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TM 11-490-5
fails to clear up the problem, equipment problems, such
multiple subscribers are reporting problems. To affect
as receiver alignment or receive module malfunctions
rapid isolation of the source of the interference, proceed
are possible. Isolate the trouble by substitution.
as follows:
(c) If a station has been informed that
(a) On the orderwire circuit, request the
their transmissions are interferring with the receipt of
distant-operator to check his transmission from the
signals by another station, certain steps may be taken at
transmit voice frequency telegraph system.  This is
the suspected station to determine whether this is
accomplished  through  appropriate  patching  and
actually the cause of the problem:
monitoring and the results are reported. If the report is
that clear copy is leaving his station, then the trouble
1. Obtain approval to take the transmitter off
lies either along the propagation path, in antennas and
the circuit.
connections, in any interconnecting cables, or within the
2. Terminate the transmitter into a dummy
local operator's own station.
load, and proceed to check whether transmitter is
(b) Monitor  adjacent  channels  to
operating normally as indicated in paragraph 4-3c(2)(b).
determine whether they are also affected.  If these
(2) Major system isolation procedures.
channels are affected, service may be restored by
indicated previously, a major point where interference
connecting to available spare channels.  Notify the
can enter a communication system is the radio link.
distant operator to do the same, after informing the
This radio link normally consists of a transmitter with
subscribers that a momentary interruption of service
some type of modulator, the receiver, the transmit and
may be necessary while patching is accomplished.
receive antenna system, and the transfer media
Equipment  (possibly  multiplex)  malfunctions  are
between the two antennas.
indicated; or if only a few channels are affected,
Although the exact methodology will vary depending
possible EMC problems are involved.  If a single
upon the equipment and type indicators or test
channel only was found to be affected, this indicates
instrumentation available, the procedure is similar for all
that other than EMC problems are involved and
corrective maintenance must be accomplished.
(a ) Receiver.
(c) A quick check can be made to
1. With the test instrumentation available,
determine whether a carrier channel is at fault by
check the receiver output signal to determine if it is
reconnecting the channel at both stations to a spare
normal for the particular system.  If the system is
channel. Care must be taken that such reconnecting is
transferring digital signals and the pulse recognition and
coordinated to avoid undue disruption of service.
pulse restoration circuits (that is, the circuits that
(d) If connection to spare channels
determine when a pulse is present and then generate a
does not restore service, the interference may be
true pulse at that time interval) are an integral part of
caused by a local transmitter or ancillary, or there may
the receiver, the signal should be monitored between
be a problem arising from proximity of the transmission
the detector and these pulse circuits. If the signal is
line/antenna  system  to  the  circuit  suffering  the
normal, the interference is not entering through the radio
interference.  In this case, a detailed check of the
antenna and transmission line would be required,
2. If the signal is not normal at the receiver
including measuring the voltage standing wave ratio,
output, the receiver input signal should be checked
unwanted  ground  connection,  corrosion,  insulation
using available signal indicators, test instrumentation,
breakdown, and the like.
and test points. If the RF signal cannot be checked, the
(e) If  problems  are  still  apparent,
IF signal can provide an indication of signal level and
checks should be made of the RF signal levels
deviation from normal. A spare receiver or broadband
throughout the system, back to the originating station.
portable radio may also be used to check the incoming
Levels which are too high will usually cause overdrive
RF signal. If the input signal is normal, as determined
problems  with  accompanying  nonlinear  distortion.
by tests on the actual receiver, the spare receiver, or the
Those which are too low cause unnecessary degradation
portable  radio,  the  distant  transmitted  signal  is
of the signal-to-noise ratio. Adjustments must be made
interference free.  If not, then the distant transmit
to ensure that levels are correct. Refer to paragraph 4-
system may be at fault. If there is no fault at the distant
3d (8).
station and there are no apparant propagation problems,
local testing will be required to determine whether the
(f) It is possible that the problem could
problem is associated with the actual receiver or
stem from nonlinearities within the voice frequency
receiver antenna system.  This can most readily be
telegraph terminal, from the carrier system, or within the
accomplished by replacing the receiver. If the
interconnecting cables.  A distortion analyzer may be
used to detect nonlinearities in the system.
(g) If reconnection to a spare channel

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