Sweep generator from SK-M-24-2. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Currently, many are replacing third-generation TVs with more modern ones. It is a pity to throw the old and faulty into a landfill. Meanwhile, simple devices can be assembled from individual blocks and assemblies of these devices. One of the examples of the unexpected use of the TV channel selector is described in this article.

From the TV channel selector SK-M-24-2, you can assemble an attachment to the oscilloscope - a sweeping frequency generator for viewing the frequency response of radio and television equipment in a wide frequency range - 0,5 ... 100 MHz. At the same time, the manufacture of the device consists mainly in soldering unnecessary parts for this device from the channel selector board and adding a small number of new ones.

This GKCH has a classic block diagram of devices of this group (Fig. 1). It has two generators G1 and G2, tunable in frequency by changing the voltage. The tuning limits of the first GKCh generator are 150 ... 250 MHz, and the second - 150 ... 160 MHz. The frequency deviation of the generator G2 is achieved by changing the capacitance of the varicap in the oscillatory circuit with a sawtooth voltage from the oscilloscope scanner. The high-frequency voltage from these generators is fed to the mixer U1, at the output of which oscillations of the difference frequency of 0,5 ... 100 MHz are formed, with a deviation of the selected center frequency up to ± 5 MHz. This voltage is fed through the emitter follower A1 and the low-pass filter Z1 to the amplifier A2, and from it through the matching stage A3 to the output of the device. The gain A2 and, accordingly, the voltage at the output of the GKCh are electronically regulated.

The schematic diagram of the GKCh is shown in fig. 2. Generators G1 and G2 are assembled, respectively, on transistors VT1 and VT3 according to a capacitive feedback circuit, which is carried out through capacitors C7 and C8. High-frequency oscillations from generators through capacitors C1, C2 and diodes VD1, VD2 arrive at the emitter of transistor VT2, which acts as a mixer. After the emitter follower on VT4, the difference frequency oscillations allocated by the low-pass filter (L3-L5, C15-C18, C21) are fed to the VT5 transistor for amplification. The emitter follower on VT6 serves to optimally match the amplifier to the load.

(click to enlarge)

The control of the central frequency of the GKCH is carried out by a variable resistor R26, and the adjustment of the frequency band under study is performed by R28. The frequency deviation of the generator is regulated by a variable resistor R29. The output voltage of the GKCH is changed by the regulator R25. It must be borne in mind that the maximum depth of deviation depends significantly on the amplitude of the sawtooth voltage supplied from the oscilloscope.

Additional details, in addition to those available in the channel selector, are shown in the diagram with thicker lines.

The described device allows tuning in a wide frequency range without using a range switch. The operating frequency range of the GKCh is limited in the range of 0,5 ... 100 MHz by the properties of the applied low-pass filter and the necessary spacing between the generator frequency and the maximum difference frequency.

In the manufacture of the device, it is necessary to compare its schematic diagram with the SK-M-24-2 diagram [1, 2] and unsolder unnecessary parts from the block. Naturally, the purpose of the pins of the board connector has been slightly changed relative to the original one. In addition to the remaining parts, transistors VT4, VT6, resistors R14, R16, R21-R24, capacitors C15-C18, C23-C26, coils L3-L5 are installed on the board. In this case, all newly installed coils and capacitors are taken from among those soldered from the board; for example, L3-L5 are "similar" coils from the input filter of the selector.

The location of the coils L1 and L2 directly on the circuit board of the block in close proximity to other parts worsens their quality factor and, therefore, reduces the stability of the output frequency of the MFC. Therefore, coils L1 and L2 are soldered from the board, and pieces of tinned wire 1 cm long are soldered into the holes formed, and these coils are soldered to their ends again, placing them between the board with parts and the top cover. The described arrangement of coils L1 and L2 is also convenient when setting up the device. They can be repeatedly soldered and soldered without violating the integrity of the printed conductors.

Variable resistors - any small-sized. Connectors XS2 and XS3, which are small-sized jacks for connecting stereo phones with a 3,5 mm plug, are installed on the walls of a tin box attached to the outside of the device body from the side of the XS1 connector. Capacitors C27, C28 (K50-12) and resistor R27 (MLT) are hinged mounted on the contacts of variable resistors and connectors.

The main generator G1 is adjusted by selecting the inductance of the coil L1 by stretching or compressing its turns, and the frequency meter checks the range of the generator overlap on the transistor VT1. At the same time, the power supply of the generator G1 on the transistor VT2 is turned off at the XS3 connector.

Similarly, tune the generator G2 in the specified frequency band, turning off the power of the other. This setting is made at the maximum voltage on the VD4 varicap.

The low-pass filter L3-L5, C15-C18 is adjusted to pass the signal in the frequency band up to 110 MHz. After adjusting the filter, the coils L3 and L5 each have 11 turns with an inner diameter of 3 mm, L4 - five turns with a diameter of 4 mm.

A schematic diagram of the detector head is shown in fig. 3, and the connection diagram of the instruments during measurements is shown in fig. 4. It should be borne in mind that the oscilloscope used in conjunction with the GKCH must provide a "decreasing" sawtooth voltage (for example, the widely used S1-94 oscilloscope). If the radio amateur has only an oscilloscope with an increasing "saw" at his disposal, then the GKCh frequency deviation must be performed using the G1 generator.

The magnitude of the output voltage of the GKCh can be judged by the following measurements. The DC voltage at the output of the detector head connected to the GKCh output is 0,9 V in the middle part of the range, and 0,3 and 1,9 V at the edges of the range. output GKCh respectively twice lower.

The appearance of the console is shown in fig. 5 (the control knobs are temporarily removed from the axes of variable resistors).

Literature

1. Elyashkevich S. A., Peskin A. E. TV sets 3USCT, 4USCT, 5USCT. Device, adjustment, repair. - First edition. - M.: MP "Symbol-R". - 1993. - 224 p.
2. Katsnelson N., Shpilman E. "Horizon C-257". Radio channel module. - Radio, 1984, No. 9, p. 24-28.

Author: N. Herzen, Berezniki, Perm region

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