علامات ضعف الشاشة الملونة أو تلفها .. كتاب التلفزيون الملون والعادي

11-19 Signs of weakness or damage to the color screen:
1 - Mixed faded colors - or the weakness of all the colors of the image - and sometimes bright bright areas appear at startup and then disappear slowly after a while. The screen can be activated with an independent transformer that feeds the screen filament with an appropriate alternating voltage, as previously explained.

2 - A negative image (negative), which becomes normal when the light switch is lowered and the contrast is reduced in a weak room light.

3- It is not possible to control the light switch due to an internal short in the screen valve, and when lightly knocking on the neck of the screen, the image appears and then goes back to disappear, and sparks may appear between the electrodes of the internal screen.

4 - The image does not appear and a short circuit is detected between the control grid and the cathode of the screen or between the control grid and the cover - and it appears using the ohm field in the ohmmeter when the device does not have a power supply. The movement of the ohmmeter pointer when it is placed between each two electrodes after removing the base feeding the screen electrodes indicates the presence of a short circuit and contact (and a capacitor discharge method is used between the two contact electrodes, as we mentioned in detail in this malfunction, Figure (11-53).

E - Separation or interruption in the heating filament of the screen - and this appears in the absence of any image or structure of lighting - and the absence of a glow in the filament despite the supply voltage reaching both ends - and this malfunction requires replacing the screen - and there is no objection to trying to weld the filament using high voltage as we explained in detail previously .

11-20 The circuits of the screen valve and its protection:
1- The circuit to protect the screen from exceeding the voltage or exceeding the beam current
Increasing the voltage of the screen elevator beyond its specified value may lead to malfunctions, as well as to emit X-rays that exceed the permissible value. Therefore, some devices are equipped with an automatic circuit that stops the high voltage, thus protecting the screen valve. Figure (11-54). The working principle of this circuit is based on the thyristor (SCR), which is Q804

If it is in the connected state, the feed of the horizontal oscillator +B decreases, so the oscillator stops working, and thus the high voltage stops feeding the screen valve elevator.

A - Preventing a high voltage increase: Wave pulses are formed from the horizontal output transformer during the return period of the horizontal scan line, and these pulses charge the capacitor C814 through the diode D813. It becomes connected, and (R818) determines the high voltage - and when D809 is connected, a positive voltage passes to the gate of the thyristor Q804, so it becomes connected - which causes the high voltage to stop and does not return to work until it is reset to the correct value.

B - Preventing the beam current from exceeding: It feeds the (ABL) voltage of the automatic illumination limiter. Automatic Brightnesslimiter) to the prevention circuit through the resistance R824 and charge the capacitor C815 when the beam current is normal, this voltage is positive and applied to the Zener anode D809 and this positive voltage on the Zener anode will prevent it from connecting - and the diode D814 installs this positive voltage at a maximum value of 1 volt ).

When the beam current increases, the (ABL) voltage decreases and becomes negative. This negative voltage is set to a value of (0.6) volts by the diode D815. The negative voltage from C815 applied to the Zener anode D809 causes a second connection of the thyristor Q804 (SCR), and disables the horizontal oscillator - and does not return to work again until the beam current drops to level Al-Mamoun.

Automatic Brightness Limiter (ABL) circuit:

This circuit (ABL) is found in many colored devices, and it is based on determining lighting and preventing its glowing. This is due to the increase in the size of the image: the screen valve elevator: this may cause an increase in the leakage of the beam current resulting from the high level of the visual signal and its illuminated background - although the screen elevator voltage is most likely regulated, the high voltage feed can go out of regulation if it exceeds The beam current limits its regular work.

The image flicker also occurs due to the increase in the amount of deviation when the anode voltage decreases (decrease in the beam acceleration). This condition is usually accompanied by some imbalance in the focus (focus) in the image, as the work of (ABL) is to prevent the leakage of the beam stream from reaching the value that causes the image to bloom.

Working principle of ABL circuit:
From the previous diagram of the Automatic Lighting Limiter (ABL) circuit (Fig. 11-55), this circuit performs the following:
A- Reducing the amplitude (duration) of the video signal and limiting the lighting.
B - Determining the value of the beam current by means of the control grid of the screen valve.
11-21 Spark gaps in the screen valve base plate:

Since the screen valve operates with high voltages that may reach (33 kilovolts) and considering that the ejectors and screen elements are located in a closed space, spark discharge and internal electric arcs may occur that damage the screen elements or circuit transistors. To prevent this from happening, the screen valve base plate is provided with gaps In all its poles except for the heating filament. These gaps stop the high voltage, which slightly exceeds the work effort. These gaps are usually for a voltage of (1500 volts to 5000 volts), and sometimes the spark gap voltage must be higher than the upper limit of the maximum voltage - in this case, the current limiting resistance may be placed with a value of about (1000 ohms) in series. With a connection point at the base of the screen valve, and the spark gap may be inside a disc ceramic capacitor or in the form of two electrodes installed in a ceramic mold.

Protection of the heating filament from short circuit with the cathode and the occurrence of sparks between them:

Most manufacturers of color television sets - in order to avoid contact between the heating wire and the screen landing pad or sparks between them, do the following:

1- The heating filament of the screen valve is fed through a special transformer whose voltage is 6.3 volts alternating (VAC), and it is isolated from the grounding.

2- The voltage +B is connected to the heater through a resistance whose value ranges between (100) kilo ohms to 1 mega ohm. This bias voltage makes the heater voltage closed. With cathode voltage, this reduces the possibility of sparks between them and then short.
3 - It is recommended that the heater be somewhat negative in relation to the cathodes, which gives better protection to the screen.