Tube receiver - a new life. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Tube receiver - a new life. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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Tube radios are still found in many families - for some they continue to work well, for others they are kept as a memory of the past, without the hope of hearing their sound again. As a rule, the reason for the "silence" of such radios is the exhaustion of the lamp life, the replacement of which is unlikely to be found in our time. Is it possible to make such receivers sound again? The author of this article offers you the answer to this question.

The pages of the magazine have already talked about the conversion of tube receivers to transistors [1]. However, the proposed option, although it made it possible to maintain the main technical indicators of the design, was laborious and required a fairly high qualification of a radio amateur. Meanwhile, it is possible to "revive" the old model of the receiver by simpler, more accessible means, by converting it into a direct amplification transistor receiver. In this form, it will be possible to carry out confident reception of local radio stations operating in the medium and long wave bands.

As an example, consider the reworking of the once common model of the Sakta receiver. From its design in the new version, a preselector with a range switch, a power transformer, an acoustic system and a number of other elements that can be used without additional modifications will work. At the same time, the possibility of a quick restoration of the initial state of the device remains, in case you are lucky enough to get "native" radio tubes.

Schematic diagram of the upgraded receiver is shown in the figure. Elements specified outside the rectangle with dash-dotted lines are used regular. The exception is the resistor R6. which replaced the variable resistor of the volume control. For the used standard elements, the reference designations that they have according to the description and scheme of the manufacturer are preserved. For greater clarity, the circuits used without alteration are shown in a simplified form with the medium wave reception range turned on.

(click to enlarge)

The double-circuit preselector "Sakty" (L14, C15, C17, C19 and L15, C23, C24 together with the coupling coil with the antenna L13) provides the direct amplification receiver with a sufficiently high selectivity. Coupling coils with input circuits are not required, since a field-effect transistor VT1, turned on by a source follower, is used at the input of the radio frequency amplifier. The high input impedance inherent in such a cascade allows you to make a complete connection to the preselector circuit without altering the factory coils.

The main amplification of the received radio signal is performed by a cascade on the transistor VT2. and from it the amplitude-modulated oscillations arrive at the detector, made on the diodes VD1 and VD2 according to the voltage doubling scheme. The load of the detector - a variable resistor R6 - is also a volume control.

The amplification of the signals of the audio spectrum is performed by the cascades of the DA1 microcircuit. included according to the standard scheme [2]. The load of the audio frequency amplifier is the dynamic head BA1. symbolizing the acoustic system of the three sound heads of the receiver itself.

The direct amplification receiver is powered by a built-in transformer T1. All its circuits for connecting to the AC mains - mains voltage switch, fuse, power switch - are used, remain unchanged and therefore are not shown in the diagram. A rectifier with a diode bridge VD3 is connected to the filament winding of radio tubes, from which the backlight pumps of the EL1 scale are also powered. EL2. The low-resistance section of the inductor L41 ("Dr" according to the factory scheme) switched here from the power supply circuit of the radio tube anodes works in the rectifier filter.

All elements of the direct amplification receiver are located on a separate small board, made by hinged or printed wiring. It can use fixed resistors MON-0.5 (R10) and MLT-0.T25. MLT-0,25 (others), capacitors of types K50-6 and KLS.

The diode assembly VD3 can be replaced by a diode assembly of a similar type with other letter indices or four KD105B diodes. Diodes VD1 and VD2 can be replaced with D311 or D2 with any letter index. As a field effect transistor VT1, you can use KP303A, KP303B. and bipolar VT2 - KTZT2, KT315. KT358. KT3102 with any letter index and other low-power high-frequency np-p transistors. Chip DA1 must be equipped with a heat sink.

Instead of a standard anode choke section (L41), you can use the primary winding of the output transformer of almost any industrial transistor receiver (Spidola-230, Giala-404, Sokol-403, etc.). In this case, the transformer should be placed directly on the direct amplification receiver board next to the capacitors C11 and C12.

When preparing the tube receiver for operation in the proposed version, it is necessary to remove the available radio tubes from the panels (except for the indicator one). Disconnect the filament circuits from the indicator lamp and the VHF unit. One of the outputs of the filament winding of the power transformer has a connection to the receiver chassis - this circuit should be broken. The conductors suitable for the step-up (anode) winding of the power transformer should also be unsoldered. Disconnect the group of dynamic sound heads from the output transformer of the receiver and switch to the output of the DA1 chip. it should be taken into account that the dynamic heads already have a galvanic connection with the chassis.

The assembled direct amplification receiver board can be attached to a variable capacitor bank (VPC) or to the closest IF (intermediate frequency) filter shield. The board should be connected to the receiver elements in accordance with the circuit diagram, and the common power bus of the transistor part should be connected to the receiver chassis. After completing the assembly and turning on the power, with the external antenna disconnected, select the resistors R2 and R5 so that the voltage between the collector and emitter of the transistor VT2 is about 1,3 V. it can be compensated by additional adjustment of the tuned capacitor C15 (the capacitance should be slightly reduced).

Literature

Prokhoptsev Yu. The second life of the "old" radio. - Radio. 1992. No. 11. p.54.55.
Ataev D.I., Bolotnikov V.A. Analog integrated circuits for household radio equipment. - M.: MPEI, 1993.

Author: Yu.Prokoptsev, Moscow

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