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 |
|
||||
Y |
Film capacitor |
|
17nF |
Horizontal stage |
Voltage multiplication/boosting |
Video amplifier |
Coupling |
||||
Z |
Ceramic capacitor |
|
9.7nF |
RF tuner |
|
IF amplfier |
|
(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.