Elecraft AF1 Audio Filter - Manual (Page 7)

7
Circuit Description
Refer to the schematic diagram on page 8. The filter circuit uses a quad operational amplifier integrated circuit that features
excellent isolation between the stages. Audio applied at J1 passes through a low-pass filter formed U1A and B. Potentiometer
R5 varies the high-frequency cutoff. U1C and D form two independent bandpass filters whose center frequencies are
controlled by ganged potentiometers R10A and R10B. These stages tune together so they are both centered on the same
frequency at all times. U2 is an audio amplifier that drives the AF output.
Rotary Switch SW1 has three sections. Section B selects the output of the low-pass filter, the output of the low-pass filter
plus one bandpass filter, or the output of the low-pass filter plus both bandpass filters. In the OFF position, SW1C routes the
audio from the input directly to the output so you don’t have to remove the filter when it is not in use. SW1A controls power
to the filter.
The power circuit allows you to use either a 9V battery or an external power supply. Diodes D1 and D2 isolate the battery
from an external supply to avoid the possibility of an external supply driving current into the battery and causing it to rupture.
Troubleshooting
If LED D3 does not light when SW1 is positioned to LP, check the orientation of diode D1 (if using internal battery) or D2 if
using an external supply. Test points 2 (+) and 1 (-) also allow you to check to be sure voltage is present at J2. Next check the
voltage across test points 7(+) and 8(-). If the voltage is above about 2VDC, D3 is either installed backwards or it is
defective.
If there is power but no audio output except when SW1 is in OFF position, turn the AF GAIN pot fully clockwise and touch a
metal test probe to the center pin of pot R15. You should hear a slight click, indicating that amplifier U2 is working. If so, the
problem is most likely in the circuits of U1A or U1B. Similarly, loss of output or function in BP1 indicates a problem in the
U1C circuit and problems in BP2 indicates a problem with the U1D circuit (see the schematic diagram on the next page). The
usual cause of problems in these stages is an incorrectly installed part, an electrolytic capacitor installed backwards or a
solder bridge across two adjacent pads. Use a magnifier and carefully inspect the board for correct component values and
solder bridges around the affected circuit.