S-COM 6K Owner's Manual page 15

4. RECEIVER #1 (REPEATER RECEIVER) AUDIO
The controller requires audio from your repeater receiver, which should be connected to pin 13 of
J2.
To get a better idea of how to interface your particular receiver's audio output to the controller, let's
examine the controller's Receiver #1 audio input circuit.
This circuit consists of op amp U22D connected as an AC-coupled inverting amplifier with two
feedback resistors and one feedback capacitor. A 50k pot, R75, serves as a level-adjust pot for the
audio going into the op amp. The input impedance of this circuit depends upon the pot setting, but
will be 14k ohms or greater.
The best place to get receiver audio is the output of the receiver's first audio preamp stage (usually
the stage that follows the discriminator). This point is sometimes available at the "high" end of the
volume control pot. The preamp stage usually provides flat (de-emphasized) audio, and the low
impedance driving circuit (op amp or emitter-follower amplifier) can generate high-level audio while
driving the capacitance of shielded cable. Don't tap into the wiper of the volume control pot, since
accidental adjustment of the volume control will affect the repeat level. Tapping into the speaker
driver as an audio source is discouraged, for the same reason. Also, speaker audio contains more
distortion than audio from earlier stages.
The controller works best when driven with flat (de-emphasized) audio at a level of 700 mV rms (2
V p-p). If you provide this level, the controller's RX1 pot (R75) will be in the midrange position and
optimum audio will be fed to the internal circuits. You can adjust the RX1 pot to accommodate
other audio input levels. The controller's input acceptance range, as shipped from the factory, is
200 mV rms (0.5 V p-p) to 2 V rms (5.6 V p-p).
If your receiver's audio output level is below 200 mV, you can increase the gain of the controller's
input by cutting a resistor. Feedback resistor R24 (100k) has one lead marked with a circled letter
"A" on the PC board. You may cut this lead and lift the resistor to increase the gain by 3. The new
input acceptance range will be 70 mV rms (0.2 V p-p) to 700 mV rms (2 V p-p). The midrange
position of the RX1 pot will now correspond to 250 mV rms (0.7 V p-p).
Some repeater owners will prefer to drive the controller with pre-emphasized audio directly from the
receiver's discriminator. The controller must then de-emphasize the audio so that the audio feeding
the DTMF decoder, phone line, etc. is flat. To accomplish de-emphasis, the controller's RX1 op
amp stage must be modified to provide a -6 dB/octave rolloff response. A simple way to do this is
to replace capacitor C45 (47 pF) with a 0.01 uF if the factory gain is used. Replace C45 with a
0.0047 uF if the triple-gain modification has been done. Note that using this larger capacitor will
decrease the op amp's gain at increasing audio frequencies; you may need to make the increased-
gain modification to offset the loss created by the de-emphasis capacitor.
After you have completed the repeater receiver audio interfacing, check the audio level at pin 14 of
U22D. This is the output of the RX1 op amp. You should read about 700 mV rms (2 V p-p) of
audio when the receiver is fed a fully-deviated 1 kHz sine wave tone from a service monitor.
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6K V1.02 03-16-91