Crystal radio (AM receiver)?

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kynix

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http://forums.radioreference.com/attachment.php?attachmentid=58468&stc=1&d=1486630244
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 http://www.kynix.com/uploadfiles/pdf8798/NE555DT.pdf).

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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WA0CBW

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Using a different voltage would require different value bias resistors. Also the gain of your replacement transistor may require different values. The diode needs to be small signal type not a Zener.
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RFBOSS

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The diode needs to not just any small signal diode, but a germanium diode (which the one specified is).

If they really want you to use an electrolytic capacitor for C3, it is installed backwards. The voltage on the base of Q1, the transistor is positive.


Electrolytic capacitors do not like to be reverse biased.

What is the resistance of your headphones? This will have an impact on the gain of the circuit (it is functioning as the collector resistor).

The tuning capacitor looks like it should tune at least the lower to upper middle portion of the AM broadcast band using the specified inductor.


Going from a 3 volt supply to a 5 volt supply likely will not change the gain enough to make a difference. It will change the amount of direct current that will flow through the headphones.



The transistor just needs to be an audio or better transistor, with decent gain, it does not need to be an RF transistor.

Are you sure that the transistors that you are using are NPN type transistors?

I thought of something else. What is supplying your 5 volts? If it is an external power supply, part of your issue may be poor power supply decoupling, causing what we used to call motor boating. I would try it with a couple of AA cells and use short leads to connect to the radio. In this circuit the power supply is in series with the audio signal.
 
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