Replace variable capacitor with varicap

For wikipedia reference: https://en.wikipedia.org/wiki/Varicap

Actually the very first circuit gives a clue how to connect. I assume using two is the most easy approach.
That assumes that L1 not directly connected to ground won't be an issue.

 

Replace VC1 with a series circuit consisting of your varicap (cathode to the 'top') and a capacitor.

Apply your tuning voltage to the junction of the two via a series current limiting resistor (100k will do).

The tuning range you get will depend on the capacitive swing of the varicap (depends on the voltage range you apply to it) and the value of the series capacitor - you effectively have TWO capacitors in series with a consequent reduction on overall value, something for YOU to calculate!

 

You need to couple it to the inductor via a capacitor. And, unless you use a separate isolated supply, you need two capacitors, one on either side of the inductor / varicap. Choose capacitors much larger than the capacitance of the varicap (say 100nF) so that it will not effect the range of the varicap by much.

Bob

 

Yes, I missed that on the schematic, which was too small to read easily.

Are you using a separate isolated supply for V4?

Bob

 

100nF is fine.

Bob

 

Sir michael1978 . . . . .

NO can do . . . . your C7 install shown will kill / block any required DC base bias from getting to Q1.

With no RF experience . . . . . .I really-really-REALLY think that you should get this circuit operative with conventional L-C components in the tuned circuit portion. And knowing that you have working circuitry, you could THEN attempt the incorporation of varactor tuning.

Tell us all about the . . . . . . . . .

L1 coil that you have constructed to use.
Is it on a ferrite rod?
Is it wound with magnet wire or Litz wire ?
How many turns is on it . . . . is it spaced out turn by turn or scramble wound into a bobbin ?
What gauge of wire ?
I am assuming that the band being covered is the conventional Broadcast band of ~550-1600 Khz ?

If low cost is a major consideration . . . .
Do you realize that you can make your own single section circular variable condenser with two blank sheets of common 1 oz, single sided copper PCB stock and the thin plastic from a clear sheet page protector . . . which is a much more efficient dielectric than air is.
A tuning shaft and a small compression spring keeps the plates pressed together. . . and THEN . . . spread on a thin coating of silicone grease / oil as the MAGICAL rotational agent and then affix the tuning knob to the shaft.

OR . . . . . after your feedback on the inductor . . . . . and if you are using a ferrite rod, I can fill you in on how to use a fixed mica capacitor and make the inductor variable , in order to accomplish the tuning.

Thaaaaaaaaatsit . . . . .

73's de Edd
.....

 

The circuit is missing an AM detector diode and is also missing an audio power amplifier.
The transistors simply amplify and are probably overloaded by the radio frequency of an AM station.

 

What is difficult about a varicap? You simply apply a reverse bias voltage and the amount of voltage determines its capacitance.
What is difficult about a simple LC tuned circuit? They resonate at their calculated frequency.

You probably hear the noise made by the transistors that have nothing to limit their gain. The first and third transistors have a voltage gain of about 170 times and the second transistor is an emitter-follower with a gain of 1. Then the total gain is 170 x 1 x 170= 28,900 times. Your earphone is killing the gain of the third transistor.

 

An audio amplifier is used to play a high impedance audio signal source into a low impedance transducer like an earphone or a speaker. An LM386 is designed to drive an earphone or a speaker.
But your circuit does not demodulate the radio signal so it has no audio. A demodulator for AM is a detector diode that your circuit is missing.