Question 2
AIM: to test the candidates’ knowledge of the various components of a television circuit.
You are provided with specimens A and B
a Identify specimens A and B.
______________________________________________________________________________________________________________________________________________________
b Draw the circuit symbol of specimen B.
c State:
(i) three physical features each of specimens A and B
Specimens A
(I) _________________________________
(II) _________________________________
(III) ________________________________
Specimen B
(I) ___________________________________
(II) ____________________________________
(III) __________________________________
(ii) two functions each of specimens A and B in a television receiver:
Specimens A
(I) _________________________________
(II) _________________________________
(III) ________________________________
Specimen B
(I) ___________________________________
(II) ____________________________________
(III) __________________________________
(iii) one stage in which the specimens identified in 2(a) are located
Specimens A
(I) _________________________________
(II) _________________________________
(III) ________________________________
Specimen B
(I) ___________________________________
(II) ____________________________________
(III) __________________________________
(iv) two ways each to determine if specimens A and B are faulty
Specimen A
_____________________________________
Specimen B _____________________________________
(v) state one effect each of the following on the output of a television receiver
Faulty EHT rectifier in specimen
A _____________________________
(II)open circuit in specimen
B ______________________________________
(d) (i) Place specimen B as shown in Fig. 3, connect the red lead of the ohmmeter to terminal 1 and black lead to terminal 2. Observe and record your readings
_________________________________________________________________________
(ii) repeat d(i) with the red lead on terminal 3 and black lead on terminal 4. Observe and record your readings
_____________________________________________________________________
(iii) Compare results in d(i) and d(ii) and make deductions.
_____________________________________________________________________________
(e) State:
(i) one function of the rubber cap on one of the terminals in specimen A;
________________________________________________________
(ii) The device to which the rubber cap is connected.
_________________________________________________________
(f) If a faulty specimen A is to be replaced in a television receiver, state two safety precautions to be observed.
__________________________________________________________________
Observation
The expected responses were:
2 (a) Specimen A
- Line transformer.
- High tension transformer.
- Flyback transformer.
- High frequency transformer.
Specimen B
- Power transformer.
- Step down transformer.
(b) Circuit symbol of Specimen B
(c) (i) Physical features of Specimen A
- It has three wires.
- It has two control knobs (focus and screen).
- It has ten or less pins.
- One wire carries an insulated cap to cover the anode.
- It is made of ceramic materials.
- It has by the side a ferrite material clipped with a metal.
- It has insulated body.
Physical features of Specimen B
- 2 input wires and 3 or 2 output wires
- Metal frame
- Insulation tape that binded the wires
- Two coils enclosed in a plastic material.
- Plastic material
- Two holes by the sides for connecting to the board.
- Ratings printed on the body.
(ii) Functions of Specimen A
- provides the screen voltage for the CRT.
- provides high voltage to the CRT.
- provides the blanking pulse.
- provides drive power for the horizontal amplifier.
- provides the heater voltage for the CRT.
- distributes voltage in horizontal circuit.
- provides high impedance matching in horizontal circuit
Functions of Specimen B
- isolate input power circuit from the mains.
- step down voltage.
- prevent electric shock.
(iii) Stages located in a television receiver
Specimen A
- Horizontal stage / horizontal scanning stage.
Specimen B
- Power supply stage.
(iv) Ways to determine if specimens are faulty
Specimen A
- Visual / physical inspection.
- Smelling.
- Observation of output of Television receiver.
- Broken ferrite material .
Specimen B
- Visual / physical inspection for breakages and open circuit
- Resistance / continuity test
- Smelling
- Burning
(v) Effect of faulty EHT rectifier in Specimen A
- No picture
- No raster
- Dull screen
- Blank screen
- Low voltage output
- Compressed picture / reduce width of picture
Effect of open circuit in Specimen B
- No power output
- Dead set
(d) (i) 1.05kΩ - 1.1kΩ
(ii) 5.2 - 8.2
Note: Ratio of resistance in d(i): d(ii) can be 200: 2
(iii) The resistance value of the primary is higher than that of the secondary (Rp ˃ Rs)
(e) (i) Function of the rubber cap
- Seals and protects the anode wire
- Provide stress relief on the anode
- Acts as electrical insulation
- Water / moisture proof
(ii) The rubber cap is connected to the Anode
(f) Safety precautions to be observed
- Disconnect the television receiver from the mains supply
- Discharge the anode
- Use insulated tools to remove the faulty specimens
- Replace with correct equivalent
As reported by the chief examiner, candidates did well in this question but lost some marks because they confused the physical features of the specimens they were asked to provide with the internal features.