Figure 48. Photocell bridge modulator circuit of Facsimile Transcievers TT-1/TXC-1 through TT-1C/TXC-1.
tional modulation envelope, consisting
pacitive reactance to ground for the
of the 1,800-cycle signal varied in ampli-
slide arm of R42, thereby permitting
tude both above and below the zero
close adjustment of the null point.
axis, first appears in a distorted form at
Variable resistor R45 (in the second
network across the input of the bridge)
the grid of stage V10.
(a) Resistor networks. Between points A
is the CONTRAST control, a front
panel operating control used in setting
and C, figure 48, where the unmodu-
the original contrast range at the
lated 1,800-cycle carrier signal is
applied, are two resistor networks; one
facsimile transmitter. This control
consists of R41, R42, and R43 in series
may be adjusted to balance the bridge
and the other consists of R44, R45,
on either maximum light (positive
and R46 in series. Resistors R47 and
transmission; minimum signal on
white, maximum on black) or on
R48 are in series across R45. The
first of these networks includes variable
minimum light (negative transmis-
resistor R42, which serves as a neutral-
sion; minimum signal on black, maxi-
izing control and is used in balancing
mum on white).
out the capacitive effect of the photo-
(b) Other components. Resistors R47 and
cell. Its effect on the circuit is the same
R48 are used in place of the proper tap
on CONTRAST control R45; they
R42 is an alignment control and is lo-
serve merely to keep point D electri-
cated beneath the chassis of Facsimile
cally at a predetermined relationship
Transceivers TT-1/TXC-1 through
with respect to R45. Resistor R49,
TT-1B/TXC-1 (figs. 128 and 129).
which may be shunted by R49A, R49B,
In Facsimile Transceiver TT-1C/
or R49C, or combinations thereof, is a
TXC-1, R42 is located on the floating
portion of a voltage divider between
panel behind the DB METER. This
the regulated B+ voltage and ground.
control is used to set the proper value
This divider keeps both anodes of the
to balance out the capacitive effect of
photocell at a slight positive potential
the photocell when the photocell is
(.3 to .6 volt) with respect to the photo-
changed; it is not used as an operating
cell cathode. This bias extends the
control. Capacitor C58 provides ca-
linear portion of the photocell curve
86
