I'm trying to add varactors to this circuit. However I can tune in no radio stations, with the varactors. If I put in a tuning cap instead, it works perfectly. Can any one help me trouble shoot, what I'm doing wrong?

 

Hello,

You did not bias the TA correctly.
I added a capacitor of 1 nF and connected the 100K resistor to pin 2:



Bertus

 

Yes, and then the 100uH should be a direct link to ground and the 1uF on the output should be 0.1uF (per the datasheet).

 

Thanks for the quick response. I had found a bare bones ta7642 circuit on the web. It worked just fine with tuning cap. So I simply added the varactor, and didn't think of looking at the datasheet closely.

 

I still am unable to get this circuit to work. When I use the tuning cap, it works great. However when I use the varactors all I get is static. I believe the tuning cap should be up to 160pf. The varactors are 435pf, and since there are 2 should be a little over 200pf. It seems they should be equivalent. Is there something else I'm missing?

 

Hello,

Perhaps the 10K is to large, try 4k7:



You could also try a decoupling capacitor (0.1 - 1 uF) on the wiper of the tuning pot.

Bertus

 

I don't see any problems once the changes mentioned above are done to the circuit. Are you sure that you have the right varactor diode?

I found a BB149 data sheet from NXP that is much lower capacitance. It seems that the 1SV149 from Toshiba is more like the part you had in mind.

What an unfortunate state of affairs that two completely different varactors would have such similar part numbers.

 

Maybe the 2k is too small and is damping the resonant circuit. As the diodes are reverse biased the current through this resistor will be very small (50nA) and so its value could be much higher without changing the voltage on diodes. Try 100k in that position.

 

Hello,

You did not bias the TA correctly.
I added a capacitor of 1 nF and connected the 100K resistor to pin 2:

View attachment 129091

Bertus

Took me a couple of looks to spot that - insufficient capacitance seemed like a good guess.

Last medium wave project I saw, used a pair of TO220 style Shottky barrier rectifiers for variable capacitance - and still couldn't span the whole band.

 

When I put the loopstick to ground, the audio completely goes away. Not even static. I am using the 1sv149. I had originally used bb149, but replaced them when I found the 1sv149.