A3Q14; resistors A3R37 and A3R38 establish the
base bias voltage for transistor A3Q13.
c. Electronic Switch A3Q12. Operation of elec-
tronic switch A3Q12 is controlled by the current
through transistor A3Q13. The voltage developed
across collector load resistor A3R39 is applied
between emitter and baee of transistor A3Q12.
With decreasing signal strength, the voltage ap-
plied to the base becomes more positive. Tran-
sistor A8Q12 is turned on when, the signal
strength falls below the threshold level, causing
THRESHOLD indicator lamp DS2 to light. The
drop in collector voltage that takes place at this
time is coupled through isolating resistor A3R35
and diode A3CR4 to the base of emitter follower
A3Q9.
d. Emitter Follower A3Q9. Conduction
through emitter follower A3Q9 is controlled by
the operation of electronic switch A3Q12 and
BREAK IN switch S2A. When the incoming sig-
nal strength falls below the threshold level and
electronic switch A3Q12 is turned on, conduc-
tion through emitter follower A3Q9 increases as
the forward bias between emitter and baee in-
creases due to the drop in the collector voltage
of transistor A3Q12. A similar result is produced
by the operation of BREAK IN switch S2A,
which causes the base of transistor A3Q9 to be
returned to O volt through diode A3CR4, isola-
tion resistor A3R36, and contacts 1 and 8 of the
switch. The increased conduction causes the emit-
ter to become leas positive, and this change is ap-
plied to the base of transistor A8Q10 in the re-
ceive circuit (para 2-11h). The ultimate effect is
to keep electronic switch A3Q11 (para 2-11i)
turned on in the receive circuit, causing the re-
ceive loop to run closed when the incoming vf
signal is below threshold level or the BREAK
IN switch is operated. Resistors A3R30 and A3-
R31 and diode A3CR3 establish the operating
point for transistor A3Q9.
a. Ring Detector A3CR1. Ring detector A3-
CR1 triggers the alarm circuit upon the receipt
of a 20-cycle alarm signal. The incoming alarm
signal is rectified by diode A3CR1, and the re-
sultant dc voltage which is smoothed by capaci-
tor A3C14, is used to forward-bias transistor A3-
Q16. Resistor A3R45 assures that only the 20-
cycle ringing signal has sufficient amplitude to
forward-bias the transistor. Vf ringing or infor-
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