TM 5-6675-319-14
The CRT is the light source, and the lens controls the focus of the light beam.
During the scanning of a line, the light beam from the CRT passes through a small
area of the negative, exposing the positive printing material.
Light passing
through the printing material is focused by the front mirrored surface into the PMT,
where it is converted into an amplitude-modulated dc signal.
I f t h e l i g h t s p o t
received is low, the PMT output is low.
When the PMT output is low, the light
spot slows down.
If the negative being scanned has low density, the PMT s e n s e s m o r e
light, the PMT output is higher and the light spot speeds up. Thus, the light spot
velocity is instantaneously modulated in accordance with the density of the nega-
t i v e .
(2) The current signal from the PMT is applied to the input of the variable
dodging circuits on card VDSF2.
Amplifiers A4B and A5 change the linear signal to a
l o g a r i t h m i c s i g n a l.
This signal is applied to amplifier A6, where it is amplified
by a factor of 15.
The signal is passed through the % DODGING control potentiometer
R129 and compared with EXP LEVEL control potentiometer R128 at amplifier A7A. The
signal continues through A8A and is sent to A4A for current amplification .
The
signal is then applied to the chopper inverter on control card 3C2. The output from
c o n t r o l c a r d 3 C 2 i s a p p l i e d t o t h e f a s t a x i s i n t e g r a t o r c i r c u i t s , c o n s i s t i n g of
operational amplifier Al and current amplifiers Q3, Q4, Q5, and Q6 on fast axis
deflection card 3D2.
The fast axis integrator integrates the signal, making the
current through the yoke change at a rate proportional to the amplitude of the
chopper inverter output.
The rate of change of yoke current and the speed of the
CRT spot are directly proportional to the amplitude of the inverter output. The
amplitude of the inverter output changes the speed of the CRT spot. Since the
current flow through sampling resistors R12 and R13 is the same as the current flow
through the yoke, the voltage across these resistors is proportional to the yoke
c u r r e n t .
The voltage across R12 and R13 is applied to the sweep failure detector on
the variable dodging card and to the fast axis raster circuits on raster edge card
3CC1.
Fast axis raster edge card 3CC1 consists of two independent voltage compara-
tors which monitor the voltage from R12 and R13. When the voltage exceeds the
levels set by right and left RASTER edge control potentiometers R135 and R136, a
signal is developed and applied to the fast axis toggle circuit on the fast axis
d e f l e c t i o n c a r d .
(3) The fast axis toggle, consisting of Q7 thru Q10, is a modified Schmitt
trigger with an output of positive and negative pulses. This output is fed back to
the chopper inverter on card 3C2 and to the input of the slow axis control circuits.
During the sweep to the right, the feedback is positive and the PMT output is
i n v e r t e d .
When the sweep is to the left, the feedback negative, and the PMT output
i s n o t i n v e r t e d .
When the spot of light reaches the left or right limits set by
RASTER control potentiometers, the polarity is reversed and the inverter causes the
spot of light to sweep in the opposite direction.
The fast axis toggle output is
also felt across keep-alive resistor R11 on card 3C2. Resistor R11 acts as a
protective circuit for the CRT.
If the PMT output decreases to zero, the voltage
dropped across resistor R11 provides a small amount of current that enables the CRT
to slowly continue its sweep.
This prevents damage to the CRT phosphor.
9-9
