
NHRC-2 User Guide
NHRC-2 User Guide Page 4 2/28/2000
Copyright © 2000, NHRC LLC
All Rights Reserved.
CAS+ and CAS- terminals must be connected to something in order for the
controller to detect the signal present indication.
Transmitter audio can be fed directly into the microphone input of the transmitter.
VR2 (marked TX LVL) is the master level control, used to set the audio level into
the transmitter. 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 (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 250mA of current be
drawn through it, because the trace on the PC board is somewhat narrow.
2. Adjusting the Audio Levels
Preset all potentiometers to midrange. Connect an oscilloscope probe or DVM to
pin 15 of U3 (the M8870 DTMF decoder). (Use the power supply ground for the
'scope's ground or the DVM's return.) Key a radio on the input frequency, send
some touch-tones, and adjust VR1 (marked RX LVL) until DTMF decoding is
reliably indicated by a 5-volt level on U3 pin 15. Disconnect the oscilloscope or
DVM. Adjust VR2 (marked TX LVL) to adjust transmitter deviation, ideally
measured with a deviation meter or service monitor. Adjust VR6 (the beep level)
to set the courtesy tone and CW tone level.
The easiest way to adjust the ISD1420 input and output level is to select the
simplex repeater mode and record messages until the audio sounds right. VR3
(marked RECORD LVL) adjusts the record audio level into the ISD1420. Adjust
this control for the best sounding record audio. VR5 (marked SPEECH LVL) sets
the ISD1420 playback level. Adjust this control for best acceptable transmitter
deviation. VR4 (marked PROC RX LVL) is used to set the receiver audio level,
and may not need to be adjusted from midpoint.
3. Initializing the Controller
To initially program your secret code into the controller, you must apply power to
the controller with the pins on the INIT jumper, (SW1) shorted, putting the
controller into the initialize mode. Remove the jumper a few seconds after power
is applied. All of the values stored in the EEPROM will be reset to defaults, and