Encyclopedia of radio electronics and electrical engineering / radio reception

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Changing the ranges in the receiver "Selena RP-405"

A few years ago, the Selena RP-405 portable radio receiver was popular. This inexpensive, compact and. at the same time. very light and practical device and today can still be found in many families. Unfortunately, he only has two bands - LW ​​and KB (25m). The LW range is not necessary in all regions, and on KB (25 m) reception is good only for a limited time of the day: a few hours in the morning and evening. Meanwhile, a minimal alteration of this receiver can provide its owners with operation on the subbands of 25, 31 and 41 m.

What is the improvement? First of all, the armature with coils L4.1 and L4.2 (LW local oscillator) is removed. then cut the tracks to the extreme terminals of the switch SA1. and the "hot" pin L3.1 is connected directly to pin 6 of the DA1 chip. The coils of the KB range of the magnetic antenna MA are desoldered and removed. Instead, six turns of PEL-0.5 wire are wound with a step of 1 mm (this will be the coil of a magnetic antenna of the 41 m subband), two turns of the same wire are wound between them (communication coil L26). They are soldered at points on the board 7-8 and 9-10, respectively.

On a fragment of the circuit (Fig. 1), capacitors with changed denominations are indicated by the sign "'", and newly introduced elements are indicated by thicker lines and an index in the reference designation. It is necessary to pay attention to the connection of KPI sections. Sections with a capacity of 3 ... 270 pF (C1.3 and C1.4) are connected to the coils L1 and L3, and one of the sections with a capacity of 3 ... 30 pF is connected to the coil L1 of the magnetic antenna. You also need to solder a jumper instead of the L4.2 coil of the remote DV local oscillator circuit. The newly introduced capacitor C16 is soldered to one of the extreme terminals of the switch SA1.

Adjustment begins with a range that covers the subranges of 25 and 31 m (switch position SA1. when C1d is disconnected from the circuit), setting C2d to the minimum capacity, and KPI to the maximum. Further, rotating trimmer L3. you should try to tune in to the stations of the sub-band 31 m and. by rotating the L1 trimmer, achieve the maximum reception volume on this subband.

Reducing the capacity of KPI. need to make sure. that stations of the 25 m subband are received. The easiest way to do this is with the help of the GSS. but for control it is enough to have a working receiver with these KB subbands. Rotating rotor S2d. achieve maximum volume on the 25 m subband.

The described operations are repeated several times until the full laying of the range boundaries (complete overlap of both subranges) is achieved for complete conjugation of the input and heterodyne circuits at two points - the middle of both subranges.

By switching SA1 to another position, they begin to tune the 41 m subband. By moving the coils along the core of the magnetic antenna, they achieve the maximum level of reception of on-air noise, radiation from a personal computer or GSS signal. Next, you need to make sure that the entire subrange of 41 m is being received. In extreme cases, you will have to change the capacitance C1A. After that, the trimmers of the L1 and L3 coils, as well as the turns of the magnetic antenna coil, are fixed with molten wax. On this setting can be considered complete.

It is worth adding that it is useful to simplify the AGC scheme. removing from it the input attenuator in the UHF circuits. This will slightly increase the sensitivity of the receiver and increase the stability of the local oscillator tuning frequency. As practice has shown, it also changes with a change in the voltage in the AGC circuit. Therefore, it is necessary to cut the track leading to pin 3 of the DA1 chip (not shown in the diagram) and connect it to a common wire (as is done in the Olympic-2 receiver).

Improving the sound quality of the Nevsky-402 receiver

Currently, radio receivers are widely used, in the path of which the K174XA2 chip is used. They have good parameters, but the sound quality leaves much to be desired. This is due to the "classic" diode detector, which introduces large non-linear distortions. In addition, the non-linearity of the characteristics of the diode (a decrease in forward conduction with a decrease in the voltage at the junction) attenuates low-level signals, which reduces their sensitivity.

The emitter detector works much better. In a receiver with a K174XA2 chip, a positive voltage is required at the detector output, which makes it possible to increase both the sound quality and sensitivity with a minimum of alterations.

A fragment of the circuit of the Nevsky-402 receiver (Fig. 2) shows a variant of the detector modification. Diode VD1 is removed from the circuit. and instead of it, a transistor VT2 is introduced. In this case, it may be necessary to change the value of R7 or. even better, replace it with a transistor VT1 of the same type as VT2. These can be KTZ01, KTZ12, KT315 with any letter indices, the VT1 transistor has a diode connection and allows you to stabilize the bias voltage on the VT2 base. In series with VT1, you may need to include a resistor R7. If the voltage at the VT2 emitter is less than 0,05 V. then the resistor should be selected so that the voltage is within 0,05 ... 0,1 V.

After such an alteration, the sound quality and sensitivity of the receiver have noticeably increased.

Author: M.Sapozhnikov

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