Improving the picture on an old TV. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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The alterations proposed by the author of the published article improve the reproduction of the image on the TV screen.

In Russia and other post-Soviet countries, there are still many televisions of the brands "Horizon", "Vityaz", "Seagull", "Youth" of the third or fifth generations in operation. They used a typical video channel on a K174XA33 (TDA3505) chip and transistor video amplifiers. In order to improve the image quality on the screens of such TVs, it is first proposed to slightly change (simplify) the video amplifiers in them, although objectively this worsens the signal type to some extent.

To clarify the proposal, consider a simplified circuit diagram shown in Fig. 1, one channel of video amplifiers, the most widely used in Gorizont TVs. Particular attention should be paid to the transistor VT3. It serves to reduce the output impedance of the video amplifier and provide a high rate of recharging of the parasitic load capacitance that exists in the kinescope between the electron gun electrodes. From the point of view of signal amplification, this is absolutely the right decision. However, from the point of view of perception of the image, it seems to be ambiguous.

Consider what happens if you remove the transistor VT3, and install a jumper instead of the diode VD1. Obviously, then, due to an increase in the output resistance, distortions of the video signal will occur along the fronts of the video pulses. At the same time, these distortions will be perceived differently, depending on the parasitic capacitance recharge current. At a voltage of 180 V across resistor R3, the resistance of which is 18 kΩ, the recharging current is 10 mA, and at a voltage of 18 V it does not exceed 1 mA. At the same time, if a low-amplitude signal of a relatively high frequency of the "meander" shape, shown in Fig. 2 with a dashed line, with a high level of the constant component, the signal will be amplified with virtually no distortion. At a low level of the DC component, it will turn into a sequence of triangular pulses (sloping front and vertical decay) of very small amplitude, shown by a solid line.

It should be repeated once again that from the point of view of the fidelity of signal amplification, this is bad. However, for the viewer, this means reducing the visibility of small signals (and, above all, noise) at levels close to the black level. There is a kind of noise reduction effect in the video amplifier. Recall by the way that the principle of negative modulation of a radio signal by a video signal in television broadcasting also serves to reduce the visibility of weak interference, since they are below the black level.

In addition to the above, it should be noted that the emitter junction of the transistor VT3 and the diode VD1 are in the common feedback (OS) circuit of the video amplifier, and during the edges of the video signal pulses, this OS circuit breaks. As a result, due to the phase delay of the OS signal, switching distortions occur in the form of voltage spikes. When amplifying a high frequency video signal, this undefined state of the video amplifier becomes very noticeable. On the screen, this manifests itself in the form of a distortion of brightness differences (for example, titles on a black background at the end of the film are poorly readable).

Schematic diagram of the video amplifier, which takes into account the above considerations, is shown in Fig. 3. It allows you to use existing video amplifiers with minimal modifications. The general OS has been replaced with a local one. By selecting the resistor R4, you can change the gain of the video amplifier, and by selecting the capacitor C1, you can achieve the most optimal correction for image clarity for perception. Additionally, it is necessary to reduce the reference voltage to 2,5 ... 3 V (common for all video amplifiers), for which the resistance of the resistor connected between the base and collector of the transistor in the cascade that forms this voltage is reduced.

Consider, for example, a specific revision of the TV set "Horizon - 51CTV441". A fragment of its circuit diagram is shown in Fig. 4. The changes are shown for the "red" (R) channel video amplifier, the other two are being modified in the same way.

(click to enlarge)

First of all, remove the diode VD11, transistor VT20 and resistors R119, R131 (marked with a cross). Instead of a resistor R102 with a resistance of 1,5 kOhm, a resistor with a resistance of 100 Ohm is installed, and instead of a VD16 diode, a jumper is installed. Next, they break (cut) the printed conductor connecting the emitters of transistors VT16 and VT19. Using the method of surface mounting, a resistor Rdop with a resistance of 470 Ohm and a capacitor Cdop with a capacity of 1300 pF are soldered into the gap. It is most convenient to do this using elements for surface mounting of size 1206 or 0805. They can be soldered right at the cut site To reduce the reference voltage, the resistor R116 with a resistance of 1,2 kOhm is replaced by a resistor with a resistance of 200 ... 220 Ohm.

In other TV models, there may be some differences in video amplifiers, usually associated with high-frequency correction circuits. Resistor R3 (see Fig. 1; in Fig. 4 - R126) can have a rating of 24 kOhm. In this case, it must be reduced to 16 ... 18 kOhm.

After the TV has been modified, the colors on the screen become more saturated and uniform, since during the transmission of the dominant color, due to the noise reduction effect, the signals of non-primary colors are suppressed. The image becomes somewhat idealized.

Finally, you can further improve the image quality in these TVs by replacing the SKTs-45 color transition correction submodule with the LZYAM-0,47 delay line in the luminance signal circuit. In this case, the inputs of the color difference signals RY and BY must be connected to their respective outputs.

The fact is that the increase in the steepness of the differences in color-difference signals, with all the usefulness of such a function, does not create a significantly noticeable visual effect of improving the image. The delay of the brightness signal Y in the microcircuit, firstly, significantly reduces the quality of the signal itself and, consequently, the picture, and secondly, it limits the range of the brightness signal at the input of the K174XXNUMX video processor, which in turn entails a limitation in the dynamic characteristics of the image.

A schematic diagram of one of the variants of the luminance signal delay unit is shown in fig. 5.

Capacitor C1 is selected according to the best image quality within 100...300 pF. Adjusted resistor R1 sets the level of the brightness signal so that whitish areas do not yet appear on the brightest image.

The delay line should be LZYAM-0,47-1150 or similar (almost a "cube"), which is used in Yunost TVs of models 32ТЦ309/312. All elements of the node are placed on an additional board so that, if necessary, you can restore the standard configuration of the TV.

Author: S. Kostitsyn, Izhevsk

See other articles Section Телевидение.

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