S-COM 7K, Manual, Page: 364 / 400

7K
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Appendix C
C-6
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If the transmitter keys when the PTT input is opened and unkeys when the
PTT input is driven low, place DIP Switch #7 in the OFF (open) position.
Repeater Receiver Audio
The controller requires audio from the repeater receiver. Connect the audio
from the repeater receiver to Pin #1 of J2 (the female DB25S connector).
To get a better idea of how to interface your particular receiver’s audio output
to the controller, let’s examine the controller’s repeater receiver audio input.
This circuit consists of an op amp connected as an AC-coupled inverting
amplifier, with two feedback resistors and one feedback capacitor. A 50K pot,
R97, is connected between the audio input and ground, and serves as an
adjustment for the audio going into the op amp. The input impedance of the
controller depends upon the pot setting, but will be 14K Ω or greater.The best
place to get receiver audio is the output of the receiver’s first audio preamplifier
stage (usually right after the discriminator). This point is often available at the
high
end of the volume control pot. The preamplifier stage provides flat audio
from a low-impedance driver, often an op amp or an emitter-follower stage.
Note: Do not
tap into the wiper of the volume control, or use the speaker driver
as the audio source; accidental adjustment of the volume control will affect the
repeat level. Speaker audio has a higher level of distortion than audio from
earlier stages.
The controller works best when driven with flat (not pre-emphasized) audio at a
level of 700mV RMS (2V peak-to-peak). At this level, the controller’s RX1 pot
(R97) will be in the mid range position when optimum audio is fed to internal
circuits. The controller’s actual input acceptance range is 200mV RMS (0.5V p-
p) to 2V RMS (5.6V p-p) as shipped from the factory, controllable by the RX1
pot.
If your receiver’s audio level is below the minimum of 200mV, you can increase
the op amp’s gain by cutting a feedback resistor.
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Resistor R30 (100K) has one lead marked “A”on the PC board; cut his lead
and lift the resistor.
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This operation changes the input acceptance to a new range of 70mV RMS
(0.2V p-p) to 700mV RMS (2V p-p).
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The midrange position of the RX1 pot will now correspond to 250mV RMS
(0.7V p- p).
Some repeater owners will prefer to drive the controller with pre-emphasized
audio from the receiver’s discriminator circuit.
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To accommodate this, the op amp stage must provide a -6dB/octave rolloff
response (de-emphasis), which results in flat audio being sent to the
controller’s circuits. (This is especially important for the DTMF decoder).
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Capacitor C67 (47pF) can be increased to 0.01µF if normal gain is used, or
to 0.0047µF if the high-gain modification has been done, to provide the
required de-emphasis.