Tuning the Receiver


Tuning the receiver involves aligning the BFO (beat frequency oscillator), VFO (variable frequency oscillator), and the input toroid for the correct frequency of operation.

A. BFO ALIGNMENT FOR 10MHZ CRYSTAL FILTERS

1. Place a jumper between the antenna input and the input to the crystal filter, which is pin 3 of L2, a T4-6T transformer.

2. Tune the "BFO ADJ" variable trimmer close to the single BFO crystal near L3, the other T4-6T transformer on the opposite side of the board.

3. If WWV is strong enough, you should hear a tone as you tune the trimmer. Zero-beat the tone and you can listen to WWV. After alignment of the VFO and front end, the tone of the background noise can be used to adjust the trimmer.

4. Set the "BFO ADJ" trimmer somewhere in between the highest and lowest tone. A simple 10MHZ crystal oscillator can be used as an alignment signal, if WWV cannot be used.

The test point for a frequency counter or scope is pin 7 on the NE602 (U2). The frequency should range to both sides of 10MHZ. A live signal should be used for final adjustment.

B. VFO ALIGNMENT

The test point for a frequency counter or scope is pin 7 of U1 (NE602).

1. Remember to place the VFO tuning capacitor at full closed or open. Closed for normal tuning and open for backward tuning. (See chart in Step 6, Part C)

2. Adjustment is made by tuning the 85PF trimmer at C32. After adjusting C32, there will be some frequency drifting. This is the trimmer "setting in" and not drift from the VFO circuit.

3. Set the frequency with the value from the charts in Step 6, Part C, according to the band that you have chosen for the receiver. You will be at the low edge of the band. Moving the tuning capacitor will raise the operating frequency.

C. FRONT END ALIGNMENT

1. Trap alignment - Move the slug with an alignment tool until WWV is not heard in the output of the receiver.

2. Input toroid - Adjust the "ANT ADJ" trimmer near the input toroid until the background noise is loudest. Tune in a live signal and re-tune.

The receiver is now ready to use. Further refinements can be studied in the following sections: Crystal Filter Bandwidth, Audio Amplifier, and Experimenter's Section. If you are having problems, read the following Troubleshooting section. For information on how it works, read the Theory Of Operation.

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Last Update: 04/20/97
Web Author: David White, WN5Y