Lesson 2/Learning Event 3
of alternate letter R and Y will approximate reversals. Alternate letters
R and Y is a good test of the teletypewriter printers because these two
letters are directly opposite in transitions, causing the teletypewriter
printer to completely reverse the position of mechanical linkage within
the selector mechanism.
Reversals are especially useful in making
distortion checks with a zero-center-scale milliammeter because of the
equal-length mark and space time periods.
d. Repeated Space Bar.
To develop space-bar signals the operator
merely holds down the space bar on the teletypewriter keyboard. When such
signals are used to measure telegraph distortion, the meter taking the
reading must be electrically weighted to accommodate the code combination
so as to give a zero reading when no distortion exists.
e. Miscellaneous.
Miscellaneous
telegraph
signals
are
those
encountered in normal traffic movement. They have no set pattern by which
the signals can be identified or compared with signals of another type.
Although miscellaneous signals are not specifically used for testing, they
do permit monitoring of telegraph loops under normal operating conditions.
Learning Event 3: PATCHING TELEGRAPH CIRCUITS
The normal idle condition for a DC telegraph circuit is a closed
circuit; that is, there is current in the circuit. All patching methods
must maintain this normal condition; that is, the circuit must never be
opened during patching procedures. The method used must therefore include
the use of normal-through jacks so that there is current even in the
absence of patch cords.
Also, in making the patch the technical
controller must patch first into the meter and then the circuit, which has
battery applied.
If the technical controller reverses the procedure and
patches into the circuit first, he will break traffic flow.
Furthermore
he will endanger his own safety because battery voltage will appear on the
patch cord plug.
a. Series Jack. When a technician patches a telegraph device into a
series jack, current continues through the device so patched because both
tip and ring in the jack contact the patch cord plug. The current enters
the tip of the plug and returns through the ring. Thus a test set sending
signals will insert them in series with the circuit.
41
Previous Page
