دارة agc في الدارات المتكاملة .. كتاب التلفزيون الملون والعادي

8-7 AGC circuit in integrated circuits:

The integrated circuit shown in Figure (8) - (13) contains four basic stages, namely:

1 - AGC circuit.

2- The jamming processor (reflector) circuit.

3- Synchronization separator circuit.

4 - Reference circuit for DC power supply.

These sections are related to the work of the AGC. As for the control signals provided by the integrated circuit, they are:

1- Non-delayed (direct) AGC voltage in the forward way of phases.

​​​​2 - AGC delay voltage in the forward way to control voter amplification using bipolar transistors.

3 - The AGC delay voltage of the voter that includes FET transistors.

Latch circuit:

Sync pulses may be absent, especially when the voter tuning is changed from one station to another. This circuit feeds the device with negative synchronization pulses during the time of the original horizontal synchronization pulses - and when the original horizontal pulses do not appear at the same time - the current passes to the ground, the AGC transistor does not work, and the AGC comparator remains in a state of stop - and when the correct synchronization pulses return to work, it returns transistor to the correct work

8-8 Malfunctions of the AGC automatic gain control circuit:

The image is negative (negative) This defect - which is the inversion of the black and white areas in a monochrome TV - is caused by the lack of AGC voltage feeding the RF and IF stages. The picture
Negative occurs when the mid-frequency signal of the image is high.

Its effect is mostly on the last amplifier in the IF stage. The possible faults when there is a negative image are:

1 - AGC setting error

2 - A transistor or valve in the AGC stage has failed.
3 - A transistor or valve in the intermediate frequency stage IF or the visual amplifier has failed
4- The arrival of a positive voltage on the AGC line.

There is no picture or sound when the screen is lit (raster):

This is caused by a complete loss of signal, often caused by too much AGC voltage applied to the stage of the RF or IF amplifiers or both - this much voltage causes the circuit to break and stop working.

This malfunction may appear in all types, i.e. the fuse - and the transistor or the one with integrated circuits - and the reason is due to the failure of a part or element of the AGC circuit. Image .

Overloaded image with overlap:

This phenomenon is easily known. By noticing the increase in blackness in the monochromatic image. If overloading affects the IF stage, it may lead to horizontal drawdown in the image. Or a vertical stop - in addition to the possibility of a buzzing sound, especially when the signal is strong - and this is due to the overlapping of 50 Hz pulses with the medium frequency signal of the sound. This malfunction occurs as a result of insufficient AGC voltage
. The fault may be simple, and it can be fixed by resetting the AGC statement. If the fault is not corrected, it is likely that there is a fault in a transistor or an element in the circuit. It is noted that this malfunction is similar to what occurs when there is a defect in the IF phase. Note: Due to the presence of similar symptoms caused by the faults of the AGC circuit, the RF phase, the IF phase, or the visual amplifier, so it is necessary to distinguish between these faults and limit them. We will discuss that later