Lesson 1/Learning Event 7
or polar signals are being passed, and whether the current is normal for the
condition at hand. To an extent, he can also determine whether test signals
or repeated letter signals are being transmitted or received.
(a) Neutral circuit (high level). A neutral circuit can be identified
by current for mark, and absence of current for space. Neutral signals can
be identified by pulsing of the meter needle to one side of zero as signals
are transmitted down the line.
If a repeated letter is transmitted, the
meter pulsing will have a repetitious pattern. When miscellaneous signals
(test signals or traffic) are transmitted, the meter pulsing will be
erratic.
If reversals (continuous stream of equal-length mark and space
pulses) are transmitted, the meter needle will hover near one-half the
current value.
Zero center scale can be used provided the technical
controller inserts a neutral-to-polar repeater between the neutral loop and
the meter.
(b) Polar circuit (high level).
A polar circuit can be identified by
current flow in one direction for mark, and in the opposite direction for
space.
Polar signals can be identified by slight pulsing of the meter
needle to either side of zero as signals are transmitted down the line. If
a repeated letter is transmitted the swing of the needle around zero will
exhibit a repeating pattern.
When miscellaneous signals are transmitted,
the meter pulsing will be erratic. If reversals are transmitted, the meter
will register zero provided no distortion is present, because the current
for equal periods of time (mark and space) will be as great in one direction
as in the other. If distortion is present, the needle will tend to favor
the mark side or the space side of the meter, showing the effect of either
marking or spacing bias distortion. In the event the technical controller
opens the circuit by mistake, he may think he is observing reversals,
because the meter registers zero. He is therefore wise to use an alternate
means to check for reversals.
A very good method is to observe the
pulsating voltage in the telegraph loop with an oscilloscope.
(c) Reverse current (turnover). There is a correlation between current
in the wire and the performance of the teletypewriter.
In a neutral
circuit, the teletypewriter should run closed when there is current, and it
should run open when the current ceases.
If the teletypewriter runs open
even though current shows on the meter, the wire pair connecting to the line
relay of the teletypewriter is probably inverted. This condition is known
as turnover. In a polar circuit, the teletypewriter should run closed for
marking current, and it should run open for current in the opposite (space)
direction. If the machine runs closed on space and open on mark, the wire
pair is obviously inverted to the line relay, and turnover exists.
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