BFO/IB cross mod info needed

[WM6]

... How could I replace the 100pf tunning cap, for a pot.

I hope to more technical and less emotional answer from our coleague mikebg.

Variable capacitors are relative unstable (not only by temperature but physicaly too) elements in oscilator circuits, so it is reasonable if one wish to replace it by a little more stable elements.

In disputed schematic we have analog oscillator circuit and there is no simple replacement variable C with pot R, if we want to maintain the equal same features of the device.

To avoid variable C (or variable L or Xtal) maybe we need to go digital like this solution ( http://mediaexl.blogspot.com/2009_09_01_archive.html ) :

[Carl-NC]

Dave,

You can make a variable cap using a varactor, which is basically a diode with a high voltage coefficient for the reverse capacitance. You then use a pot to apply different voltages, resulting in a variable cap. The problem in this circuit is, the cap is applied differentially, so a varactor approach would be tough to do. Looks like Maikl found the better solution.

- Carl

[satdaveuk]

Thanks

Hi Maiki, WM6, and Carl-NC
Maiki, I wasnt so much angry, just fustrated after asking a very simple question, and started to think maybe I shouldnt have bothered to ask in the first place.
But since you and the others on here have done the business giving me the information I was looking for.
So for that I am very greatful, and a very big thank you

All the best to you all

Warm regards

Dave

[WM6]

Solution as Carl suggested (not exactly one resistor replacement):

[Skippy]

You could try adding two 10pF capacitors from pin 2 to ground and from pin 6 to ground. If the circuit still seems to work OK, then try replacing the pin6 cap with a varicap diode. The diode anode goes to ground, the cathode to pin6 via 220 pF. Then apply the reverse DC bias to the cathode, for example put a 10K pot across the 12V supply, and take the wiper to the diode cathode via a 100K resistor. The varicap needs to be the 2 -20 pF approx. type. Worth a try.

[satdaveuk]

Yes that is worth a try Skippy.
I seem to remember many years ago when i was a apprentice we used to be able to produce a simple tuning circuit with a coil incl slug, small capacitor and a pot.
Setting up was pot mid position , adjusting slug to frequency required, then the pot for tuning there after.
But for the life of me i cant remember the circuit configuration, was 35 years ago mind you, and it was mainly valve/transistor.
Regards
Dave

[Skippy]

Seems my previous attachment was an epic fail, so thought I'd try again.

[satdaveuk]

Thanks Skippy, when im in the workshop next I will try it and see what happens, keeps the brain active.