Radio,Television And Electronic Paper 3, WASSCE (SC), 2018

Question 2

AIM: To test the candidates’ knowledge of capacitors in a CRT Television receiver.

  • You are provided with specimens X, Y and Z.
  •  Identify and record in Table 2 the type of capacitors labelled specimens X, Y and Z.
  •  Sketch in Table 2 the circuit symbol of specimens X, Y and Z.
  • Measure and record in Table 2 the capacitance of each capacitor.

Table 2


Specimen

Type of capacitor

Symbol

Measured capacitance with unit

Stage(s) found in TV receiver

Function(s) performed in the stage

X

 

 

 

Power stage

 

Vertical stage

 

Y

 

 

 

Horizontal stage

 

Video amplifier

 

Z

 

 

 

RF tuner

 

IF amplifier

 

  • State in Table 2, one function performed by the specimens in the indicated stages.
  • Connect specimens Y and Z in series.
  • Measure and record the total capacitance of the connection in (f).
  • Calculate the total capacitance in (f).
  • Compare and comment on the results in (g) and (h).
  • Reconnect specimens Y and Z in parallel.
  • Measure specimens Y and Z in parallel.
  • Calculate the total capacitance in (j).
  • Compare and comment on the results in (k) and (l).
  • State two ways of identifying faulty specimen X.
  • State the steps involved in replacing a faulty specimen X.
  • List three tools required in replacing a faulty specimen X.
  • State two precautions taken to ensure the accuracy of your readings.

Observation

The expected answers were:

(d)                                                       Table 2


Specimen

Type of capacitor

Symbol

Measured capacitance with unit

Stage(s) found in TV receiver

Function(s) performed in the stage

X

Electrolytic capacitor

4.5 – 4.9 µF

Power stage

Smoothening/removing ripples/filtering

Vertical stage

  • Coupling
  • Bypass

Y

Film capacitor

17nF

Horizontal stage

Voltage multiplication/boosting

Video amplifier

Coupling

Z

Ceramic capacitor

9.7nF

RF tuner

  • Tuning
  • Coupling

IF amplfier

  • Tuning
  • Coupling

                                               
(g) 6.23nF                                                                                                                              
(h) Total capacitance                               


(i) There is a slight difference in the two values. Indicating that tolerance of the capacitors does
not sum up in series.                                                                                                        
(k) 28.5nF                                                                                                                              
(l) Total capacitance              


(m) There is a slight difference in the two values. Indicating that tolerance of the capacitors
does not sum up in parallel.                                                                                               
(n) Ways of identifying faulty specimen X

  • Swollen/burnt/burst/broken legs/removed legs
  • Reading continuity when tested with meter/shorted
  • Non-charging/inability to store charge

(o) Steps involved in replacing a faulty specimen X

  • Desolder/remove from circuit
  • Replace with one of the same rating/equivalent in the correct polarity              

(p) Tools required for replacing faulty specimen X

  • Soldering iron
  • Pair of longnose pliers
  • Desoldering gun
  • Lead sucker
  • Side cutter
  • File
  • Tweezer
  • Brush
  • Screw driver

(q) Precautions taken to ensure accuracy of readings

  • Ensure that the meter is set to the appropriate range.
  • Ensure tight connections.
  • Avoid body contact with leads to reduce error.
  • Ensure the meter is adjusted to zero before taking readings.
  • Avoid error due to parallax.

Question 2 required candidates to identify the various types of capacitors found in a CRT television receiver and apply some level of theoretical knowledge of them. It was reported that majority of the candidates that attempted Question 2 were unable to identify tolerance, in capacitance, as the major difference between the measured capacitance and the calculated one. More so, quite a number of the candidates were reported not to correctly deal with the use of prefixes such as nano.