Generator - AF probe. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Generator - AF probe. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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The proposed generator is useful for troubleshooting various electro-acoustic equipment. It allows you to apply the voltage of the audio signal to various points of the device under test, starting from the microphone input, and, thus, to search for an idle cascade or channel of a stereo device.

The generator provides at the output rectangular pulses with a duty cycle of 2 (meander) and an average frequency of the audio range (1000 ... 1200 Hz). Their amplitude is fixed: 1 mV, 100 mV or 0,7 V. To control stereo channels, the generator has two outputs with independent switching of the signal. The signal amplitude at the two outputs is the same.

The rectangular waveform allows, in the presence of an oscilloscope, to judge the amplitude-frequency and phase-frequency characteristics of the device, as well as its tendency to self-excitation [1].

The generator circuit is shown in fig. one.

Generator - AF probe

The generator is assembled according to the scheme of a symmetrical multivibrator [2] on transistors VT1 and VT2. It is desirable to select transistors with a current transfer coefficient of at least 50. The generation frequency is determined by the elements R3, R4, C1 and C2. The collector load of the second transistor is made up of three resistors, forming an output voltage divider. Switches SA2 and SA3 set the amplitude of the output voltage, switches SA4 and SA5 signal is fed to the desired output. Toggle switches of the MT-1 type were used as switches.

The HL1 LED performs several functions: together with the resistor R1, it stabilizes the multivibrator supply voltage, indicates the generator is turned on and the battery status. A decrease in the brightness of its glow indicates that it is time to charge the battery.

The generator is powered by two D-0,25 disk batteries. This provides a clean output signal free of any impurities and AC hum. For the same purpose, the generator is placed in a metal case, and the output wires with probes are shielded.

To charge the batteries, a device is used, the circuit of which is shown in Fig. 2.

Generator - AF probe

The quenching capacitor C1 in the AC circuit limits the charging current to an acceptable level, and the resistors R2 and R3 limit the possible current surges when turned on and voltage drops in the network. The VD5 zener diode limits the voltage at the output of the charger if contact with the battery is broken or if it is mistakenly connected to the network without batteries.

The design of the charger should exclude the possibility of touching any current-carrying conductors with your hands. Capacitor C1 (type K73-17 or MBM) is selected for an operating voltage of at least 300 V. The rectifier bridge can be used ready-made, for example, type KTs405E. Battery charging time - 15...20 hours.

Most of the generator parts are mounted on a printed circuit board made of one-sided foil fiberglass, a sketch of which is shown in fig. 3.

Generator - AF probe

The board has no holes - the leads of the parts are superimposed on the printed conductors and soldered to them. Conductive traces can be cut with a sharp knife or cutter along a ruler without resorting to etching. Unnecessary foil between the conductors is removed in any way, for example, using the same knife. To facilitate this work, it is useful to warm the foil with a soldering iron.

The fabricated printed circuit board is placed in a case of suitable dimensions, made of any material. In the author's version, a metal box measuring 100x60x30 mm, pasted over with a decorative film, was used. On the top cover there is a power switch SA1 and switches for output signals SA2 - SA5. Any output connectors XT1, XP1 and XP2 are suitable, you can do without them altogether by leading three output wires through a hole in the side wall of the case. The wires must end with probes or crocodile clips.

The generator, assembled from serviceable parts, practically does not require adjustment, it starts working immediately. If desired, you can set the desired generation frequency by selecting capacitors C1, C2 or resistors R3, R4. It is also useful to adjust the output voltage levels by selecting resistors R5 - R7.

Literature

Ivanov B.S. An oscilloscope is your assistant. Appendix to the magazine "Radio". - M., 1991.
Borisov G., Partin A. Practicum radio amateur in digital technology. Appendix to the magazine "Radio". - M., 1991.

Author: I. Gorodetsky, Moscow

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