TM 5-6675-324-14
the blank exposure, SIEXP = O; thus, K201 is de-energized. To provide high inten-
sity light for scanning exposure, the high LINT input is the output of Q209-11 via
pins 4 and 7 of K201. When SIEXP = 1, K201 is energized. Sample V1 is applied to
the input of UNITY gain voltage follower Q209/1. The input to voltage follower
Q209/2 is raised by applying Sample V2 to one end of a voltage divider, R210 and
R209, with Sample V1 at the other end, using the potential at Q209/2 as an input.
The two signals have the same relationship at origin as they do at Q209, but are
shifted in level. The outputs of Q209/1 and 2 provide reference voltages for expo-
sure settings 1 and 9 of the exposure control. The reference voltages are applied
to the two ends of the resistive track of VR601 (exposure control) which acts as a
voltage divider that is tapped by a slider. The output of the slider goes through
Q209/3. So output Q209/3-11 = LINTT. The resistivity (sensitivity) of the photo-
sensitive cadmium sulfide (CdS) layer of the drum changes when it is exposed to
light after a long idle period and reverts to its original dark resistivity if left
idle. This change is not instantaneous but takes place over a period of time in
each direction. The change in resistivity affects the toner image, so compensation
must be provided. This is the function of the Sensitivity Compensation circuit
(Lamp Control Timer circuit) for it controls light output and the effect varies with
time. It reduces light intensity during the first 30 sec of a continuous copy. It
reduces the level of LINT to the greatest extent at the start of the copying cycle
and reduces its effect at the end of 30 see, so LINT rises to its normal level. If
the copier is not used for a while, the effect of the Sensitivity Compensation
circuit is slowly reintroduced, reaching a maximum after 5 min. Prior to copying,
RMH1 = O, LINTT = O, and C204 and C205 voltage is approximately 24 V. When copying
is initiated, RMH1 = 1, Q210/1 and Q210/2 conducts, thus driving the base of Q210/3
and generating current flow, LINTT, from the emitter of Q210/3. The current source
for Q210/2 and Q210/3 is from C204 and C205. The output LINTT is applied to one end
of VR206 and a fixed 10 V dc to the other. When LINTT is high (initially as much as
24 V), the base current of Q209/4 is increased, lowering the output of Q209/3-11
(output of LINT). As LINTT decreases, its effect on Q209/4 is decreased. When
LINTT falls to O, Q209/4 is controlled by VR206. Q209-4 and Q209-5 pass portions of
the current. VR205 controls the amount of current flowing as LINTT. This varies
the voltage appearing at the LINTT input end of VR206. VR206 causes an overall
shift in the light output. At the end of copying, RMH1 = O, Q210/1 is cut off, and
C204 and C205 charge slowly to 24 V. This period lasts 5 min. If copying is
resumed during this recharging period, a lower level LINTT output will be supplied
until the capacitors again can no longer provide current. The LINTT output is
designed to correspond to the sensitivity of the drum at any moment so that the
light output during scanning will be correct for producing an image of proper
density, whether or not the copier is in constant or intermittent service.
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