

synchronization between sample send as burst and generated sub carrier is

necessary. This is achieved by APC detector (automatic phase control) circuit which compare the.

generated carrier phase with the burst phase obtained from the chroma gain amplifier. Its threshold level is extemally

controlled by 'tint or hue' control. The correction voltage appeared at its one output is send to the carrier generator to

coincide the phase. Its other output is send to the) burst gate circuit called the 'burst clean'. If the burst gate opens at

wrong signal such noise or Y signals it will be cancelled out by this feedback loop.

Failing of it cause the colour sync problem where colour beat seen upon the picture. The output of chromagain control amplifier is send to the colour matrix decoder stage one input directly (via R507, C51 a), whilethe other is send through a quartz delay line (X502). This delay line also called as 'PAL delay line' alternatephase of C signal with each line (64uS). The third input of matrix receive control signal from the PAL switchcircuit. The PAL switch is a gating circuit gated from killer detector output and signal from colour sub carriergenerator. The output of PAL switch is altered one by one through T type, flip - flop which is triggered witheach H line signal obtained by keying pulse. Thus the PAL switch signal control the matrix decoder for eachH line between direct and delayed (phase alternated) input. This create the required phase difference signalat the matrix output which is send to R - Y and 8 - Y demodulators. The G - Y signal is created from addingthe R - Y and 8 - Y signals with respective proportions, (G - Y) = (0.51 R - Y + 0.19 8 - Y).The (G - Y )signal is inverted and obtained as G - Y signal. The demodulated R - Y. G - Y. 8 - Y signals are now said R.G. 8 signals only. These signals are fed to video amplifier transistors base with respective cathodeconnections, Le. R with red cathode video amplifier, 8 with blue video cathode amplifier and G, with greenvideo cathode amplifier.


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The signal from microphone is amplified and fed to recordhead,which in turn magnetize tape with signal pattern. During playback the playheadgenerate electrical signals and amplified output drive loudspeaker to producesound.In this manner the sound signals are recorded and reproduced.

1.Playback Preamplifier:The signal output at head is very low in order few mV.It is required to amplify it to

improve signal to noise ratio and to compensate(equalize) preior to final amplification.In

fig 1,the preamplifier circuit is shown.It is two stage high gain direct coupled amplifier,

where noise reduction performed by the series voltage ve feedback.SelectingRIAA mode

the network switched to R-Cfilter circuit(R3,R8,C7) with time constant 120s and

3180s.The overall voltage gain of this stage is200(at 1000Hz inRIAA mode)and 250 inflat mode.


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2.A.F Output amplifier:The signal output at preamplifier has sufficient amplitude(about 1Vpp), it is notappreciable to drive the loudspeaker.Hence it is further amplified by the output amplifier

prior to send,speaker voice coil.The circuit is complementory push-pull amplifier with one

more amplifiying stage.The circuit is shown in fig 2.


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3.Power Supply:The built in dc power supply circuit is shown below. The input ac is stepped down by t1,

full wave rectified, filtered and passed through three terminal regulator7812.to minimize

ripple. D included for safety of REG78


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s

4. Recording amplifier:The recording circuit divided in three parts,(1)The signal amp (2)ALC (3)Bias.

1.The signal amplifier boost the input signal from microphone output(in order few mV)to

sufficient level ,thus recording current should enough strong to magnetize tape.It is a

three stage amplifier, where first stage is simple voltage amplifier, built around

transisterQ8, raise the input voltage60 times.The output signal of this stage further goes to

input of final amplifier, which is two stage direct coupled amplifier(comprising Q3,Q7

Circuit),with negative feedback.Here it should be note that emitter bypass circuitaroundQ3,has high frequency boost circuit C14,R16.As stated in earlier text(pre-

emphesis) it is required toboost high frequency signals during` recording.The recording

current(200A) is set by R18.If this current is more than it the head core will be

saturate,which result in top and bottom clipped recorded signals. To avoid it automatic

level control(A.L.C) Circuit used.

The ALC circuit around Q4, tooks a small input signal from the output Q3, amplify it

nearly 150 times and half wave rectify with diode D3,D4. This dc voltage which is in

propotion with output signal amplitude feed to base of Q6,which acts as variable

resistance.More the dc voltage produced the transistor impedance of Q6, between

collector-emitter falls and signal atten


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3.The need of bias required to eliminate the crossover distortion, produced at the

headside.To describe it more clear lets observe the illus 12a,where B-Hloop of aferromagnetic material is shown . It is clear from illustration that, first time magnetizing

field(Hc) applied to it (material) cause to move the curve O-A. When field is bring back to

minimum,it does not follow the same O-A.curve but there is some magnetic flux

remain,Shown as point B.It is called the renitivity of ferromagnetic material.To remove it

Hc quantityO-C required.The same replica shown in Hc and +Hc to bring this renitivity

back to point O,whenever magnetic field Hc=0,obtained by adding some kind of ac

voltage to the field winding.


