Radio,Television And Electronic Paper 3, May/June 2015

Question 1

AIM: To test candidates knowledge in different stages on colour Cathode Ray Tude (CRT) Television.

You are provided with a 20” Colour CRT Television Receiver.
Let the supervisor remove the back cover of the television receiver.
List two tools used be the supervisor in opening the back cover of the television receiver.
State the model of the television set provided from the back cover.
Copy Table 1 into your answer booklet.

Table 1

Stage	Component	Function of the component	Fault associated with the stage
Power


Horizontal Sync


Audio Output

Identify the power stage in the receiver.
From the power stage, identify three components and record them accordingly in Table 1.
State and record the function of each component identified in step (g) as specified in Table 1.
Repeat steps (f) to (h) for the other two stages listed in Table 1.
List two faults associated with each stage as specified in Table 1.
Locate the high tension transformers, count and record the number of connecting pins at the base in your answer booklet.
State one safety precaution to be taken in replacing a faulty line.
The candidates were expected to let the supervisor open the back cover of the radio receiver while taking note of steps taken.

Observation

The candidates were expected to answer as followers:

(c)Tools used for opening back cover of television receiver.
      -Screw driver    →    long nose pliers
      - allen key
(d) Correct model of television
Table 1

Stage	(g) Component	(h) Function of the Component	(j) Fault associated with the stage
Power	Transformer/ chopper transformer	-generates pulse output -distributes voltage - isolate low voltage from high voltage -steps voltage up or down	- No sound - No picture - Dancing picture - Distorted sound - Dead set -No output (sound and picture) - low audio output - unsteady picture Output
	Capacitor/ smoothening choke	-filtering/removal of ripples -generate pulses at required frequency with chopper transformer
	Diodes, Zener diode	-rectifier -switching device -protective device -voltage reference source -voltage stabilization
	Fuse	Surge protector/over current protection
	Power transistor	-provides large required current with voltage regulators -provides low output resistance, used as voltage comparator
	Switch	Make or break the circuit
	Thyristor	-Switching element -provides pulse output (rectification)
	Thermistor	Temperature sensing device, heat control
	Power resistors Jumper	-for voltage dropping -regulation For transferring current from one point to the other.
Horizontal Sync	Horizontal I.C	-amplification - provides horizontal output at 50H2 signal -provides horizontal scanning signal output at 50Hz	- No raster - sound on picture - white dot at the centre of the screen - Dark patches by the side of the CRT. - Bright vertical line across the screen/ CRT. - No picture - split screen
	Resistor/Potentiometer Variable resister	-regulator, Horizontal Control regulation -Voltage dropping
	Capacitor	Combines with inductor to generate scanning frequency signal.
	Heat sink	Prevents heat accumulation
	High tension transformer	-generates high voltage required to liberate electrons from the CRT
	Power transistors	-supplies the required current to drive the circuit
	Inductors	-combines with capacitors to generate scanning frequency signal
Audio Output	Audio I.C	Amplifies the audio signal	-loss of sound -distorted sound -humming sound - Noisy audio output
	Capacitor	- filtering a.c -coupling - blocks dc when necessary
	Resistor	-for biasing -to provide reference value
	Transistor	-as amplifier -regulator/controlling element
	IC Protective sockets	IC protection

(k)14pins/16pins
(l) Safety precautions to be taken in replacing a faulty line transformer:
- discharge the anode of the transformer to avoid risk of electric shock

- avoid excessive heat while soldering
- handle the leads carefully to avoid breakage
- use the right tool for the right job.
- replace the transformer with the correct equivalent
- avoid leaving the soldering iron too long on each pin when soldering
- avoid partial contacts or dry joints

The candidates in most cases only stated the general functions of components instead of the functions as applied to the purpose for which they were used. Candidates did not show familiarity with the faults associated with Television receiver.