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the AC power circuit conductors running in parallel with the communication circuit conductors. At
times this voltage can read 50 volts or more. This voltage is of no consequence because the page and
channel lines are balanced twisted pair audio circuits.
In an SelectFone system, the audio voltage across page line conductors or across any channel line
conductors (L1 and L2) is of the order of 0.5 to 0.75 volts peak when someone is speaking into the
handset microphone. The voltage will be proportional to the person's voice speaking into the handset.
Hum or Buzz
When one of the two conductors in the page or channel lines short circuits to ground, the audio
circuit becomes unbalanced. If there is leakage to ground from one of the conductors, the audio
circuit becomes unbalanced.
When audio lines are balanced twisted pair, the induced noise voltage is the same on both conductors
and, therefore, cancels out before it is amplified. But when the circuit becomes unbalanced, the in-
duced noise voltage is not the same on both conductors and is, therefore, amplified and causes an
audible hum or buzz. To solve the problem, locate the source of the ground by checking each conductor
and moving from one junction point to the next and disconnecting the effected circuit. Improperly
grounded circuits are often the result of poor termina tions such that small strands protrude and touch
adjacent terminals. In many instances, the presence of water or moisture and deposits in a junction box
form a conductive solution that causes a ground fault or leakage to ground.
Low Level Audio At All Stations
This problem can occur when there is a dead short across any channel or page line. The system will
still function but the audio level will be the lowest at the stations closest to the short. Locate the short
by checking with an ohmmeter at the junction boxes and disconnecting the effected sections.
Cross Talk
Channel and page lines are twisted pair wires and balanced such that when circuits are accidentally
crossed during installation interference between two lines will occur. Cross talk can also occur when
there is leakage or a ground fault on two of the channel lines or when there is a missing or open line
balance resistor. The resistance between the channel line circuits should be very high or infinite.
Inoperative Station
In a properly operating system an inoperative station indicates miswiring or a problem with the
station itself. Check the wiring connection. If the problem is not caused by miswiring, swap the
plug-in modules of the effected station with one that is operating properly to identify the defective
module.
VI. MAINTENANCE.
SAFETY MESSAGE TO MAINTENANCE PERSONNEL
Listed below are some important safety instructions and precautions you should follow:
• Read and understand all instructions before installing or operating this system.
• Any maintenance to the SelectFone station must be done with power turned off.
• Any maintenance to the unit must be performed by a trained electrician in accordance with NEC
Guidelines and local codes.
• Never alter the unit in any manner. Safety in hazardous locations may be endangered if additional
openings or other alterations are made in units specifically designed for use in these locations.
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