Question 1
-
AIM: To test the knowledge of candidates in fault finding and their causes in a Cathode Ray Tude (CRT) Television set.
- You are provided with a colour CRT television set and a television indoor antenna.
- Copy Table 1 into your answer booklet.
- Connect the indoor antenna to the television.
- Plug the television to power source and put it ON.
- Tune the television to available station in your locality.
- Observe and record in Table 1 the output signal quality in terms picture and sound.
- The supervisor should monitor activities from steps (h) to (u).
- Put OFF the television set and unplug from power supply.
- Carefully open the back cover of the television set.
- Locate the CRT.
- Carefully remove the small panel at the base of the CRT.
- Count the number of pins at the base of the CRT and record into your answer booklet.
- Repeat steps (d) to (f).
- Compare and comment on the results obtained in (f) and (m).
- From your results, state one function of the base panel.
- State the model of the television provided for you.
- What is the name of the uninsulated copper wire wound round the neck of the CRT?
- Mention one function of the wire in (q) above on the picture and sound output of the television.
- What is the name of the black cable around the edge of the CRT?
- State one function of the cable mentioned in (s).
- Fix the base panel of the CRT back to position and replace the backcover.
- State two precautions taken when carrying out the activities listed above.
Table 1
Condition of the television
Output quality
Picture
Sound
When the base panel of CRT is firmly connected
When the base panel of the CRT is removed
Observation
The candidates were expected to let the supervisor open the back cover of the radio receiver while taking note of steps taken.
The candidates were expected to answer as follows:
(b) Table I: correctly copied Table 1.
Condition of the television |
Output quality |
|
Picture |
Sound |
|
When the base panel of CRT is firmly connected |
|
|
When the base panel of the CRT is removed |
|
|
(l) Number of pins – 8/10/12/14 depending on the TV model.
(n) Comparison and comment
- With the base panel firmly connected, TV set showed clear picture and gave audible sound
- When the base panel was removed, no picture, no raster was seen and only hissing sound heard.
(o) Function of base panel
- Provides required voltage to the electron gun
- Amplifies the chrominance signal
- Amplifies the video output
- Provides the required screen voltahe
- Controls the focus.
(p) Any appropriate/correct model
(q) Scanning coil/yoke/deflection coil
(r) Horizontal and vertical synchronization of the picture signal.
(s) Degauzzing wire/ earth wire.
(t) i. Colour intensity
ii. Lightening protection
iii. Neutralizes magnetic effect on the colour
iv. Hold the CRT in place.
(v). Precautions taken
- I ensured the use of right tools for the right job
- I avoided breaking of the panel
- I avoided breaking of the CRT pins
- I ensured that the set is off while working on it.
The candidates were required to explain the functions of CRT, scan coil, degaussing wire and most importantly the function of base panel of a CRT in a TV receiver. Many candidates responded quit well to this question.