Question 2
-
AIM: To test the student in different stages in superheterodyne radio receiver.
- You are provided with superheterodyne radio receiver set.
- Let the Supervisor open the back cover of the radio receiver.
- List two tools used by the Supervisor in opening the back cover of the radio receiver.
- Copy Table 2 into your answer booklet.
- Identify the radio frequency amplifier stage in the radio receiver.
- In the radio receiver frequency amplifier stage, identify three components and record them accordingly in Table 2.
- State the function of each component listed in (f) as specified in Table 2.
- Repeat steps (e) to (g) for other two stages listed in Table 2.
- List two faults associated with each stage listed in Table 2.
- Locate the audio I.C., count and record the number of pins on it.
- State on safety precaution to be taken in replacing a faulty audio I.C.
Table 2
StageComponent
Function of the component
Fault associated with the stage
Radio Frequency Power Amplifier
Local Oscillator
Audio Frequency Output
The candidates were expected to let the supervisor open the back cover of the radio receiver while taking note of steps taken.
Observation
The expected answers were:
- (a) Tools used to open the back cover of radio receiver
- Screw driver
- Allen key
- Long nose pliers
Table 2
Stage |
(f) |
(g) |
(i) |
Radio Frequency Power Amplifier |
Inductor |
Combines with capacitor to produce a tuned circuit |
-weak reception/faint reception - no output signal - interference from |
Capacitor |
Combines with inductor to produce a tuned circuit |
||
Resistor |
Voltage divider/biasing |
||
Transistor, I. C. |
Amplifies signal |
||
Local Oscillator |
Inductor |
Combines with capacitor to determine frequency |
-Q-point drift |
Capacitor |
Combines with inductor to determine frequency |
||
Resistor |
Voltage divider/ biasing |
||
Audio Frequency Output |
Capacitor |
Coupling, dc blocking |
-loss of audio/sound |
Resistor |
Voltage Divider/biasing |
||
IC/transistor |
Amplification |
||
Diode |
Protection |
(j) 8 pins/14pins/16pins [3marks]
(k) safety precaution to be taken in replacing a faulty audio I.C
- Ensure the polarity of the pins are correct
- avoid excessive heat while soldering
- handle the leads carefully to avoid breakage
-replace with the same type of I.C/or its equivalent
- use the right tool for the right job.
- observe all necessary workshop safety rules.
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 radio receiver.