the tuner section of the tv has a number of problems that can cause the viewer of the television to have a bad reseption and picture.Some of the problems will be highlighted include:
- No Picture, No Sound But the Raster Is Evident
This is the most common tuner problem, and the most basic. It could translate into lack of a signal at the antenna, so one should first check all stations to verify whether its an external problem. Trace the signal from the antenna to the tuner. If the antenna system is fine, isolate the problem in the tuner by injecting a signal and trace it through the RF amplifier, the mixer and the AFT section and verify expected outputs progressively. The cause of the problem will not give the required output signal.
A defective RF amplifier will definitely cause this symptom; one should thoroughly test it before proceeding to other sections. Any defective part will require replacement.
This could also be caused by a defective oscillator. Test the output of the oscillator to determine whether it’s producing the required frequency at the specified amplitude.
- No Picture, No Sound, But on Selected Channels
This will definitely not go beyond the local oscillator, but its advisable to trace the signal from the antenna to eliminate a defective antenna system. Then follow confirm the oscillator signal to verify its validity. This will probably prove that the problem was caused by a misadjusted oscillator. However, solder could be loose on one of the tuner components.
- Weak, Snowy Pictures
This may be a consequence of an insufficient signal to the antenna, so one should check other channels. This could also translate to an excessive AGC voltage, or insufficient oscillator voltage; a defective oscillator.
In some sets the tuner section could have a coupling transformer, and this can cause this problem. This setup would be accompanied by coupling capacitors which could be defective or open.
- Poor Picture Quality
Misadjusted or defective RF amplifier could be the cause of this. It could also be as basic as the wrong antenna positioning, or a poor quality coaxial cable. If these have been eliminated, inject a signal and trace it through the tuner. Check also for open coupling capacitors or an open AGC isolating resistor.
- No Color
This could be a consequence of fine tuning, assuming the obvious causes are eliminated, i.e. color control and reception. Check the AFT current and verify its required level.
- Ghosts
This means a second picture superimposed the main one. The ghost picture is usually to the right. This indicates problem which is external, i.e. two signals of the same transmission received at the same time. One should try adjusting the antenna,
- No UHF, But VHF is Functioning.
One should start with the antenna and check the UHF section of it for possible causations. If this is eliminated, one should proceed to the UHF strip in the tuner and inject a signal for verification. The problem should be encountered here.
Conclusion
Troubleshooting a tuner sometimes may mean the replacement of whole modules, if that is the case and the apparent problem extensive but isolated, it is advisable to replace whole parts because tuner circuits are precision oriented at attaining this by part replacement could be hard.
Reference:
Samuel M. Goldwasser, Notes on the Troubleshooting and Repair of Television Sets, Copyright © 1994-2007 Samuel M. Goldwasser, http://www.repairfaq.org
Alvin A. Liff, J.A. Sam Wilson, Color and Black and White Television, Theory and Servicing, 3rd Edition©1993 Prentice Hall Inc. upper
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