إشعاع المذبذب المحلي واستقرار تردده ..كتاب التلفزيون الملون والعادي

E-13 The radiation of the local oscillator and the stability of its frequency:

This problem used to appear in the past, as the electromagnetic waves of the oscillator radiate their waves and confuse the neighboring devices. Therefore, the RF amplifier circuit in the voter prevents these waves from leaking into the antenna - and the use of a dedicated connection cable and isolating the oscillator stage within a closed space has blocked these radiations. As for the stability and stability of the oscillator frequency, it is very necessary. The frequency shift occurs due to the effect of the change in the ambient temperature, and the bias may reach 1 megahertz, so the sound and image signals are completely lost. Fortunately, the selection of well-made components and the presence of the (AFT) circuit, i.e. the automatic fine-tuning circuit, prevents frequency shifting, in addition to the presence of the (AFC) circuit, i.e. the automatic frequency tuning circuit, and the AFC circuit in the horizontal scanning circuit.
Source voltage change:

A change in the source voltage changes the DC voltage and is affected by poor voltage regulation. All of this leads to a change in the bias of the oscillator - it could be a change of 3 volts.

Continuous, causing the frequency to change by 1 megahertz. However, modern devices contain good regulating circuits that provide a stable continuous voltage (DC), regardless of the change in the source voltage or the load current.

the noise :

Electrical noise occurs from electrical external machines close to the device. Or because of sunspots, as noise occurs from the same circuit elements. Thermal noise is generated in the resistors because of its atomic structure, which is disturbed by the effect of heat. Likewise in transistors. This noise has nothing to do with frequency. RF amplifier circuits are designed to have good signal-to-noise amplification. Otherwise, the image will have snow spots on the screen

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5-14 Manifestations of defects caused by the voter and their repair: 1- Presence of a linear structure (flat on the screen and lack of sound and image).

2 - The sound is good and the picture is of poor quality in all settings of the tuning key

3- Overlapping in the image, or the presence of snow spots, or a negative image, or in it

Ghosts, lines, or loss of synchronization. The cause of voter malfunctions may be contamination or poor contact of the seams, which require good cleaning and lubrication of the moving section. Likewise, the presence of pieces in a file or a break in a slide. Or damage to one of the elements in the voter circuit, such as a resistance or a capacitor, may lead to these malfunctions.

To determine the location of the fault in the voter, a TV signal generator is used, which is connected to the entrance of the mixer stage. If the image appears, the mixer and the local oscillator circuits are good, then it is connected to the output of the RF amplifier - then to its input, and every time the image appears on the screen, this indicates that the stages The signal passed by is valid.
The fault is confined to the last stage to which the signal of the signal generator was entered, if a good image does not appear on the screen.

The antenna wire can be used when there is no signal generator, as the internal wire of the antenna cable is connected through a capacitor with a capacity of (300-500 microfarad) to the test point, and the other wire is connected to the chassis of the device, and we perform the same stages previously mentioned in the signal generator method. In order to renew the broken element after limiting the fault at a certain stage, the method of measuring voltages and resistances is used in comparison with the device diagram, and there is a transistor test device inside the circuit that is very useful and quickly in examining and testing transistors without dismantling them from their circuit. It is useful to know that the failure of the AGC automatic gain control may lead to the complete absence of the image, and therefore it is preferable to disconnect the AGC line from the voter before starting to repair it.

UHF voter malfunctions:

The faults of the UHF voter are simpler because it consists of only two active components: the detector diode and the local oscillator. As for the connecting wires, they are well fixed on holders. As for the capacitors, they rarely malfunction - and if a defect occurs, it is usually due to a mechanical reason or the accumulation of dust on the capacitors - and care must be taken when repairing the faults of the voter, as increasing the length of the connecting wire may lead to a malfunction in its tuning, and a distorted image appears in UHF channels only. It is due to the weak oscillator voltage. The interface between the oscillator and the mixer also needs attention.

Resetting or changing the voter requires referring to the manufacturer's instructions. A displacement or twisting of the capacitors may disrupt the voter's work.

Faults of the selector with capacitive diodes (farctor):

The diode-capacitive tuning method has dispensed with the use of moving mechanical elements for tuning - therefore, the symptoms of malfunctions differ from those
In other constituents, common faults are:

The instability of the combination.

2- The inability to synthesize.

3- Shift in potential (AFT).

The reasons for the instability of tuning or the lack of tuning are as follows:

1- Incorrect DC supply voltage.
2- A defect in the tuning voltage or the voltage divider.
3- The mixer in the VHF voter is not correct.

As for the AFT shift, it is caused by the tuning voltage shift. The DC voltage field is measured. If a defect is found in the AFT circuit, it must be isolated from the voter and examined as an independent circuit