VFO Fine Tuning


If you are out of range, either change the value of C35 or add turns (lower frequency) or remove turns (raise frequency) from the VFO toroid. The tap does not need to be changed for minor changes in total turns. Remove or add turns at the "VFO" side of the coil.

VFO maximum frequency range is 3 to 10MHZ for this board. Experimentation is encouraged outside this range. Lower frequencies should be no problem. Higher frequencies are limited by the lead lengths on the board and layout.

C32, C35, and C30 are used to tune the frequency of the VFO.

For VFO's in the 3 or 4MHZ range that are used in this version of the 40 and 20 meter receiver, greater VFO range is obtained by connecting the stator of the 35PF tuning capacitor to the VFO side of L1, marked "WIDE" on the silkscreen. In every case, a small tuning range is obtained by connecting the stator to the "VAR CAP" pad, and a large tuning range by connecting the stator to the VFO side of L1, marked "WIDE" on the silkscreen.

If a general coverage receiver is used, place a short antenna lead near the VFO and tune the receiver for a signal. Adjust C32 and walk the VFO to the proper frequency.

If a signal generator is used, place a short lead from the output of the generator close to the receiver. Tune C32 until the signal is found. A crystal oscillator can also be used to generate a calibration signal. For setting up a 20 meter receiver, set the generator at 14MHZ and tune C32 with the VFO tuning capacitor closed. For setting up an 80 meter receiver, set the generator at 3.5MHZ and tune C32 with the VFO tuning capacitor open.

The circuit board is also drilled at L1 so that a shielded coil similar to the "TRAP" can be used. Shielded coils used for 10.7MHZ IF's can be used for VFO's in the 5-10MHZ range. This layout also accepts the L46 series of Amidon shielded coils. The L46-6 is recommended. The 85PF trimmer can be removed or replaced with a fixed value, since the slug in the coil can be used to set the frequency.

If no test equipment is available, turn the trimmer at C32 a little bit at a time while peaking the "ANT ADJ" trimmer. Your tuning capacitor should be connected to the wide range pad. Tune the VFO tuning capacitor and listen to the signals. When you hear some familiar signals, you will be close to the band your looking for. Ham radio communication is unique, and should be easily picked out.

Only four capacitors are needed to vary the oscillator from 3 to 6MHZ, then change coil size and range from 6 to 10MHZ with the same capacitors. See chart in Step 6.

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Last Update: 05/07/97
Web Author: David White, WN5Y