a. Telephone Operation Between Sites.
(1) Ringing. The operator at the local site is signaled when the RINGER button is pressed and released
rapidly several times. This action activates magneto G201 to generate a ringing signal. When the RINGER
button is pressed, switch S203, which normally connects buzzer DS201 (and a call lamp circuit in models A, B and
C) to audio line transformer T201, is actuated to connect the magneto to T201. The ringing signal is coupled
through T201, through the line to the local site. Incoming ringing signals are coupled through T201 to activate
buzzer DS201. In A-, B- and C- models, incoming ringing signals are also applied to the call lamp control relay to
activate the call lamp visual signal. For operation of DS201, TEL-RAD-RAD/SPKR switch S202 may be in any
position. However, it must be set at TEL for the operator at the remote site to hear the local operator's voice.
(2) Telephone communication. Transmitted voice signals generated in the microphone of Handset H-138/U
are amplified by audio amplifiers Q201 and Q202 in the preamplifier and are coupled through T201, through the
line to the local site. Incoming voice signals from the local site are coupled by T201 to the H-138/U earpiece
b. Operation of Radio Set From Remote Control Unit.
(1) Transmitting. For this type of operation, the TEL-RAD-RAD/SPKR switch is in either the RAD or
RAD/SPKR position. Voice signals developed in the handset microphone are applied to audio amplifiers Q201
and Q202. The amplified output signals are coupled by audio line transformer T201 to the LINE binding posts.
Oscillator Q204 produces a 3,900-hertz (Hz) signal tone which is applied to audio amplifier Q205. Output signals
of Q205 are coupled by T201 to the LINE binding posts. The voice signals and the 3,900-Hz signal are
transmitted over the field wire to the local control unit. The 3,900-Hz signal tone activates circuits in the local
control unit which keys the transmitter portion of the radio set.
(2) Receiving. Audio signals from the radio set receiver are returned to the remote control unit through the
local control unit and field wire.
(a) Audio signals appearing at the LINE binding posts are coupled by audio line transformer T201,
switch S202 (RAD position), filter FL201, VOLUME control R208, to audio driver Q203. Output signals of audio
driver Q203 are coupled by audio transformer T202 through relay K201 (deenergized) and switch S202, to the
(b) When the TEL-RAD-RAD/SPKR switch is set to RAD/SPKR, battery voltage is applied to power
amplifiers Q206 and Q207. The audio signals are coupled from audio driver Q203 to power amplifiers Q206 and
Q207 by audio transformer T202. The output of power amplifiers Q206 and Q207 is coupled to loudspeaker
LS201 by audio transformer T203.