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enter image description here

I've made this circuit on a breadboard. It doesn't work properly.

I hear the sound of an oscillator (similar to the sound of 555 timer).

I changed C6 and L1 to change the frequency but I hear the same sound.

When I use 3K ohm instead of R2, the frequency increases (the tune of the sound is higher).

The transistor in the circuit is not available for me. so I used other transistors (general purpose, switching transistors and amplifier transistors).

I'm using 5 volt instead of 3 volt.

I want to know what is the problem? and should I use a zener diode or an ordinary diode?

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    \$\begingroup\$ What happens if you remove C3, leave the ends open? That way you can identify which end is likely to be the cause of the oscillation. Same trick works with D1 \$\endgroup\$ – jippie Aug 30 '14 at 13:45
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    \$\begingroup\$ AA121 is a germanium diode. Did you use the same type? \$\endgroup\$ – venny Aug 30 '14 at 13:47
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    \$\begingroup\$ BTW it's an AM receiver, not FM \$\endgroup\$ – Andy aka Aug 30 '14 at 16:32
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It definitely needs a germanium diode because of its small forward volt drop to rectifying signals (i.e. the incoming antenna signal) - a schottky might work but stick to the circuit is my advice and use the correct votlage power supply too. With a 5 volt power supply the transistor may get a little over-biased on the base and cause unforseen problems. Same as using a 3k for R2 - stick to the 820k because it is providing the correct (presumably) amount of bias on the transistor AND the transistors dc/low frequency (aka audio frequency) gain is initially determined by R2 being so high.

I believe the 4.7nF capacitor to be a mistake - it will set the 3dB amplification point of the transistor at about 41 Hz and this is way-off for audio - try a 47pF instead - this raises the 3dB point to something like 4kHz.

L1 can be critical too - do you have a picture of it?

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  • \$\begingroup\$ Thank you sir for answering my question, I've made some of the coils by my self. This is the picture: link I don't know their values because my multimeter doesn't measure coils. \$\endgroup\$ – Michael George Aug 30 '14 at 20:19
  • \$\begingroup\$ The coils pictured in the link are far to small (too few turns) to work at AM (~1 MHz) frequencies. \$\endgroup\$ – gbarry Jan 17 '16 at 6:28

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