Industrial receiver - hearing aid. Encyclopedia of radio electronics and electrical engineering

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To listen to an interesting radio program, a hearing-impaired person needs a receiver with an output to a headphone, and for free communication with others, a hearing aid is indispensable. Having both devices with you at once, switching the head phone from one to the other, is rather inconvenient. To combine both functions in one device, some radio amateurs (see, for example, V. Petrov's article in "Radio", 1997, No. 1, p. 21) assemble a direct amplification receiver with a switch that allows you to optionally connect a radio signal reception unit to the UZCH or microphone.

A similar problem can be solved easier and faster if you use the factory dual-band superheterodyne of previous releases. In this case, in one position of the band switch, the device will operate in the normal mode of receiving radio signals (medium waves), and in the other, it will amplify the audio frequency signals coming from the microphone. In this case, the amplifying properties of not only ultrasonic frequencies, but also radio frequency cascades are used. The essence of the latter is that the local oscillator of one range (for example, long-wavelength) is tuned to a fixed frequency equal to the intermediate one, and the LW preselector "breaks away" from the converter and is replaced by circuits with the microphone turned on.

The audio signals entering the converter input (see figure) modulate the intermediate frequency oscillations, which freely pass through the lumped selection filter (FSS) tuned to a frequency of 465 kHz, are amplified in the IF cascades and after the usual detection in the ultrasonic frequency. This construction of the sound channel allows you to get more gain than with the operation of one ultrasonic frequency converter, as well as use the AGC action.

The figure shows a fragment of a typical input part and a converter for circuits in the DV range (the Gauja receiver, factory circuit, with earlier modifications of the receiver, transistors with the npn structure were used, the polarity of switching on oxide capacitors and diodes was changed, power supply with a grounded minus current source). In two places, the dashed line marks breaks in regular circuits, which are performed by cutting through the foil of printed conductors. The newly introduced circuit elements and their circuits are highlighted with thicker lines.

What to be guided by when tuning the frequency of the local oscillator? It is known that on the LW its lower frequency is 615 kHz. Therefore, the required frequency reduction factor will be K = 615/465 = 1,32. Without touching the loop coil L4 (its inductance is 580 μH), it will be necessary to increase the capacitance of the heterodyne circuit capacitor by a factor of K2 \u1,75d 5 (taking into account the capacitance of the installation). To do this, we will replace the standard capacitor C25 with a capacitor with a larger capacity (instead of a capacitor with a capacity of 51 pF, a similar XNUMX pF should be used).

Then, having broken the conductor coming from the range switch to the L3 coupling coil with the DV antenna circuit, you should connect the BM1 microphone circuit and an additional resistor R5 to connect to the receiver power circuit. The latter is necessary for the operation of an electret type microphone (MKE-332B). The microphone can be placed next to the dynamic head.

Having completed the revision, we proceed to adjust the work of additional elements. To do this, connect a headphone to the receiver and set the range switch to the "DV" position and turn on the power. Place a sound source near the receiver, for example, a radio broadcast loudspeaker operating at a low sound level. By rotating the rotor of the tuned capacitor C6, at some point you can hear the work of the wire broadcasting program. The correctness of the setting is controlled by the maximum playback volume in the head phone - in this case, the frequency of the upgraded local oscillator will exactly match the existing setting of the FSS circuits - in no case should you touch the latter, in order to avoid detuning the radio channel. If the adjustment by the capacitor C6 is somewhat rough, a finer adjustment can be made with the coil trimmer L4, rotating it no more than one turn. If the trimmer has to be rotated more, then we recommend changing the capacitance of the capacitor C5 to the nearest nominal value in the series and repeat the adjustment.

The schemes of other receivers may have some differences from the one given, but the principles of refinement remain the same. Receivers with one band, as well as models with a common unswitched coupling coil with antenna circuits of two bands, can be converted without the introduction of additional switches, only in a hearing aid with a loss of radio reception function. However, the need to enhance the audibility of the interlocutor is sometimes more important, and here the proposed method of alteration will be useful.

Author: Yu.Prokoptsev, Moscow

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