Question 1
- State three:
(i) parts of a Cathode Ray Tube (CRT) television receiver that can cause electric shock;
(ii) safety precautions in handling the CRT of a television receiver.
- Table 1 shows some symptom of faults in a CRT television receiver.
Table 1
|
Symptoms |
Two faulty stages/sections |
1. |
No raster, no picture but sound normal |
|
|
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2. |
Raster and picture normal but no sound |
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|
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3. |
Raster normal but no picture and no sound |
|
|
Complete Table 1 by stating two stages/sections to which each symptom of fault is associated.
Observation
The expected answers were:
(a) (i) Parts of a Cathode Ray Tube (CRT) television receiver that can cause electric shock
- Anode cap and connector of flyback transformer
- High voltage cable/power cord
- Yoke assembly
- Flyback transformer
- High voltage capacitors/filter capacitors
- Heat sink
- CRT chassis/degaussing coil
(ii) Safety precautions in handling the CRT of a television receiver
- Turn off the power and disconnect the a.c. power cord before removing the back cover of the receiver.
- Remove any metal jewelry on your body and work area.
- Do not work in isolation. Always have someone nearby in case of an accident/emergency.
- Wear appropriate personal protective/safety devices.
- Avoid holding the tube by the neck.
- Discharge high voltage capacitors.
- Avoid water spillage on the floor and work area.
- Avoid overcrowding the work area with tools.
(b) Table 1
|
Symptoms |
Two faulty stages/sections |
1. |
No raster, no picture but sound normal |
EHT circuit |
Scanning coil |
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Vertical output |
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Line output stage |
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Picture tube |
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Biasing of picture tube |
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Socket connections |
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Video output circuit |
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2. |
Raster and picture normal but no sound |
Sound/audio section |
Sound I.C./power transistor/audio amplifier |
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Loudspeaker |
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3. |
Raster normal but no picture and no sound |
Antenna |
Feeder line |
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RF tuner circuit |
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Video section |
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IF amplifier |
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Video detector |
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AGC |
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Sync separator |
Only a few candidates attempted question one and the responses of these candidates were reported to be poor. Safety precautions in handling a CRT television receiver, faults and troubleshooting should be well taught by teachers as this is an important aspect of the subject.