delay amplifier A2Q15 produces a dc level out-
output of A2CR2 couples through emitter follow-
put that activates the alarm circuit as described
er A2Q6 to reset multivibrator A2Q8-Q9. The
in j above.
new output of A2Q8-Q9 couples through S5A-
7-5 to release the inhibit from the send circuit.
l. NORM-REC-SEND Switch S3. For half du-
The new output of A2Q8-Q9 also couples
plex operation on a 2-wire telephone channel or
through buffer amplifier A2Q7 and S5A-3-1 to
for full duplex operation on a 4-wire telephone
lock the send circuit in the space hold condition.
channel, S3 is left in its NORM position. For
The break-in (detect) function can be disabled
half duplex operation on a 4-wire telephone
by strapping terminals 1-2 at the input to A2Q7.
channel such as a radio-telephone channel, S3 is
set to SEND for transmission and to REC for
switch S4 set to 20 and RING switch S1 de-
reception. When set to SEND, contacts S3-9-12
pressed, the 20 cycle ringer is activated by +18 v
provide a closure at CONT binding posts E1-E2
applied to it via S4-3-1 and S1B-6-4. The 20-Hz
to operate the remote radio transmitter if re-
ringing signal couples through the vf interface
quired. Also contacts S3-1-4 make to enable the
network to either binding posts E4-E3 or E5-E6
send vf output by disabling AND gate A2Q12-
depending on the setting of S5. With S5 set to
Q13. Contacts S3-5-8 make to bypass capacitor
4W the signal path is from the 20 cycle ringer
C13 in the vf interface network thus providing a
through lamp DS3, S1A-6-4 and S5C-5-6 to E3.
dc path for the external circuit common battery
The return is from E4 through S5B-6-5 and
holding function if required (C13 is also by-
S1A-7-9 to the 20 cycle ringer. With S5 set to
passed with S3 in NORM position when COM-
2W the 20-Hz signal path is from the 20 cycle
BAT terminals 5-6 are strapped). When S3 is set
ringer through lamp DS3, S1A-6-4, S5C-5-7,
to REC, contacts S3-1-3 make to inhibit the send
and S5C-3-1 to E5. The return is from E6
Vf output.
through S5B-1-3, S5B-7-5 and S1A-7-9 to the
20 cycle ringer.
m. 4-Wire Half Duplex Home Copy Circuit.
NOTE
When S5 is set to either 4W or TEL the output
If S4 is left in the 20 position output
of the send circuit, at amplifier A1Q7, connects
transformer T3 is disconnected from
through S5A-12-9 or S5A-10-9 and 4W HC
telephone channel at contacts S4-7-8.
strap terminals to the input of the receive circuit,
at emitter follower A3Q1.
C i r c u i t (Vf). W i t h
VF-20
i. Signaling
switch S4 set to VF, contacts S4-1-3 are open to
n. Power Supply Circuit. The power supply
disable the 20 cycle ringer. When RING switch
furnishes all required operating voltages for the
S1 is depressed, contacts S1B-7-8 open the send
TH-22/TG from any of three primary power
loop, thus locking the send circuit in the space
sources: 230 volts ac, 115 volts ac, or 26 volts
hold condition. The space hold signal is trans-
dc. A switch adapts the power supply circuit to
mitted through the vf network into the telephone
operate with any of these input voltages. When
channel in the same manner as any send vf sig-
operating from an ac power source, the source
nal.
voltage is applied direct to the input of power
transformer T1. Transformer T1 produces both
is received via the telephone channel, it couples
high and low ac voltage outputs from the input,
through the vf interface network as would any
The high voltage output of T1 is rectified and
receive vf signal. At the output of input trans.
filtered by high voltage rectifier CR5-CR4 and
former T2, however, it couples to ring detector
is applied to the +108-volt bus; the low voltage
A2CR1, the resultant dc level at the output of
output is applied to low voltage rectifier CR2-
A2CR1 couples through differential amplifier A2-
CR3, where the ac voltage is rectified and fil-
Q16-Q17 to enable freerun multivibrator A2Q-
tered. The filtered output is applied to the input
18-Q19. The audio rate output of A2Q18-Q19
of low voltage regulator Q4-CR6, which main-
couples through af amplifier A2Q20 to produce
tains the output voltage at +18 volts on the +18-
an audible tone at speaker LS1
volt bus. When operating from a 26-volt dc
k. Alarm Circuit (Vf). When a space hold
-
power source, input power is applied through a
signal is received via the telephone channel it separate connector to overvoltage protector Q1-
couples through a vf interface network into the
CR1, which prevents damage due to transients.
receive circuit as would any receive vf signal. If
Output of the overvoltage protector is applied to
the space hold persists for 2 seconds or more
inverter Q2-Q3. The inverter develop an ac
2-6
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