Wednesday, December 14, 2016

Varactor tuning circuit.

Here is the basic circuit with some things to consider when sizing parts. I have seen 10k pots in tuning circuits. There is really nothing wrong with this but the 100k will use 1/10th the current thus conserving battery. The 1 meg resistor should be fine with a good diode but if it is leaky it could drop voltage and cause problems. This can also be referred back to the pot. If the diode is leaky it will draw current threw the portion of the pot being tapped and cause excess voltage drop thus limiting the tuning range. I will build it as shown and see how it functions.

I built the basic circuit (RF, Detector, and oscillator) it is very strong. I used jumpers to connect some test coils and a tuning capacitor to it. I received some broadcast band and short wave. The J112s work very well. It has four knobs to turn and takes a little adjusting. More on that after I get the tuning section built.
EDIT 1: The four knobs have become 6 knobs. Now with a volume control on the AF amp and the slug tuning knob.
EDIT 2:  The slug tuning gives about 100KHz for two revolutions. With a one inch or larger knob it gives a sensitive adjustment.

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