Elecraft AF1 Audio Filter - Manual (Page 6)

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The battery holder many be installed on the top or the bottom of the board. Normally you will want to install
it on the top so the finished unit will sit flat on its feet. However, if you plan to install the filter in an enclosure, you can
mount the battery holder on the bottom of the board. Solder pads are provided to mount the holder on either side as
indicated on the circuit board. Also, if plan to supply only external power to the filter, you can leave the battery
holder off entirely.
Install the battery holder as follows:
Position the battery holder on the board within the outline. Be sure you have it on the correct side of the board.
Unless you have a special need, you will want to put it on the top side along with the other components. Do not
solder yet.
Attach the holder to the board with the 4-40 screws in four places, using lock washers and nuts. The screw heads
should be on the holder side so the battery will fit correctly. You may need to bend the battery holder terminals
slightly to align the screw holes with those in the board.
Solder the two terminals to the pads and trim them flush.
Attach the four feet to the corners of the board on the bottom. If you plan to build the filter into a rig or separate
enclosure, you may leave the feet off and use the four corner holes for mounting the filter.
Attach the four knobs to the rotary switch and potentiometer shafts. Line up the index mark on the knob for the rotary
switch so it is pointing to OFF when the switch is rotated to the fully left position. The potentiometer shafts may wobble a
little because long shafts are required to approximate the height of the rotary switch shaft.
Checkout and Operation
If you have difficulty with the following steps, refer to the circuit description and troubleshooting notes in the following
sections.
Install a 9-volt battery in the holder or connect a d-c supply between 9 and 14VDC to J2 (positive to center pin).
Connect an audio source to the AF IN jack (J1). This can be a headphone output from your receiver or other audio
source.
Connect your headphones or a small loudspeaker to the AF OUT jack (J3).
Position the LPF, BP and AF GAIN pots to minimum (counter-clockwise).
Turn SW1 to LP (low-pass) position. The red LED D3 should light.
Rotate the AF GAIN clockwise for a convenient audio level in the phones or loudspeaker.
Rotate the LP pot clockwise. You’ll hear high frequency sound increase in volume as you turn the pot. This is raising
the cutoff frequency from minimum (fully counter-clockwise) to maximum. Leave LP fully clockwise.
Rotate SW1 to BP1 (bandpass filter 1) position. If possible supply a single frequency tone to the input (such as tuning in
a CW signal on your rig).
Rotate the BP pot. The volume of the signal will rise abruptly as you pass its frequency.
Rotate SW1 to BP2 (bandpass filter 2) position and repeat tuning the BP pot. You should notice that the adjustment is
now noticeably sharper. BP 2 adds a second bandpass filter to further improve the selectivity.
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Be sure the LPF is always set so it will pass the frequency you’ve chosen for the bandpass filter. You can set
the LPF cutoff frequency close to the BPF frequency so it aids in rejecting high-frequencies above the bandpass
frequency.