It would be tricky to bias a varicap - you'd have to stabilise the decoupled collector rail.

If the decoupled rail is "tight" enough - you might be able to get away with grounding one end of the varicap.

The inductor will obviously shunt any DC for a varicap, you have to DC block it with a series capacitor big enough to have little effect on the total capacitance. The varicap is used reversed biased, so there's only a tiny leakage current - you can feed it with a high resistance that doesn't clobber Q very much.

Pedantic theorists will tell you that the RF which is an AC waveform, will sum with the varicap bias and degrade tuning accuracy - millions of TV tuners don't worry about it and work just fine. High end FM tuners sometimes have twin or double ended varicaps which cancel out the AC offset.

So it should work if I replace the variable capacitor with a varacap assembly with a red LED in the place of the varacap?

 

But it is difficult to arrange a suitable biasing arrangement in that oscillator circuit as neither end of the tuned circuit is at a fixed voltage.
Maybe this would work?

 

the diode in the circuit can be an LED or a silicon diode, right?

 

In principle it can be any diode. Really only experiment will determine which will work well in your circuit.

 

So it should work if I replace the variable capacitor with a varacap assembly with a red LED in the place of the varacap?

I've never tried LEDs for that use, they're probably too much capacitance anyway. They usually have a low reverse voltage rating which limits the swing.

Most people use things like 1N4001 etc, but they're probably too much capacitance for band II as well.

The varicaps in TV/PVR tuners might be nearer the ball-park - you might need to use a few in parallel to get enough capacitance, but you can add or remove devices to get in the right tuning range.

 

What values should c1 and c2 be?

 

What values should c1 and c2 be?

They are effectively in series with the diode capacitance so their series value should be small compared to the diode capacitance but it would not be critical. I would suggest 220pF.

 

Would 1 nanofarad capacitors work? I have a good amount of them.

 

I would think so, if they are ceramic.

 

They appear to be ceramics. I'll try it when I get a chance later today.

 

But it is difficult to arrange a suitable biasing arrangement in that oscillator circuit as neither end of the tuned circuit is at a fixed voltage.
Maybe this would work?
View attachment 126077

If the supply decoupling is stiff enough, you can ground one end of the varicap - its still in parallel with L for all AC intents and purposes.

But you have to stabilize the rail.

 

They are effectively in series with the diode capacitance so their series value should be small compared to the diode capacitance but it would not be critical. I would suggest 220pF.

You have that back to front - take another look at the series capacitance formulae.

2 equal capacitors in series will be half the capacitance - a big & small capacitor in series will be a tiny bit less than the small capacitor.

 

Can i use the equation for resistors in parallel for capacitors in series?

 

Can i use the equation for resistors in parallel for capacitors in series?

Its the same procedure - just as parallel capacitors & series resistors.

 

If the supply decoupling is stiff enough, you can ground one end of the varicap - its still in parallel with L for all AC intents and purposes.

But you have to stabilize the rail.

But that 'rail' is where the aerial (sorry, antenna) is connected.

You have that back to front - take another look at the series capacitance formulae.

2 equal capacitors in series will be half the capacitance - a big & small capacitor in series will be a tiny bit less than the small capacitor.

Yes, my bad, but I knew what I meant.

 

But that 'rail' is where the aerial (sorry, antenna) is connected.

.

Looks like a regen - not my favourite.............

 

The speaker isn't making any noise at all. I tried an LED (made sure the LED worked using diode setting on multimeter) and a silicon diode. Nothing it heating up, so thats one good thing. Any ideas on what might be the problem?

 

Variable Inductor ? back to the 60's car radios.

You don't have to go *THAT* far back.

They're probably all frequency synthesis tuning now - been I while since I last saw a variable capacitor.

 

The speaker isn't making any noise at all. I tried an LED (made sure the LED worked using diode setting on multimeter) and a silicon diode. Nothing it heating up, so thats one good thing. Any ideas on what might be the problem?

The schematic you posted shows a B/E junction shunting the rail after the 22k resistor - so not much getting to the collector of the other transistor on the RF side.

Can't quite work out whether its a regen or not - it should have some sort of regeneration control.

Cranked up regeneration causes the RF stage to oscillate and produce a loud hiss in the speaker - and also interference to other listeners.

 

Should I lower the resistance of the resistor?