The vertical section of the tv is responsibel for the diflection of the beam in the vertical direction. The main problems are discussed belo winclude:
- Loss of Vertical Sweep
This is characterized by the raster collapsing to a single horizontal line in the center of the screen. This could be an inoperative deflection oscillator in the vertical strip. It could also mean a defective vertical amplifier or an open yoke. The best way to approach this problem is to inject a 60Hz signal from the oscillator output and trace it back to the deflection yoke. This will determine where the problem occurs.
- Insufficient Height of Vertical Deflection, But Linear
This could be a cause of misadjusted height and linearity controls in the vertical sweep amplifiers, increased sawtooth time constant in the sawtooth generators or a low amplification level at the output.
If the sawtooth generator peak to peak levels are as required, then the trouble must be in the amplifier. The compliment of this is true.
Poor Vertical Linearity
This could depict misadjusted linearity controls at the vertical strip. It could also be faulty output transistors or leaky bypass capacitors for the cathode or defective output transformers in the sawtooth forming circuits.
This calls for an internal adjustment which adds a signal to the horizontal deflection to compensate for the geometry of the CRT/deflection yoke. It could also be faulty capacitors in the horizontal deflection power supply.
- Loss of vertical synchronization
This may be caused by insufficient amplitude or a change in the free running vertical sweep frequency. If the vertical hold is able to stop the picture from rolling, then its possible that the oscillator if functional. But to check through each component in the vertical strip will save time in isolating this problem.
- The Keystone Raster
The vertical deflection windings of the yoke are placed horizontally on either side of the neck of the picture tube. These windings are responsible for creating a uniform horizontal magnetic field inside the neck of the picture tube. If a shorted turn were to develop inside one of the windings, the magnetic field closest to it would be weakened. The electron beam would receive little vertical deflection when the horizontal deflection brings it near the weak magnetic field. The height of the raster near the weak magnetic field would be less than that of the opposite side, thus producing the keystone raster.
But shorted turns are not the only cause for this problem, open windings are consequently shorted through the damping resistor to the other good winding and cause this problem.
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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