NHRC NHRC-4, Installation And Setup Gude

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NHRC-4 User Guide
Receiver audio can typically be taken from the high side of the squelch control. This audio
must be de-emphasized with the controller's de-emphasis circuit, which provides a -
6dB/octave slope. Optionally, audio can be taken from later in the receiver's audio chain,
where it is already de-emphasized. Care must be taken that this source of audio is not subject
to adjustment by the radio's volume control. If the receiver audio has not been properly de-
emphasized, either in the receiver itself or on the controller board, the repeater will have a
very "tinny", unnatural sound to it. The NHRC-4 repeater controller is shipped without the
de-emphasis circuit populated on the printed circuit board, for "flat" audio response. To
install the de-emphasis filter, two 100K ohm resistors must be removed, and a 51K ohm, a
510K ohm, and a 6800pF capacitor must be installed on the board. Consult the NHRC-4
Repeater Controller (Audio) schematic for modification instructions.
The receiver must provide a signal present indication (also called CAS, COR, RUS) to the
controller. The controller requires an "active-high" signal here. If your radio only has
"active-low" signaling available, a simple inverter can be constructed with a 2N3906 and a
4.7K resistor. Connect the emitter of the transistor to a source of positive voltage, the
collector to the controller's CAS terminal, and the base to the active-low signal through the
4.7K resistor.
Transmitter audio can be fed directly into the microphone input of the transmitter. VR5 is
the master level control for the primary radio, used to set the audio level into the transmitter.
VR2 is the master level control for the secondary radio. The transmitter's deviation limiter
(sometimes called IDC) should be set such that the transmitter cannot overdeviate,
regardless of input signal level. One way to adjust transmitter deviation is to set the
transmitter deviation limiter wide open (unlimited), adjust the controller's master output until
the transmitter is slightly overdeviating, then set the transmitter's deviation limiter to limit
just below 5 KHz deviation. Then reduce the controller's master output until the transmitted
audio does not sound compressed or clipped. Transmitter deviation should be adjusted with
a service monitor or deviation meter.
Transmitter keying is provided by a power MOSFET (Q2/Q6) configured in an open-drain
circuit. This can be used to key many transmitters directly. The MOSFET essentially
provides a closure to ground for PTT. For other transmitters, the MOSFET can drive a small
relay to key the radio. Although this MOSFET can handle several amps, we recommend that
no more than 500 mA of current be drawn through it.
Copyright
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2001, 2005, NHRC LLC. All Rights Reserved. Page 3