Electronic – Troubleshoot Crystal Radio

That's a very very very very minimalist circuit and very 'laid back' construction. It will work with a suitably high impedance ear-piece and proper construction and enough transmitter power and close proximity and ... ie It MAY work but it's not surprising if it doesn't as there are too many potential pitfalls.

• The earpiece looks like it is low impedance - either 8 ohms or perhaps a few hundred ohms.

  Without a tuned circuit you need all the sensitivity and lack of loading that you can get. Use of a very very high impedance "crystal earpiece" will make a significant difference.

• Your general construction is excessively rough for what you are trying to do. You MAY make it work but one bad join may doom it and stability is unlikely.

• Your wiring lengths and stray capacitance and inductance are long for use at 145 Mhz.

• As MikeJ-UK noted in a comment - if your transmitter is FM (frequency modulated) then you will probably not hear anything as the diode acts to "recover" or "demodulate" the amplitude variations in the carrier wave of an AM (amplitude signal).

• Adding a tuned circuit at the frequency of interest - as seen in almost all crystal set designs you will see, is probably the single best improvement that you can make.

• Clip leads of the sort you are using usually use crimp construction. They can be high resistance or open circuit or have a thin skim of oxidation at the wire to clip contact so that you need a certain level of current flow before they will conduct.

  Soldering the core of the circuit with short lengths of wire is far preferable.

  Clip leads of the sort shown should ALWAYS be soldered at the wire to clip join. Failure to do this will give you many hours of harmless and unproductive fun. When I buy leads like this I put a loose knot in the wire and remove it once the lead has been soldered.

• The diode MAY be a 1N34a. That's a Germanium small signal diode. Rare and not cheap. More usual would be a 1N4148 or 1N916. Most small glass diodes are probably OK. Do NOT use a low Voltage power diode.(1N400x family etc)

• The "rabbits ears" antenna leg is OKish but a length of wire is just as good

• Power outlet ground MAY be safe as safe and MAY kill you when you put the earpiece in your ear.

The voltage from pin 8 of SCTX2BC changes the DC component of the voltage on the base of Q2 transistor, modifying its quiescent IC current and its gain. So this is how the RF signal amplitude is modulated.

The RF signal from the Q1 based oscillator is connected via the capacitive voltage divider consisiting of C5 and C6.

The bias of Q1 may be found by the following equation:

$$V_{bias} = R7\,\frac{I_E}{\beta+1}+0.65V+I_E\times R8 = 9V$$

$$V_{CE} = 9V - I_E \times R8$$

$$I_C = \frac{\beta}{\beta + 1}I_E$$

C4 assures the positive feedback.