NHRC-2 User Guide
NHRC-2 Troubleshooting Guide
Page 3
3/16/2000
Copyright © 2000, NHRC LLC
All Rights Reserved.
nice square wave on it, at the 3.58 MHz clock frequency. If the clock is
not found at pin 15, look for it at U3 (M8870) pin 8. If the clock is present
on U3 pin 8, but not on U1 pin 15, verify the installation of C20, a 33pF
capacitor. If the clock is not present on U3 pin 8, verify that power is
applied to the U3, that the crystal Y1 is properly installed and that the
crystal is not shorting from the case to the trace just under it. Lifting Y1
up slightly off the PCB or using a dab of RTV or “hot glue” under the part
will correct this.
Audio problems
First, note that the controller should pass audio through to the transmitter
only when the CAS signal is present. If the audio passes through when
the CAS signal is not present, and un-squelched audio is transmitted
during the hang time, then it is likely that either you have forgotten to
remove the "INIT" jumper (SW1), or are overdriving the audio gate FET.
Remove the jumper or reduce the signal applied with VR1.
The next common problem to cause the audio path to malfunction is the
failure to use dipped tantalum caps for C2, C7, C8, C9, C10, C11, C14
and C17. The tantalum caps have a very low ESR (effective series
resistance). The use of any other type of cap will cause the output of the
op amp to sit at the rail (+12v). Voltages around U5 (LM358) measured
with a DVM should be as follows: Pin 3 = +6.5v, Pin 1 = +6.5v, Pin 7 =
+6.5v. If pin 1 or 7 is reading higher (about +12 volts or more) you have a
leaky cap or an open in the feedback path. If pin 3 isn't reading around
+6.5v check the values of R1 and R2, should be 10Kohms. Also note that
you have installed the tantalum caps in the board with the proper polarity.
The square pad indicates the positive side of the cap. Reverse voltages
easily destroy tantalum caps, if you put it in backwards, throw it away and
use a new one.
If the audio out of the controller is low, check to make sure that VR2 is a
500Kohm pot and R15 is 10Kohms.
DTMF decoding problems
If DTMF tones do not mute completely or not at all, try lowering the level
at VR1 (RX LVL). You may be over-driving the audio muting gate FET
(Q1) and it is starting to turn on. Verify the installation of all the
components in the muting circuit: R6, R7, R8, Q1, and Q2.
If DTMF tones do not mute and the controller doesn't respond to
commands, can' t load password, make certain that VR1 isn't adjusted to
low. You should have 2v P-P at U5 pin 1, ignoring the DC level.
Check the audio circuit for malfunctions as above. If the audio through
