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GEETA INSTITUTE OF MANAGEMENT & TECHNOLOGY

KANIPLA (KURUKSHETRA)

Department of Electronics & Communication Engineering

Laboratory Manual

Semester : 8th Subject: Audio Visual Lab Subject Code: ECE-410E

Page No.

Experiment-02

Aim: Study of tuner section of a color TV.

Requirements: Training Kit and Accessories.

Circuit Diagram:

4. STUDY:

The RF section mainly consists of RF tuner. This tuner has RF amplifier, a mixer and a local

oscillator. The antenna receives RF waves form the atmosphere. It converts them intocorresponding signal variations. These RF variations are fed to the RF tuner. The input

impedance of RF tuner is 75 . The antenna system and coaxial cable should be properly

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matched. IF a coaxial cable of 75 impedance is employed. There is no need of any extra

matching device between the cable and antenna socket at the receiver. But in case of a feederwire of 300 is employed, then an impedance matching transformer (300 75 ) is added

between the feeder and the antenna socket at the receiver. There is also a need of impedance

matching between output terminals of antenna and co axial cable or feeder wire. The RF tuner

selects the signal of the desired channel amplifies it and converts in to Intermediate Frequencies

(IF). The video IF is at 38.9 MHz. The sound IF is at 33.4 MHz.

Tuner section has +12 V supply voltage at TP 14 (MB). This voltage is used for all the

operations in this section. +12 V is provided from horizontal output section. Transistors TA02,

TA03, TA04 (BC558) are used to select the desired band from the tuner section. To switch ON

these transistors 10.8 V is provided at their base. Output is obtained at IF terminal (TP20). AGCvoltage of 2.8 V is obtained from pin 11 of IC 7680 (IC101). This voltage is used for the purpose

of automatic gain control. AFT voltage is provided from pin 13 of IC7680. It is available at

TP16. Tuning pulse output from system control IC is fed in to the base of the transistor T903(BC547). Which switch ON the transistor. So tuning voltage is 0-33 V approx. is obtained at TV

terminal (TP19).

Following various terminals of RF tuner are provided in the form of Test Point (TP) in the trainer

kit.

UB The system control IC provides the UHF band selection voltage. This voltage switch

ON the transistor. Hence, UHF band is selected.

HB VHF band III selection voltage available from system control IC through switching

transistor at HB pin.

LB VHF band I selection voltage available at LB from system control IC through

switching transistor.

TU Channel selection voltage is available at this pin through transistor. It is varying from

0-33 V during channel selection.

AGC Automatic Gain Control voltage is available at this point from IC 7680.

AFT Automatic Fine Tuning voltage is available at this point from IC 7680.MB It is tuner sections Power Supply pin. Here 12 V is available.

IF Intermediate Frequency output signal from tuner is available at IF pin.

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STUDENTS ACTIVITY

5. OBSERVATIONS

6. RESULT7. VIVA VOCE QUESTIONS

(i) What is a Tuner?(ii)Draw the labeled PIN diagram of IC 7680.(iii)Distinguish between AGC and AFT.

(iv)What is IF?(v)What is a tuner card?

ASSESS YOURSELF: .

ANY SUGGESTION FOR IMPROVEMENT: ..

..

TEACHERs REMARKS: .

TEACHERS SIGNATURE

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IF 7680

Functions :

1. VIF2. SIF3. Video Pre-amplifier4. Video detector5. IF AGC6. RF AGC

Pin

No.

FUNCTION Voltage

1. Volume control 0-6.7

2. Noise filter biasing 2.7

3. Audio output 7

4. Ground (-Ve supply voltage) 0

5. AGC detector 7.3

6.

Decoupling capacitor for

DC feedback loop of IF amplifier

4.9

7. Balanced PIF input 4.8

8. Balanced PIF input 4.8

9.

Decoupling capacitor for

DC feedback loop of IF amplifier

4.9

10. RF AGC 6.4

11. RF AGC output 3.9

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12. PIF ground 0

13. AFT output -1 7.2

14. AFT output -2 5.8

15. Video signal output 3.6

16. AFT tank 4.2

17. Video tank IF detector circuit 7.818. Video tank IF detector circuit 7.8

19. AFT tank 4.2

20. Positive (+Ve supply voltage) 11.7

21. Sound IF input (5.5MHz.) 4.6

22. Sound IF detector (5.5MHz.) 4.6

23. De-emphasis(audio output 5.3

24. Sound IF detector (tank circuit) 4.5

410expt02

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