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TM 11-5895-878-14&P
isolation procedures at other TCF's which handle the
associated alarm output) to verify that no failure has
defective circuit.
occurred in the loop current distribution system serving
the subscriber loop. If a blown fuse alarm is received,
(2) If the noise is not encountered in (1)
the line should be checked for a short circuit.
above, then the fault is in the subscriber's terminal
equipment or cable.  Maintenance should be notified
(2) Open connection at subscriber terminals.
and an alternate cable pair should be sought.
If no failure in the loop battery supply is detected, the
station technical controller should:
(3) If patching was necessary in some leg of
the circuit, the technical controller, upon notification that
(a) Locate the jack appearances of the
the / malfunction has been corrected, coordinates with
subscriber circuit at the DC Patch bay and patch a
technical controllers or subscribers involved to restore
voltmeter into a monitor jack appearance of the
the circuit to the normal-through configuration.
Transmit circuit (Receive circuit of Subscriber) to
measure the loop voltage.
(4) He then completes station records, trouble
reports, and log entries.
(b) If a zero reading is obtained, by
patching a voltmeter into the subscriber circuit "REC
c. Fault Isolation of a VF Subscriber Circuit by the
MON" jack appearance at the DC (Low Level) patch
Loop-Back Method. The technical controller is notified
bay, the technical controller will be checking to
by a local subscriber that the circuit level is not correct.
determine that a steady mark condition is not being
The technical controller decides to patch the circuit out
received from the VFCT. Refer to maintenance.
of service since a spare is available to the originating
subscriber or switch. The technical controller clears his
(c) If a steady mark indication is
own station and then proceeds to isolate the fault, step
obtained, he should isolate the subscriber loop from the
by step toward the distant end (toward the end
in-station circuitry by inserting a shorting-type dummy
originating the signal).
plug-in the line transmit jack appearance at the DC
The following procedure could be used:
Patch Bay. A + 6 vdc will be applied to the line.
(1) From the subscriber through the local
(d) If the receiving device continues to
TCF can be cleared as discussed in b above.
run open, request that user maintenance personnel
check the continuity of the subscriber loop.
(2) The next TCF toward the signal source
should be notified to loop back the circuit under test.
(e) Upon verification by maintenance
That distant station patches the 0 dBm Rec to the 0
that the trouble has been located at the subscriber
dBm Send of the same channel under test at the EL
terminal (for example, an open connection), and has
patch bay. When the local technical controller inputs a
been corrected, he should ensure that the normal
1000 Hz test tone on the Send Circuit at a level of 0
interface  equipment  has  been  returned  to  the
dBm, he will receive the same test tone back on that
subscriber's receive channel. Remove the dummy plug
channel's receive circuit. If the level is correct (0 dBm),
from the line jack appearance at the DC Patch bay to re-
that leg of the channel is cleared. If the level is wrong,
install the subscriber loop into the in-station circuitry.
the fault has been isolated between the two TCF's.
(f) Complete station records, trouble
(3) If the tone looped back was received at
reports, and log entries.
the proper level of 0 dBm, the distant TCF should be
(3) Defective cable.  The station Technical
requested to remove the loop back patch and the next
Control activity is notified by maintenance that the
TCF further toward the signal source should be
failure in the subscriber loop is caused by a break in the
requested to perform a similar patch. In this way, the
cable between the station and the subscriber location.
trouble can be isolated between any two EL patch bays
The report indicates that the cable will be out of service
in the channel.
for some period of time. The area circuit records have
been reviewed by the station technical controller and
3-13. Examples of DC Circuit Fault
they indicate that an alternate link is not available, or
Isolation Procedures
feasible, between the two locations.  He has further
determined that it is not practicable to copy the
a. Open Circuit Condition. The station Technical
incoming traffic at the station for use by the subscriber.
Control activity is notified by a connected telegraph
The station technical controller should:
subscriber that incoming traffic has been interrupted and
that the page printer has started to run open. Circuit
(a) Contact the distant VFCT terminal
layout record cards at the station indicate that the
and request that the technical controller stop traffic to
subscriber circuit is connected by cable to a nearby
the subscriber until the fault has been cleared.
military installation. The technical controller, based on
(b) Insert a dummy plug in the line jack
his knowledge that a break in the DC loop is the most
appearance of the subscriber's receive circuit ("Line
likely reason for the condition reported, reasons that the
Trans" Jack) at the DC Patch bay to isolate the
communications failure could be caused by a failure of
subscriber loop from the in-station circuitry.
the loop battery, an open connection at the subscriber
(c) Upon notification by maintenance
terminal, a defect in the cable, or an electrical failure of
personnel that the cable has been repaired, he should
the receiving device.
remove the dummy plug from the line jack appearance
(1) Loop battery check.
The technical
at the DC
controller should check the appropriate fuse panel (or
3-11

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