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The cyclic current flow due to ac excited field bring the loop smaller and

smaller.This`phenomenon is base of , feeding bias voltage to the head. However if these

bias voltage does not given to the head,it cause to bring loop back in the fig, and recorded

signal will be distirted (may be removel of one half or other half or both).The recording

signal(information)is superimposed upon bias voltage,which overcome the problem and

recording is performed in linear region of B-H loop.The minimum bias frequency should

be kept twice of highest audio frequency 40 KHZ.Bias trap is used for two purpose ,1

st

avoid loading record amplifier,2nd, to maintain its level constant,sinceQ3 impedence is

lower than bias network,L2,C11,(rejector circuit) brings> impedance at bias frequency

high avoid it.

Procedure:s1.Insert a prerecorded cassette.R/P switch at play (RECmode commence by red LED)

2.Press PLAY key .Keep volume control midway and MONITOR switchON.

3.Resume playback ,pressingPAUSE key again. Connect CRO TP1 and observe audio

signal present there.

4.Connect CRO withTP2 and observe same signal with large amplitude present.

5.Select FLAT mode by S3 and observe the sound pitch change (it is sharper due to extra

amplification of high frequency signals).Observe the signal at TP2, it has sharp peaks andpossible clipped at peaks.

6.Insert a blank cassette.R/Pswitch at RECby pressing given switch (RECmode commence

by red LED).Switch off built in (OSC) test oscillator.Keep ALC switch on.


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7.Connect CRO with TP5 and ground ,observe there is high frequency(40-50Kc)signal. It

is bias signal.

8.Switch off ALC(S1).Measure the amplitude of signal at TP3.

9.Switch on ALC (S1).Repeat the above step and observe the signal variation atTP3

within limit .The ALC circuit try to kept the record signal within limitto avoid saturation

of recording current.

RESULT:- The electronic protion of Tape Recorder is studied.

Objective:- To study the power supply section of colour TV.

Appartus Required :- Colour TV Demonstrator kit ,CRO ,Connecting Probes.

Theory:- The Switched Mode Power Supply(SMPS):-

The CTV circuit operates on 100-110vdc power supply whitch is obtained from switch

mode power supply regulator whitch in short called theSMPS.This name designated it to

its functioning, where the regulator device does not conduct continuous whether it regulate

in on off-on manner.Basically it has a pulse width modulator the output of whitch,is

variable pulse width , make the power regulatorn on-off.The input ac line is rectified by

bridge rectifier consistsD806-808,and filtered by C815to obtain dc unregulated power

supply.The +ve of unregulateddcis fed to regulatorQ804, through primary winding of

smps transformer pin 5-1.The-ve of unregulated dc is fed to emitter through R805 whitch

is included for current sense. The Schmitt (pulse width) modulator is formed by Q802andQ803,the output of whitch is capactively coupled through C807 with base of Q804 .The

base of Q804is pulled to +unregulated dc through high value resistor cobina onR804+803.

The back emf generated by the primary of smps transformer is effectively suppressed by

R817-C810 snubber circuit.The other windings of smps transformer to primary sides are

feedback winding pin 7-8,and auxillary power supply winding pin 9-10.The secondary

side has two windings designated for 110 and 22Vdc supply.

The Switching action:-When ac line is connected with mains and power applied to the bridge rectifier ,the

unregulated dc supply applied to collector-emitter of Q804 via primary of SM transformer

and R806 .A small forward voltage applied to the base of Q804, through the seriescombination of R803,804 force to conduct it on every half cycle .The on condition of

Q904 bring the magnetic saturation in SM transformer core ,and when the cycle goes to


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zero this leads to produce counter emf in secondary winding.The voltage across the 2nd

winding of primary side pin 7-8, has such termination that a positive voltage appear across

the C807 through diode D804 and D803A.the base of Q804 effectively leads to cut off by

C807. As this time the Schmitt trigger formed by Q802,803 is on the C807 discharge

through it force Q804 to switch on .This successive action develope the voltage across

third winding of primary side pin 9-10,which are rectified and applied to an error detector

formed around Q801. The voltage at its emitter which is refrenced by zener diode D801,is

comparedwith the wiper end of VR801 potential divider and the difference is applied to

the input of Schmitt trigger The Schmitt trigger on time depends upon these

voltages.When the secondary side is being loaded the load reflected at primary side,thus

the error detector input voltage is dropped and voltage at the collector of Q801, rise +ve

which cause to trigger Q803,804 late thus Q804 on for long time .When load current is

low the voltage at error detector increase thus less +ve voltage applied at Q802 ,cause to

cut-off Q804 QUICKLY.Thus the output voltage can be adjusted by adjusting the

threshold level of Schmitt trigger input by mean of VR801, the error detector.

microprocessor.

The +110 volt supply is switched to H output stage through a relay contacts which is

switched on-off by system control.

HARYANA COLLEGE OF TECHNOLOGY & MANAGEMENT, KAITHAL

LABORATORY MANUAL

EXPERIMENT NO. : ECE-AV--EX-11 ISSUE NO:04 ISSUE DATE:11.12,06

REV. NO. :REV. NO. :

PAGE NO.02/02

DEPTT. : ELECTRONICS &

COMMUNICATION DEPARTMENT

DEPTT. : ELECTRONICS & COMMUNICATION

DEPARTMENT

DEPTT. :

ELECTRONICS &

COMMUNICATION

DEPARTMENT


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Result:-The power supply section of colour TV is studied.

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