down to -10 dbm or lower to prevent
amount of signal voltage that can be ap-
plied to the line and, in effect, place a
overloading the receiving transceiver.
If more than one circuit is brought into
heavy load across the line when the input
the same terminal controlling the trans-
signal goes above 0 dbm.
mission of facsimile signals, cross talk
(a) If more than one commercial line is
tests should be made before operation of
being supplied by Facsimile Trans-
the facsimile circuit is begun with +26
ceiver TT-1(*)TXC-1, the output
dbm going into the line.
level at the LINE terminals will drop
(2) Transmission over open-wire telegraph
down below 0 dbm.
lines is sometimes practical, without
(b) If only two lines are being fed, the out-
danger of cross talk into dispatcher cir-
put at the LINE terminals can be
brought up to 0 dbm by shunting re-
cuits, with a +26 dbm signal going into
sistors R84 and R85 (fig. 169) (in
the line. As a further precaution against
series with the LINE terminals) with
cross talk in this type circuit, transposi-
tion of the wires at -, -, or 2/3-mile
300-ohm resistors.
intervals would be advantageous.
(c) If more than two lines are being fed,
(3) A line that has low-loss characteristics
the output at the LINE terminals may
may furnish a received signal which is
be brought up to 0 dbm by shunting
higher than the LINE binding post input
R84 and R85 with resistors that have
of the transceiver can accept for satis-
a lower value than 300 ohms. In
factory reproduction. This is indicated
either case, the final adjustment should
by the necessity of setting the GAIN
be reached with a DB METER reading
control near zero to obtain the standard
of +2 db.
+2 db indication required for proper
Note. If a DB METER is not available for
operation of the transceiver. When this
measuring the input at the coupling coils,
condition is encountered, for receiving
temporary connections can be made to the
DB METER in Facsimile Transceiver TT-
only, connect the line to a plug and insert
1(*)/TXC-1. The regular connections to the
the plug into the jack on the transceiver
meter must be removed after setting the
m a r k e d RADIO RCVR. To reduce
GAIN control for a normal output reading
hum, connect a wire from the GND ter-
of +2 db on the DB METER. Eight db
minal to the REC CT terminal. Leave
must he added to the meter reading when
measuring the signal voltage present across the
the plug out of the RADIO RCVR jack
coupling coil because the DB METER in
except when receiving copy. When trans-
Facsimile Transceivcr TT-1(*)/TXC-1 is cali-
mitting, the line must be reconnected to
brated at the old reference level of 6 mw into a
the LINE terminals.
600-ohm line. For 0 dbm at the coupling coil,
(4) For transmission over a single commercial
the meter therefore will read -8 db. If
measuring with a vacuum-tube voltmeter, the
wire line, the standard 0 dbm (.78 volt)
correct reading is .78 volt.
signal is available at the LINE terminals
on Facsimile Transceiver TT-1(*)/TXC-
ductive coupling to a telephone line is required,
1 when the GAIN control (figs. 13
use the UC coupling coil furnished with Facsimile
through 16) on the front, panel is adjusted
for a reading of +2 db on the DB
Set AN/TXC-1(*) except with Facsimile Set
AN/TXC-1D. This coupling (feed) coil is de-
METER.
signed to fit over the receiver of the handset used
Caution: When operating into AT&T
on Telephone EE-8 and other telephone handsets
(American Telephone and Telegraph
Co.) lines (fig. 11), do not apply more
than 0 dbm to the line since these circuits
plug at the end of the cord from the UC coupling
are adjusted for proper operation for
coil is connected to the polarized LINE JACK
(J5) on the transceiver terminal strip. Approxi-
inputs at this level. To limit the signal
applied to their lines, AT&T requires
mately 30 db is lost when using this method of
connection. When using a line of Wire W-110-B
that a 104A or 105A coupling coil be
connected between the facsimile equip-
or WD-1/TT between transceivers, if there signal
ment and the line. These coils limit the
voltage is required than that obtained by using
