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.

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

      Table 1



      Function of the component

      Fault associated with the stage











      Horizontal Sync









      Audio Output








    7. Identify the power stage in the receiver.
    8.  From the power stage, identify three components and record them accordingly in  Table 1.
    9. State and record the function of each component identified in step (g) as specified in Table 1.
    10.  Repeat steps (f) to (h) for the other two stages listed in Table 1.
    11. List two faults associated with each stage as specified in Table 1.
    12.  Locate the high tension transformers, count and record the number of connecting pins at the base in your answer booklet.
    13. 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.


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



Function of the Component

Fault associated
 with the stage







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







Capacitor/ smoothening choke

-filtering/removal of ripples
-generate pulses at required frequency with chopper transformer

Diodes, Zener diode

-switching device
-protective device
-voltage reference source
-voltage stabilization


Surge protector/over current protection

Power transistor

-provides large required current with voltage regulators
-provides low output resistance, used as voltage comparator


Make or break the circuit


-Switching element
-provides pulse output (rectification)


Temperature sensing device, heat control

Power resistors


-for voltage dropping
For transferring current from one point to the other.




Horizontal I.C

- 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
-  Bright vertical line
across the screen/
-  No picture
- split screen

Variable resister

-regulator, Horizontal Control regulation
-Voltage dropping


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


-combines with capacitors to generate scanning frequency signal



Audio I.C

Amplifies the audio signal


-loss of sound

-distorted sound

-humming sound
- Noisy audio output


- filtering a.c
- blocks dc when necessary


-for biasing
-to provide reference value


-as amplifier
-regulator/controlling element

IC Protective sockets

IC protection

(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.