Vermes
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The radio works on ferrite antenna. The systems high frequency and intermediate frequency are original. Only low frequency amplifier was changed into a circuit based on UL1497. To start the receiver, a multimeter, voltmeter and signal generator are needed. In absence of the signal generator, the properly tuned, factory produced receiver can be used. For maximal simplicity of the receiver, the middle wave can be abandoned. Self-turning is done by using a miniature trimming.
Stage I – self-oscillating mixer
T1 transistor has three functions in the input circuit: it acts as high-frequency amplifier, heterodyne and degree of conversion. As the high-frequency amplifier it works in a common emiter circuit, and as the heterodyne it works in a common base circuit. The input system is wound on a ferrite rod, according to data from the figure. The winding should have 1/10 to 1/15 assemblies. A local generator (heterodyne) works in system with feedback between the collector and emitter. In order to reduce attenuation of the input circuit, the emitter is connected to the oscillation circuit coil tap. L6 coil tap adjusts the resonant circuit resistance to the resistance of the input transistor T1. Collector coil winding is connected in series with the first intermediate frequency filter. Intermediate frequency may be for example 465kHz. In order to separate the DC, emitter circuit is connected to the condenser coil. Resistors R1, R2, R3 and R4 form a system stabilizing the operating point of transistor T1, making it independent from the influence of temperature changes.
In order to increase the stability, the heterodyne coil should be wounded on the body. Automatic gain control input stage (AGC) isn't usually applied, because it may cause a change of heterodyne vibration frequency, or change the positoin on the scale of the receiver station. To compensate the heterodyne waves amplitude in the whole range, a dampling resistor with a resistance of 50 to 200 Ω can be turned on with a capacitor (3NF).
Stage II – intermediate frequency amplifier with AM detector
An intermediate frequency amplifier amplifies the signal coming out from the mixer and improves the selectivity of the receiver. The intermediate frequency filter F1 was included in series in the T1 collector circuit with L5 oscillator coil. Next transistor (T2) acts as an intermediate frequency amplifier. The intermediate frequency signal arising in the F1 filter winding coupling L8 reaches the T2 transistor base. After the amplifying, the signal is emitted in the second intermediate frequency filter F2. A detection LED D1 is attached to the L10 winding of this filter. Arised by demodulation signal at acoustic frequency is then moved through the R16 resistor into the low frequency amplifier. The operating point of T2 transistor is set and stabilized in the same way as it is for T1 transistor. The filter coils can be self-wounded , but it needs use of proper bodies. That is why the fabric radio coils are recommended.
Stage III – low frequency amplifier
A low frequency amplifier was made on an UL1497 integrated circuit, which works in a typical application.
Description of the construction
The receiver was activated in three stages, starting with the low frequency amplifier. Next step is the intermediate frequency amplifier consisted of two filters and a LED detector (blade germanium diode). The whole circuit mounted was tuned up to optional intermediate frequency 465kHz by connecting with a fabric receiver. Then by turning the core of the oscillator coil, the radio can be tuned up to a radio program. A condenser soldered directly to the coil wounded on antenna, the loudest receiving can be set.
Despite the quality of receiving is good and comparable to fabric radios, it is sensitive to interferences. The radio works properly on power supply of 9~ 12V.
The first version, from which are left: antenna, heterodyne coil and filters.
The second version.
The diagram:
View from the elements' side, firstly an UL1480 was used, but finally because of the need of mounting the radiator, it was changed into an UL1497.
View from the print's side, electrolyte condenser soldered to add the reflex system is visible.
The ferrite antenna with a tuning capacitor and a solid ceramic capacitor soldered 180pF.
Link to original thread - Radioodbiornik z przemianą częstotliwości