TM 11-5805-689-14
R76 and diode CR47. The 425 Hz tone gate circuit
discharges through diode CR19 and transistor Q13
will also be explained later as it does not come into
holding transistor Q15 off until the capacitor charges
play at this time. The collector of latch transistor
to the proper potential to turn transistor Q15 on.
Q30 is also coupled to the SRI and activates the
This time period is determined by the charge time of
indicator, indicating a request-for-service to the
capacitor C8 through resistor R36 and is set at a
operator. (In this Converter, all the oscillators are off
n o m i n a l time of 1 second. When capacitor C8
at all times with the exception of the seize/DTMF
charges to the proper value, transistor Q15 con-
oscillator.) The function of the 570 Hz oscillator will
ducts. The collector of transistor Q15 switches to a
be described at this time. To avoid repetition, only
logic 0 condition and is coupled directly to the
emitter of transistor Q14. This causes transistor Q14
one of the oscillators will be explained since all
to conduct. The collector of transistor Q14 then
oscillators are of the same basic design. The collector
of the 570 Hz oscillator transistor Q104 is coupled
switches from a logic 0 condition. The collector of
transistor of Q14 switching to a logic 0 removes the
through Q103 to positive battery potential. Base
base current from transistors Q16 and Q17 to switch
drive for transistor Q104 is derived through the
transistor Q14 off. The collector of transistor Q14
switching to a logic 0 also attempts to turn off
oscillator tank coil connects to the voltage divider
network consisting of resistors R108 and R109.
transistor Q17. However, before transistor Q17 can
turn off, the capacitor C9 (between the collector of
C a p a c i t o r C105 tunes the tank to a resonant
transistor Q16 and the base of Q17) has to discharge
frequency of 570 Hz. Initially, when power is turned
through resistor R43, transistor Q14 and transistor
on, transistor Q104 conducts. The DC current flow is
Q15
to
ground.
Thus
Q17
stays
in
f r o m the positive voltage source through the
t r a n s i s t o r Q103 emitter to collector and Q104
conduction for the time period established by the
collector to its emitter, through resistor R110, and
discharge of capacitor C9 and resistor R43. When
capacitor C9 discharges, transistor Q17 turns off and
through the oscillator transformer feedback winding
to negative battery. Base current flows from the
changes the potential on the emitter of transistor
Q17 from a logic 0 to a logic 1. The emitter of
p o s i t i v e voltage source through resistor R109,
through part of the tank coil winding, to the base of
transistor Q17 is coupled directly to the base of
transistor Q104. W h e n transistor Q104 starts
transistor Q27. This switching off of transistor Q17
conducting a voltage is built up across the feedback
removes the necessary base current to keep tran-
winding in the emitter circuit. A portion of this
sistor Q27 in conduction. The collector of transistor
voltage is inductively coupled to the base via the
Q27 then switches from a logic 0 to a logic 1. The
tank circuit winding. As this signal is in phase with
collector of transistor Q27 is coupled through diode
t h e signal on the emitter of transistor Q102,
CR44 to the base of transistor Q28. Transistor Q28
oscillations are initiated. Oscillations are sustained
now starts conducting. The collector of transistor
as long as power is applied to the circuit. Diodes
Q28 is coupled through resistor R78 to the base of
CR109 and CR110 form a clipping network across
latch circuit transistor Q31. This provides a path for
the tank circuit which keeps the AC swing constant
the base current for transistor Q31 which causes Q31
at the base of transistor Q104 due to the saturation
to conduct. The collector of transistor Q31 is coupled
of the diodes on the peak of the AC swing. Therefore,
through resistor R81 and to diode CR51 to the base
the AC voltage remains essentially constant on the
latch transistor Q30. Transistor Q31 is a PNP
emitter of transistor Q104. The output is taken from
transistor and the conduction of transistor Q31
the emitter of transistor Q104 thus minimizing the
causes the collector of this transistor to switch to a
loading effect produced by the tone gate circuit.
logic 1. This logic 1 is coupled to the base of tran-
Coupling capacitor C107 couples the 570 Hz signal to
sistor Q31 and causes it to conduct. The collector of
the base of tone gate transistor Q25. The tone gate
transistor Q30 now holds transistor Q31 in con-
t r a n s i s t o r is enabled at this time because the
duction and the circuit is "latched up." The collector
collector of the detector output transistor Q28 is
of transistor Q30 is coupled through diode CR49 and
providing bias to the emitter of transistor Q25
resistor R53 to the base of voice gate inhibit tran-
through resistor R67. The 570 Hz tone gate is a one-
sistor Q19. This circuit will be explained later since it
stage gate designed to provide a loss, together with
is not utilized until the operator inserts the plug into
the jack. The collector of transistor Q30 is also
the inherent loss of output transformer T1 of ap-
proximately 11 dB (with respect to the signal on the
coupled directly to the emitter of the 425 Hz tone
gate control transistor Q29 and would enable this
base of transistor Q25). The collector of transistor
transistor to conduct at this time, but an inhibit is
being held on its base by the output of the detector
570 Hz signal appears on the four-wire send pair at
output transistor Q28 through diode CR46, resistor
14 dBm 2 dB as per specification. When the
3-5