Arado 130

[Silver Dollar]

What about a capacitance multiplier? Be varying the gain you can vary the capacitance...

http://en.wikipedia.org/wiki/Capacitance_multiplier

[6666]

http://www.geotech1.com/forums/attachment.php?attachmentid=17162&d=1319371214

You can see the cap in this picture just rat an old radio

[Sean_Goddard]

I think the object of the exercise id to "automate" the nulling by automatically varying the capacitor.

Capacitance multipliers do not quite work in the way you would think and I'm pretty sure would not be appropriate in this application.

[Thomas]

a portable radio tuner uses a 10-365p variable cap. O just saw 6666 mentioned it.

[satdaveuk]

Variable Capacitor

Hi Everyone
The Variable capacitor in the original was made by Jackson its value is 150pf , so not something we can dig out of a typical old radio.
Had chance of a similar one made by Jackson, value was the same but a bit bigger.
The dealer wanted £16 for it and he wouldn't budge on price so that far as I got.
Got everything else including the chip but have not progressed as we don't know the specification of the search head.

Regards


PS the smiley's were put on here by my daughter

[6666]

so not something we can dig out of a typical old radio.

Why not ?

If you rat a cap out of an old radio say its max value is 365-465pf You could simply mark the 150pf max value, it might be coarse tuning, but the circuit probably tunes with some value under 150pf.

Or You can use an old ham radio trick to get a smaller cap value, by putting c's in series, to get your max value of 150 pf

so if the main variable cap is 465 pf add a 220 pf in series =150 pf max, this gives you more tuning range and less coarse.

http://www.calctool.org/CALC/eng/electronics/cap_series

[Davor]

Can someone please place a schematic in question so that I can see what's all the fuss about?