When there is no desired volume control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Телевидение

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Many users have probably encountered malfunctions of the volume controls in radio equipment. This drawback can be eliminated by applying an original solution, which is described in the material published here.

Volume controls in household equipment wear out more often than other parts. Rustling, crackling noises appear in the dynamic heads, and sometimes the sound even disappears. Attempts to completely restore regulators are usually unsuccessful. It's good to have spare variable resistors. And if not? After all, the whole difficulty often lies not in finding the desired design and rating, but in ensuring that the change in resistance from the angle of rotation of the engine axis occurs along curve B, shown in Fig. 1. The task is further complicated by the fact that in many cases the regulator is associated with a switch.

In domestic equipment, in particular in televisions where there is a K174UR1 microcircuit, it is proposed, for example, to use a variable resistor of group A (Fig. 1) with electronic correction of resistance changes as a volume control. In this case, it becomes possible to use it even when it is coupled with a switch (curve B in Fig. 1).

The essence of this non-standard solution is that the variable resistor is supplemented with a field-effect transistor connected to the control circuit of the attenuator of the K174UR1 integrated amplifier according to the circuit shown in Fig. 2. Most often, pin 5 of the microcircuit is connected to the common wire through a 10 kOhm resistor, which is removed. Since pin 5 is specifically designed for electronic gain control, it would be possible to build a completely electronic regulator, but this solution may not always be considered satisfactory. And, first of all, here's why: when, say, a TV's audio IF amplifier is electronically closed, there is still a significant level of hum and noise at its output.

In the proposed embodiment, the variable resistor remains, which significantly reduces the signal along with the noise mentioned above before the “electronics” come into action. In other words, the correction of the adjustment curve of the variable resistor occurs mainly in its lower (according to the circuit) part, i.e. at low sound volumes. In addition, pairing the volume control with the switch is easy.

It should also be noted that the use of an electronic regulator also requires a significant change in installation, but in the proposed version this is not required.

The non-standard solution lies in the combination of two adjustments at the same time - traditional resistive and electronic from the engine of the same variable resistor. When the engine is in the upper positions according to the diagram, the “electronics” are turned off, since the closing voltage is supplied to the gate of the field-effect transistor VT1 through the divider R3R4.

As the slider moves down (according to the circuit), when the level of the signal taken from it is reduced approximately threefold (depending on the selection of elements), the field-effect transistor begins to open and reduce the signal already at the output of the microcircuit itself through the attenuator built into it. As a result, the noticeability of noise in the traditional control circuit is significantly reduced and the necessary smoothness of adjustment is ensured.

By selecting resistors R1 and R3, you can achieve timely and smooth volume control at the desired area of ​​rotation of the control knob. Such a selection will be required in any case due to the spread in the input characteristics of the field-effect transistor used.

The proposed control unit is used in a portable TV "Electronics 404", which has a power switch connected to the volume control from an external battery. For guidance when setting up the unit, the diagram (Fig. 2) shows the limits of voltage change at pin 5 of the microcircuit from the open state of the attenuator (+2,1 V) to its complete closing (+1,45 V). The voltages indicated at other points may be different, depending on the parameters of the transistor used.

Note that when using the described unit, a problem arises with the inclusion of oxide capacitors before and after the volume control. When turned on normally, they were connected to the common wire through a variable resistor. In the proposed version, some positive voltage is supplied from it to the capacitors. Therefore, it is necessary to ensure that there is no change in the polarity of the voltage on the capacitors. Otherwise, you will have to change the polarity of their inclusion.

Installation of this relatively simple unit can be done on a small board with two pins, which are soldered in place of the remote resistor that connected pin 5 of the microcircuit to the common wire. The conductor connecting the slider of the volume resistor R4 to resistor R2 does not need to be shielded, since pulse and RF interference is eliminated by filter R2C1.

The node can use almost any variable resistor of suitable dimensions and rating. If a switch is needed in the device on the regulator, then the variable resistor is modified: the screen cover is removed, the existing slider is replaced with a slider from the old controller and covered with a cover from it, on which the switch is mounted. After this, resistors R1 and R3 are selected so that the complete loss of signal at the output of the node occurs before the switch operates (see curve B in Fig. 1).

Author: S. Tuzhilin, Shchelkovo, Moscow region

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