modulator circuit is shown in figure 47. In the
74. Photocell Circuit
simplified circuit, the 1,800-cycle carrier signal
from the fork oscillator unit is fed to points A and
Photocell tube V24 is the heart of the photocell
C of the resistance-capacitance bridge. Output
bridge modulator circuit, which is used only on
from the bridge is the voltage developed across
transmitting. Tube 1645 is used as the photocell
R50, between points D and B. This voltage is
in Facsimile Transceivers TT-1/TXC-1 through
the grid to cathode or input voltage of V10. A
TT-1C/TXC-1. Tube 5652 is used as the photo-
cell in Facsimile Transceivers TT-1D/TXC-1,
and active anode of V24, and this is effectively
TT-1E/TXC-1, and TT-1F/TXC-1. The photo-
balanced out by the neutralizing, capacitance
cell and its associated circuit (fig. 47,48, and 49)
cause amplitude modulation of the 1,800-cycle signal
the cathode and the second or neutralizing anode
supplied from the fork oscillator unit. Amplitude
within the photocell. When the bridge is per-
modulation follows the lights and shadows on
fectly balanced, there is no output of 1,800-cycle
that portion of the transmitted copy being
signal to be applied to V10; but when increased
scanned. After modulation of the 1,800-cycle
light shines on the photocell, it causes increased
signal is accomplished in the photocell bridge
electron emission from the cathode, producing a
modulator circuit, the modulated signal is fed to
resistive unbalance of the bridge and a greater
the grid (pin 6) of V10, the first signal amplifier.
1,800-cycle signal.
Tube V10 and the following signal amplifiers not
b. Optical System. The light that reaches the
only amplify the modulated signal, but use a
photocell can be determined at any instant by the
series of differentiating circuits (small coupling
shade (from white to black) of the small area of
capacitors and large grid resistors) to remove
transmitting copy being scanned. Light from the
undesirable low-frequency signal components
constant source of illumination (the exciter lamp
generated in the modulator circuit. Photocell
shown in the transmitting block diagram (fig. 31))
tube 5652 in Facsimile Transceivers TT-1D/
is concentrated onto a small portion of the copy
TXC-1, TT-1E/TXC-1, and TT-1F/TXC-1 per-
by the condenser lens system. The illuminated
mits the use of larger values of coupling capacitors
area is approximately the size of the filament in
than photocell tube 1645, which was used in
the exciter lamp. The objective lens focuses an
previous models. The circuit that uses tube 5652
image of the illuminated area of copy onto a
is inherently balanced for the modulating fre-
screen having a small hole or aperture that permits
quency, and therefore requires no discrimination
only the light from an area approximately one
against low frequencies in its amplifier strip as
ninety-sixth of an inch square to pass onto the
required in previous models.
photocell in back of the aperture. As the drum
a. Simplified Bridge Modulator Circuit. A
and copy move during transmission of a picture,
simplified equivalent of the photocell bridge
each scanned element of the picture reflects a
different amount of light back to the photocell.
The photocell and bridge modulator circuit change
each level of light into its equivalent electrical
signal.
c. Schematic Diagram:
(1) Facsimile
Transceivers TT-1/TXC-1
output of the bridge (the voltage across,
R60) is a varying dc signal that consists
of the keying or modulating frequency
which is superimposed on the 1,800-cycle
carrier frequency. This varying dc sig-
nal is always of such polarity that the
grid end of R50 is positive with respect to
the other end. This is caused by the in-
creased emission from the photocell when
increased light strikes it. The conven-
85
