أعطال مكبرات التردد المتوسط للصورة (if) .. كتاب التلفزيون الملون والعادي

6-10 Malfunctions of IF amplifiers:

The mid-frequency stage of the image must pass the following signals:

1 - Image information signal

2 - Color information signal

3- The horizontal and vertical synchronization signal.

4 - The extinguishing signal (erasing) for the horizontal and vertical return lines.

e - sound information signal

When there is a defect at this stage, the defect appears in the image, sound, or
in the scanning process or colors.


Malfunctions can be divided into three categories:

1- Total or partial loss of signal.

2- A glitch in adjustment and calibration.

3 - Malfunctions of the AGC automatic gain control circuit.

The manifestations of faults are similar whether the device is a valve, a transistor, or an integrated circuit, and they differ only in the amount of effort and the elements of the circuit.

1- Missing the reference:

A - Complete loss of signal.

The screen shows the illumination of the linear structure (raster), and there is no picture. The reason for this malfunction is the following possibilities:

1- Absence of B+ nutrition at this stage.

2- A malfunction in a transistor or an integrated circuit


3 - A malfunction in the AGC automatic gain control circuit, which causes some transistors to be cut off.

B - partial loss of signal.

The picture shows poor contrast and the sound is good. It may be distorted by the 50 Hz sync pulse signal.

If the voter is healthy and does not have any defects, the photograph must appear without any snow spots. These spots may appear: due to a malfunction or not adjusting the AGC properly, or due to a malfunction in the RF amplifier circuit in the voter.

It is possible to distinguish and locate the fault as follows: If it is in the IF stages, then the sound appears normal or distorted with 50 Hz synchronization pulses. That is, the sound does not disappear when the defect is a partial absence of the medium frequency signal of the image.

​​​​​​​​2- Defective adjustment or calibration: The adjustment of the medium-frequency system may be disturbed due to normal use, or when changing one of the elements (valve - transistor - (IC). Also, exposure of the elements to heat or shocks when transporting or moving may make re-adjustment necessary.

Misalignment may cause the image to appear smudged. Adjusting the FTC adjuster also changes the amount of image smearing. Because it changes the position of the image carrier on the intermediate frequency response curve. The smearing of the image arises from a phase shift due to the increased sensitivity of the response curve to low frequencies, and any shift in the medium frequency circuit of the image can affect the color signal. It appears in incorrect places in the image - and if the smearing of the image is not affected by calibrating the fine frequency adjustment switch, then the greatest possibility is that there is a defect in the image detector or the image signal amplification circuit.

The incorrect setting of the IF phases of the image can appear by losing the fine details of the image due to the loss of high frequencies of the visual signal. Also, the color signal may be completely or partially lost due to a malfunction of the IF stage setting. Also, crosstalk may occur when the IF stage is faulty, especially when the trap is used to get the correct sharpness of the response curve.

Excessive response to high frequencies in the middle frequency curve may cause buzzing in the amplification circuits, and give ghosts or ghosts that follow the vertical lines of the image. It is distinguished from antenna-induced fanfare in that the buzz changes with the change of the instrument's fine-tuning tuning knob

Some transistors of the IF amplifier stage use negative feedback to reduce the possibility of oscillation, and to widen the frequency response curve of the amplifier.

The voter is on a non-working channel, so you must use special measuring and calibration devices recommended by the company and according to its instructions. Likewise, when the stages are reset due to the accuracy of this process and its impact on the quality of the image, sound and colors.

AGC circuit faults: AGC circuit faults have the same symptoms as faults in the intermediate frequency phase of the IF image. AGC faults appear on the device screen in the form of a faded image with weak contrast and faded colors, or in the form of an overloaded image with excessive contrast, and the reason is due to the increase and weakness of the AGC voltage in the first case. In the second case.

The difficulty of dealing with faults and adjusting the AGC comes from the linkage of the AGC and the IF stage in the form of a loop that is part of the feedback circuit - the AGC voltage is related to the correct operation of the IF stage and the correct operation of the IF stage is linked to the AGC voltage applied to the circuits it governs. Thus it is difficult to separate the AGC and IF phase breakdowns.

The preferred way to determine the location of the fault is to disconnect the AGC circuit and replace it with a DC bias voltage to do its work, as well as to secure an independent DC bias for the voter in place of the AGC voltage. If the device works correctly, this indicates a malfunction in the AGC circuit - but if the defect remains as it was, the location of the malfunction continues in the voter stage or the IF stage.