CCWN 75:18
                                                       
     I accidentally omitted four resistors in the layout of the
board, so these  will have to be hand-wired into the circuit. All
of them are lOOK resistors, and  they all are associated with the
LM324 in the upper left hand corner of the board as  shown in
Figure 2. Carefully solder these resistors to existing leads of
other  resistors or to plated pads on the board as suggested in
Figure 3:       Next, on the bottom side of the board, connect
all together the four center  pads of the row of six. (This is
the "center voltage" node of the filter.) See  Figure 3 above.    
  The lO-position phase-select switch I used is a Centralab
PA-1001. As supplied  from the factory, the unit can switch to
two positions. Follow the instructions  included with the switch
to increase the positions to lO.       Drill holes for the
phase-select switch and the five external connections  on the
front and rear sides of the (CU234 or similar) metal box. Mount
the  circuit board on the bottom of the box using low standoffs
to make sure none of  the IC leads or component leads touch
bottom. Mount the connectors and the  switch.       Wire up the
five connectors to the leads as indicated in Figure 2. Mount the 
lO-position switch. Rotate the switch fully counterclockwise and
note which terminal  makes contact. Solder an insulated wire from
this terminal to the pad labeled "l"  (lower right-hand corner of
Figure 2). Then rotate the switch one step clockwise,  noting the
contacted terminal, and wire it to the pad marked "2." Continue
in this  manner up through 8, 9, and 0. Then wire the common
terminal of the switch to the pad  near the right-hand center
edge of the board as shown in Figure 2.       In normal operation
of the filter, there is a small continuous audio tone output 
even when no signal is present. The level of this output is about
20 dB below the  normal signal output when the filter is operated
in the form you have built it so far.  By a simple balancing
process the contrast between this continuous level and the 
desired output can be increased to about 40 dB. This is a
balancing process very  similar to obtaining carrier balance in a
single-sideband transmitter. To provide  balance control, wire
two potentiometer and six resistors as shown in Figure 4. The 
potentiometer can be mounted on the side of the filter box.      
The LM324 you make these connections to is the one in the lower
left hand  corner of Figure 2.       Now we are ready for
tune-up. Connect 12 volt power only just briefly and  check that
the circuit does not draw more than lO ma. Connect all inputs
except the  incoming signal. Then apply power again and listen
for the steady audio tone output.  (If you do not have an audio
amplifier for the filter output, you can connect high-; 
impedance headphones to the point shown at the top center of
Figure Q.