TV as an oscilloscope. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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TV as an oscilloscope. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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Attachment (Fig. 1), turns any TV into large screen oscilloscope. On it, you can observe low-frequency oscillations, and with the help of a sweeping frequency generator (GKCH), you can visually tune the IF amplifiers of radio receivers. The set-top box can be thought of as a miniature television transmitter.

Despite the relatively simple circuit, this transmitter generates a complete television signal, which differs from the standard one only in the absence of equalizing pulses.

Frame sync pulses are formed from the reference sinusoidal voltage by the limiting amplifier VT1, the differentiating circuit R8C4 and the threshold amplifier at VT4. Their duration is about 1,9 ms.

The blocking oscillator (on transistor VT5) generates horizontal sync pulses. These are not the main pulses of the blocking generator, but collector voltage surges that occur immediately after the main ones. Between the collectors of transistors VT4 and VT5, an isolation diode VD3 is connected. At the time of generation of the main pulse, the collector of the transistor VT4 closes on the chassis through the open transistor VT5 and the diode VD3. As a result, insets appear in the vertical sync pulses, which, as required, precede the horizontal sync pulses. The windings of the VT1 transformer of the blocking generator are wound on a toroidal core made of oxyferite (F-1000). The outer diameter of the core is 10 mm, the thickness is 2 mm. Windings I and III contain 100 turns each, and winding II contains 30 turns of PELSHO o0,1 wire.

At the beginning of the horizontal scanning period, the voltage pulse of the blocking oscillator quickly charges the capacitor C6 through the diode VD2. During the rest of the period, it slowly discharges through resistor R6. The resulting sawtooth voltage is fed to the base of the transistor VT2. Here it is added to the input voltage. The three-stage amplifier, due to the large gain (50000-100000), operates almost in a relay mode, characterized by a certain threshold.

The set-top box parameters are chosen such that in the absence of the investigated voltage, the axial line is in the center of the screen. If necessary, the image on the screen can be shifted in one direction or another by changing the resistance of the resistor R3. To improve the clarity of the line image on the TV screen, the amplifier (VT2, VT3, VT6) is covered by positive feedback from the collector of the transistor VT3 to the base of the transistor VT2 through the capacitor C5. This greatly increases the gain in the high frequency region and therefore increases the steepness of the output pulse edge. Visually, this is manifested in the increased sharpness of the transition from white to black.

Personnel, horizontal and video pulses are added at the input of the emitter follower VT7, which is a modulation amplifier of the VHF generator VT8. The latter is assembled according to the capacitive three-point scheme. The generation frequency should be chosen equal to the carrier frequency of the image of a free television channel. Otherwise, the set-top box may interfere with neighboring TVs. The required generation frequencies can be obtained by selecting the number of turns of the coil L1.

Rice. 1 (click to enlarge)

When tuned to the second television channel (59,25 MHz), the L1 coil contains 5 turns of PEV 0,6 wire, the coil diameter is 9 mm. The modulated RF voltage is fed to the output of the set-top box through the divider R18-R19, which lowers the voltage to 3 mV to avoid overloading the RF path of the TV. The output of the set-top box is connected with a coaxial cable or a twisted double wire to the antenna input of the TV.

Construction and adjustment

All parts of the set-top box, with the exception of the VHF generator, can be placed on the circuit board in any order. Parts related to the VHF generator (SP-S15, L1, VT8) must have short leads, they should be connected to each other with short conductors and grouped in one place. No shielding of the set-top box is required. If the pulse frequency of the blocking oscillator does not lie in the frequency range of the TV lines, it is necessary to enter it into this range by changing the resistance of the resistor R14 within a small range.

It should be noted that the synchronization of the TV scans from the set-top box usually turns out to be very stable, so poor synchronization when setting up the set-top box indicates some kind of error in the installation. In order to achieve fine tuning of the VHF generator of the set-top box to the selected television channel, you have to stretch or compress the turns of the winding of the L1 coil, i.e. change winding pitch. When set correctly, the line on the screen is sharply defined. The settings of the set-top box are chosen so that the largest image size on the TV screen corresponds to an input voltage of about 0,3 V. The sensitivity of the set-top box can be adjusted by changing the resistance of the resistor R2. To test the sensitivity, an alternating voltage of a known value is applied to the input or from a sound generator.

Literature

Radioamator №6 1999

Author: Shronin, Kremenchug, Poltava region; Publication: cxem.net

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