Hello, how can i replace variable capacitor with varicap

Thread Starter

michael1978

Joined Jun 29, 2014
309
hello, can somebody help me? i found this am receiver, but is using variable capacitor,
how can i replace of do with varicap this circuits(i hava varicap bb112)
receiver.png
 
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bertus

Joined Apr 5, 2008
20,109
Hello,

To adjust the varicap, one must have a DC path.
I do not see how to create this DC path.

You could have a look at the attached PDF, where several circuits with varicaps are given.

Bertus
 

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Thread Starter

michael1978

Joined Jun 29, 2014
309
Hello,

To adjust the varicap, one must have a DC path.
I do not see how to create this DC path.

You could have a look at the attached PDF, where several circuits with varicaps are given.

Bertus
i remove only variable capacitor, and bb112 is 500pf and something, so i connected in parralell varicap and inductor and i make dc pach, with wiper 100Kohm, so i hear only noise
 

LesJones

Joined Jan 8, 2017
2,363
You need to post a schematic of EXACTLY how it is connected. My interpretation of your description is the the varicap is directly in paralled with the inductor. If that is the case then any DC tuning voltage you are trying to inject will be shorted out by the inductor.

Les.
 

Thread Starter

michael1978

Joined Jun 29, 2014
309
You need to post a schematic of EXACTLY how it is connected. My interpretation of your description is the the varicap is directly in paralled with the inductor. If that is the case then any DC tuning voltage you are trying to inject will be shorted out by the inductor.

Les.
but i dont know how to make antenna thnx
receiver1.png
 

takao21203

Joined Apr 28, 2012
3,696
It should be noteable that LEDs have quite some reverse junction capacitance. Usually some 30pF.

And also maybe the AM range is pretty dead at your place, no more stations. In the nights you can get stations from far away, but in the daytime, not much is audible.
 

Audioguru

Joined Dec 20, 2007
11,251
The first two transistors amplify the radio frequencies (a few stations) with no gain control so they are overloaded when there are strong local stations. The third transistor amplifies the radio frequencies even more, again with no gain control. There is no AM detector which is usually a diode.

What do you have connected to its output?
 

Thread Starter

michael1978

Joined Jun 29, 2014
309
The first two transistors amplify the radio frequencies (a few stations) with no gain control so they are overloaded when there are strong local stations. The third transistor amplifies the radio frequencies even more, again with no gain control. There is no AM detector which is usually a diode.

What do you have connected to its output?
i connect this one (how they call) one small speaker to put in your ear
 

Audioguru

Joined Dec 20, 2007
11,251
i connect this one (how they call) one small speaker to put in your ear
It is called an "earphone" and you need a power amplifier to drive it.
The output resistor in your circuit is 10 thousand ohms. Then it can drive a 10 thousand ohms load with a voltage loss of half the signal. An earphone today is only 32 ohms so the 10 k and 32 ohms divider reduces the signal 314 times.
But the "radio" does not have a detector diode to play AM radio sounds. Instead its output is the radio frequency of about 1 million Hertz.
Please learn about how an AM radio and an audio amplifier work. Here is an AM detector diode from a search for it in Google:
 

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Thread Starter

michael1978

Joined Jun 29, 2014
309
It took a few minutes for this website to post my reply so I tried again and again. I deleted the copies.
thank you a lotttttttttttttttttttttt for time, but please can you help just a little bit, is good connected varactor? and i need a diode, and power amplifier,and why in this circuits they don use diode, "what for am radio is" and one more question i use this inductor is problem???? look$_58.jpg
 
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MisterBill2

Joined Jan 23, 2018
4,066
hello, can somebody help me? i found this am receiver, but is using variable capacitor,
how can i replace of do with varicap this circuits(i hava varicap bb112)
View attachment 148083
This receiver looks a lot like a regenerative receiver with R2 being the feedback element. What I can offer is that adding the varicap circuit shown in the second circuit the feedback portion is greatly disrupted. Note that neither end of the tuned circuit is near ground potential and so it is not a good candidate for changing to varicap tuning. For that to be possible you need a circuit with one end of the tuned circuit grounded..
 

Audioguru

Joined Dec 20, 2007
11,251
R2 applies DC negative feedback. Any signal in R2 is shorted to ground by C1 but if it isn't shorted then its negative feedback would produce the opposite to regeneration.
 

Thread Starter

michael1978

Joined Jun 29, 2014
309
This receiver looks a lot like a regenerative receiver with R2 being the feedback element. What I can offer is that adding the varicap circuit shown in the second circuit the feedback portion is greatly disrupted. Note that neither end of the tuned circuit is near ground potential and so it is not a good candidate for changing to varicap tuning. For that to be possible you need a circuit with one end of the tuned circuit grounded..
hello, just to ground one wire from tuned to ground and that is? thnx mannn...;-)
 

MisterBill2

Joined Jan 23, 2018
4,066
hello, just to ground one wire from tuned to ground and that is? thnx mannn...;-)

OK, so move the end of the 230K resistor up to the base of Q1, and then move the 100Nf capacitor to between the base of Q1 and the top of the tuned circuit. Next ground the tuned circuit. NOW you can substitute the varicap diode for the tuning capacitor, and if the tuned circuit happens to resonate at the frequency of some radio station you may hear it. That is, if the signal is strong enough to drive one of the amplifier stages into non-linear operation. Otherwise yo will indeed need a detector, which the easy kind is a diode and then a small filter capacitor. Use the posted detector circuit, it should work.
 

Thread Starter

michael1978

Joined Jun 29, 2014
309
OK, so move the end of the 230K resistor up to the base of Q1, and then move the 100Nf capacitor to between the base of Q1 and the top of the tuned circuit. Next ground the tuned circuit. NOW you can substitute the varicap diode for the tuning capacitor, and if the tuned circuit happens to resonate at the frequency of some radio station you may hear it. That is, if the signal is strong enough to drive one of the amplifier stages into non-linear operation. Otherwise yo will indeed need a detector, which the easy kind is a diode and then a small filter capacitor. Use the posted detector circuit, it should work.
do you mean R2?
 
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MisterBill2

Joined Jan 23, 2018
4,066
do you mean R2?
YES. R2 is the 120K ohm resistor. And C1 is the capacitor that I was referencing. But now here is a question that needs an answer: What frequency are you hoping to hear??? Also, those inductors may or not be the correct value. In addition, those inductors are physically very small and I am wondering if they are an appropriate value for tuning to the frequency of whatever radio stations you wish to hear.
 
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