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Old Electronics

Philips 09RB208 AM radio picks up only one station

fernandortp

hace 5 años

Hello everyone. I consult transistors for this radio that currently picks up only one medium-wave station in the 900 kHz. Under this frequency, the radio begins to emit a kind of rapid pulses and no more stations can be heard. Before this nothing was heard, since it had cut one of the wires that reach the coil of the ferrite antenna. The radio operates with 220 volts of the network through an internal transformer of 220 VAC-9 VDC and also with 6 batteries of 1.5 volts each. It carries 6 transistors: 2SA341 (2); AC127 (2); 2SB496 and 2SD96. By injecting an intermediate frequency signal of 452 kHz into the antenna input, the whistle is heard without problems in the speaker, as well as by injecting it into the base of each of the transistors. I would appreciate guiding me in solving this problem. Best regards.

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jorgedeclaypole1

5.589

hace 5 años

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hace 5 años

Those fast clicks that you listen to would be a parasitic oscillation, which, as they are stronger than the frequencies of the stations, are not heard. Should we check why this oscillation is, and why was one of the ferrite coil conductors cut? check all connections of the input and oscillator coil and their welds. regards

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Nico4706

3.081

hace 5 años

The local oscillator does not work

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fernandortp

hace 5 años

Thank you very much for your collaboration. A scheme of a similar radio would help me to locate where the oscillating coil and the transistor in charge of the frequency conversion are, since you also suggest that the local oscillator does not work. In the ferrite there are 2 windings, one long and one short. The cut wire was that of one end of the long coil, that was the one I welded. I made it work again and now no station is heard, only whistles and interference. Thank you.

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Nico4706

3.081

hace 5 años

It is difficult to get the circuit, since it is a Chilean PhilipP radio, but if it has 6 transistors, it seems to be a common circuit, which has a converter transistor, two IF, an audio preamp, and 2 output transistors. It is connected to the transmitter transistor emitter, and the antenna coil, which is the ferrite, to the base of the converter, which appears to be the 2SA341. The IF signal is output from the collector, which is also connected to the primary winding of the oscillating coil, which is a coil similar to those of the IF. If the local oscillator does not work, the co does not occur

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Nico4706

3.081

hace 5 años

The necessary conversion is needed to obtain the IF signal in the collector, and without IF the radio does not work, there is no RF carrier, and therefore it does not carry the audio signal that modulates the RF signal.

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fernandortp

hace 5 años

Based on your instructions, I checked the connections and circuits well. I demolished the 4 terminals of the ferrite antenna to verify if their windings had continuity, since they connected gave me illogical values. The longest measured 4 ohms and the shortest 1 ohm. As Nico4706 pointed out, the antenna coil, in this case the shortest of 1 ohm, is the one that reaches the base of transistor 2SA341. And among all this review, I detected that in the band switch (OL-OC) there was a little leg that was touching a neighboring contact. I just had to separate it and the radio was working.

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fernandortp

hace 5 años

So I thank you a lot for your help. The radio is Chilean and it is true that here it is very difficult to obtain schemes of these models. I wanted to upload some pictures from the radio but I don't know how to do it. Greetings from Chile.

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