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Dual Band FM stereo tuner. Part 2

The total resistance (R1Z + R14 + R15 + R16) should be in the range of 20 - 50 kOhm. We draw the reader's attention to the fact that deviations of time constant from the stated value by more than 2% lead to deterioration of stereo channel separation.

Fine tuning indicator NL2 is controlled by the key on bipolar transistor VT3. Circuits R17, C35 and R22, C37 are de-emphasis circuits.

Chip D3 is a frequency synthesizer. It receives the local frequency variations of the oscillator at its input (pin 5) and its output signal is a tuning voltage for capacitance diodes (pin 13). A special feature of the switching on of the chip is the usage of output 8 (BS) to switch between bands. The signal from the output controls the key on the transistor VT2.

Chip D4 is a microcontroller, which manages the frequency synthesizer and liquid crystal display D6. This node does not have any features. Shifting register D5 is used as an auxiliary for the indicator D6.

We would like to say a few words about the power supply of tuner nodes. The power supply circuit is built on elements D7, D8, D9, and the stabilizer D7 provides a voltage setting (+12), stabilizer D8 feeds the analog part and the stabilizer D9 feeds the digital one. Each device is equipped with a local bypass capacitor. This scheme eliminates the interference from the digital portion of the work on the small signal analog stages.

Picture 3 shows a stereo tuner circuit board; picture shows an assembly drawing. The board is made of single-side foiled glass-fiber laminate. The dashed lines indicate the boundaries of the conductive screen. It needs to be made of thin sheet metal, copper, brass. The screen additionally protects the radio receiving part from electromagnetic interference.

In the tuner structure you can use any small-sized items, such as capacitors K10-17, KM-5, KD-2, K50-35, K50-68; resistors S2-29, S2-33.

Coils L1 and L2 are wound on the smooth scaffolds 5 mm in diameter. Number of coils: L1 - 4 coils, L2 - 3 coils. Wire PEV-2, PETV or equivalent diameter of 0.3 - 0.45 mm. The coils have of brass adjusting cores with the length of 5 - 6 mm, with thread of M4.

When assembling, it is recommended that you only should install the chips D7, D8, D9. Having switched on power supply, check the voltage of +12 V on the positive output of the capacitor C11, and the voltage of + 5V on the same outputs of the capacitors C15 and C2. After that, install the chip D1, connect an external LFA to output 13 of the chip, as well as the antenna. Solder a temporary variable resistor (10-22 ohm), connecting it between the voltage of + 12V and the common wire, and the output of the engine - to the top output of the capacitor C8 (according to the scheme). Connect the base of the transistor VT2 a connection element to the common wire.

After switching on power on the engine, set the voltage of 1.2-2.1 V on the variable resistor (with respect to the common wire). Rotate the adjusting cores in the coils L1 and L2 to achieve the reception of the lowest station on the FM range (in St. Petersburg it is the frequency of 88.00 MHz). Control the signal level on LED HL1, having achieved its maximum emission. You can also connect a multimeter instrument alongside with the LED, set for the measurement of the voltage, and achieve maximum readings. Through the use of the ready-made filters Z1 - Z3, you don’t need to configure the intermediate-frequency circuit and the frequency detector.

After the operations described above you can check the reception of the highest station of the FM range (in St. Petersburg it is 108.00 MHz). After removing the connection element between the base of the transistor VT2 and common wire, check the frequency agility on the domestic FM band. Domestic range is smaller than the range of FM, therefore, it fits in the borders of agility.

Restore the connection element in the transistor VT2 and mount the chip D2. Tune in an FM station and the rotate the resistor R20 to light LED HL2. The sound should be stereo. You can now remove the connection elements from the transistor VT2 and tune in to the station in the FM band. The sound (on condition that the station broadcasts in stereo) should be stereo, LED NL2 should signal this. Setting the analog part is over. The temporary variable resistor can be unsoldered.

Set the chip D4, D5 and display D6. Of course, the microcontroller has to be upgraded by this point of time. The filenames are termo_0801.sav for an 8-character display, and termo_1601.sav – for a 16-character display. After switching on power supply, the frequency rate that is set in the first location of the EEPROM should appear on the display.

Press S1 (“record”), S2 (“continuous/fixed”), S3 (“-”) and S4 (“+”) to move through the range, change between the continuous tuning mode and fixed mode and vice versa, write fixed settings to EEPROM (30 positions). Storing smooth setting should be implemented as follows: first, the desired frequency of reception is tuned in the continuous tuning mode (number of continuous tuning mode between the symbols “>” and “<” is not displayed). Then set the number of continuous tuning mode in the fixed mode and press Record. After you release the Record button, the setting is updated, and its value is stored in non-volatile memory of the microcontroller. When you change from band to band “FM” icon will appear in the left side of the display.

At the end you need to solder in chip D3 and make sure you can tune the frequency with the help of S3 and S4 buttons. Set the card in a suitable case, attach the controls and indicators. The tuner is ready for operation.