Generator for the repair of radio equipment. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Generator for the repair of radio equipment. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Measuring technology

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When repairing an audio amplifier or household radio at home, it is often necessary to trace the passage of the signal through the cascades. The one shown in Fig. 1.23 diagram of a simple two-frequency generator. It is assembled on just one CMOS chip and does not contain winding units, which makes the device easy to manufacture, configure and operate.

(click to enlarge)

This generator makes it possible to check not only the audio amplifier, but also the intermediate frequency amplifier (IF) path of the radio receiver. The generator also allows you to adjust the IF circuits of the radio receiver according to the maximum signal level.

At the output (X2) of the device there will be radio pulses with a frequency of 465 kHz, modulated by a low-frequency signal - 1 kHz (100% modulation). In this case, if you turn on SA1, then only a low-frequency signal will appear at the output - pulses with a frequency of 1 kHz.

The high-frequency generator operates at a frequency of 465 kHz and, in order to obtain high stability from it, is made using a piezoceramic filter (ZQ1) of the FP1P-022 type in the negative feedback circuit of the DD1.2 microcircuit element. Such filters are more accessible and cheaper than quartz resonators for the corresponding frequency.

The audio range pulse generator (DD1.1-DD1.3) is assembled according to the classical scheme and needs no explanation. On the DD1.4 element, two frequencies are mixed and fed to an emitter follower made on a transistor VT1. The transistor matches the high output impedance of the microcircuit with a possible low resistance in the load circuit.

The generator provides operation in a wide range of supply voltages (4...15 V) and consumes a current of 3,7...26 mA. In this case, the frequency of the high-frequency oscillator changes over the entire range of supply voltages by no more than 400 Hz, which is quite acceptable.

In order for the output signal level of the oscillator to not strongly depend on the supply voltage of the circuit, there is a limiting diode VD1 at the output. The output signal after capacitor C4 will have a maximum amplitude of about 0,3 V, and with the help of resistor R6 it can be reduced to the required value.

Diode VD2 prevents the erroneous supply of the polarity of the supply voltage to the circuit.

In the circuit, you can use a piezofilter (ZQ1) of type FP1P-022...027. Adjusting resistor R6 type SPO-0,5, and the remaining resistors are MLT and C2-23. Capacitors: C1 - K53-1 for 16 V; C2...C4-K10-17.

The circuit is quite simple, which makes it easy to mount it on a universal breadboard.

The setting consists in setting the selection of the resistor R2 (with closed contacts SA1) to a frequency of 1 kHz at the output. After that, using the frequency meter, we check the frequency of 465 kHz ± 0,5 kHz.

In order to make it convenient to measure the frequency, we turn off the RF signal modulation, which can be done by applying supply voltage to the outputs DD1 / 12, 13.

If, due to the spread of the parameters of the logic elements (internal capacitance of the microcircuit), the ZQ1 piezofilter does not work accurately at a frequency of 465 kHz, then it may be necessary to install an additional capacitor C2 with a capacity of about 100 ... 470 pF, as well as select a resistor R3, which will allow you to shift the operating frequency generator within a small range.

Publication: cxem.net

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