TRAFFIC CONTROL TELEPHONE SYSTEMS Circuit - Sam Hallas

Page 36
The “low” and “release” potentiometers are provided for the adjustment of the
current through the selector for low and release tests.
Lever type keys for applying the various test conditions.
A centre zero micro-ammeter, with shunts for measuring test currents and
observing the impulse ratio.
A mounting with spring contacts which automatically connects the selector in
the test circuit.
Two terminals L1 and L2 for connecting the set to an outside line, and three
terminals K1, K2 and K3 for connection to an external selector key, key case
or external source of impulsing.
When a key case is mounted in this manner it must be entirely disconnected
from its associated selector apparatus case.
The apparatus is mounted on a removable wooden panel in a wooden case with
cover and carrying handle. Inside the cover is pasted a schematic (LP. 15500) of the
test circuit, together with step-by-step instructions for operating the set, similar to
the following :-
Operating Instructions
Connect a battery of 240 ± 8 V D.C. to the terminals so marked, and connect
another battery of 10 ± 1 V D.C. to its marked terminals.
Insert the selector key to be tested under the five spring terminals at the back of
the selector tray, and operate the main switch.
To Adjust the Test Set
The Selector Key provided is a general call key, and will step any selector to its
ringing position. The speed and ratio of this selector key should be checked in the
following way :-
Speed. — Operate the selector key and make sure that the time taken for the
mechanism to run back, from start to finish, is between 7½ and 8 seconds. If
the timing is not correct the governor springs of the key must be adjusted
either inwards or outwards to obtain more or less speed respectively.
Ratio.—Throw the Ratio Key (KIR on diagram). Again operate the selector
key, and observe that when impulses are being transmitted the needle of the
meter oscillates equally about the zero mark. If this is not so, variations can
be eliminated by turning with a suitable screwdriver the head of the ratio
rheostat in the centre of the panel (YJ on diagram). When this adjustment has
been made restore the ratio key.
Test Current.— Adjust the test currents through the selector by proceeding as
follows :- Throw the "Low" key (KL on diagram) and hold the "Meter" key
(KM on diagram) operated, then adjust the knob of the rheostat marked
"low" till the micro-ammeter reads 125. The meter multiplier in this case is