turn on electronic switch A3Q12. The switch
T2, and emitter follower A3Q1 to limiter ampli-
lights THRESHOLD lamp DS2 and couples a
fier A3Q2-Q3. The limited vf signal couples
ground through emitter follower A3Q9 to lock
through emitter follower A3Q4 and discrimina-
the receive loop in mark hold. When the vf level
tor driver A3Q5 to discriminator network Z2,
exceeds the predetermined value, the mark hold
CR7-CR10. The discriminator converts the vf
telegraph frequency-shift signal to a dc telegraph
is released. Either of two predetermined values
voltage signal. It converts a 1317.5-Hz tone to a
(-25 dbm, -45 dbm) are strap selected at limiter
mark and a 1232.5-Hz tone to a space, The mark-
amplifier A3Q2-Q3.
space voltage signal couples through emitter
e. Vf Interface Network. The network has
follower A3Q6-Q7 and differential amplifier
three major configurations, selected by 4W-2W-
A3Q8-Q10 to operate electronic switch A3Q11.
TEL switch S5.
(1) 4-wire telephone channel interface.
The electronic switch keys the receive loop. A
receiving device connected to REC jack J3 prints
With S5 set to 4W. input transformer T2 con-
out the received message. Where external battery
nects to binding posts E5-E6. The signal path is
is to be used the battery is connected to EXT
from E5 through S5C-1-2 to T2. The return is
BAT jack J4. Internal battery loop current may
from T2 through C13, S5B-2-1 to E6. Output
transformer T3 connects to E3-E4. The signal
be adjusted at R12.
d. Threshold Circuit. Signal detector A3CR6-
path is from T3 through C15 and S5C-5-6 to
E3. The return is from E4 through S5B-6-5 and
CR7 develops a dc level output in proportion to
input vf signal level. When the vf level drops
S4A-8-7 to T3.
below a predetermined value, the dc level couples
(2) 2-wire telephone channel interface.
through differential amplifier A3Q13-Q14 to
With S5 set to 2W both transformers connect in
2-4
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