DIAPHRAGM-CONTROL CIRCUIT
In the S mode, IC2 tells you w hat
diaphragm opening you should get. But
IC1 provides the actual calculation and
control. Fig. 63. The comparator inside
IC1 compares the shutter-speed informa
tion w ith the light-level and film-speed in
formation to make its decision — the
decision when to turn on T13, Fig. 63.
Remember that S7 remains closed until
the M1 armature moves away from the
core. W ith S7 closed, the base of T13
connects to ground through
R64.
However, when S7 opens, the base of
T13 sees a positive voltage. The voltage
applied to the base of T13 is the sum of
the voltage drops across each of the
diodes shown next to S7 in Fig. 63 —
around 1.2V. In the camera, you can see
the tw o diodes next to H-IC, Fig. 48.
T13 now has the positive voltage it
needs at its base. However, the transistor
still can't conduct. That's because the
voltage at pin 9 of IC1 — the output of the
comparator — remains high. In effect,
opening S7 just prepares T13 to turn on.
T13 then remains at the ready, waiting
only for the voltage at its emitter to
switch low.
The comparator inside IC1, Fig. 63,
controls the emitter voltage of T13. A t pin
11, the comparator sees the shutter-
speed information from RTV. The com
parator sees the film-speed and bright
ness information at pin 8. As long as the
voltage at pin 11 remains less positive
than the voltage at pin 8, the output of
the comparator stays high.
However, as the diaphragm stops down,
the voltage at pin 8 decreases. Why?
Because the SPC sees less light. When
the voltage at pin 8 reaches the voltage at
pin 11, the comparator switches states —
its output goes low. The voltage at pin 9
of IC 1, Fig. 63, then switches to OV.
S witching the emitter of T13 to OV pro
vides the transistor w ith a forward bias of
1.2V; remember, T13 already has the 1.2V
signal at its base. T13 now turns on,
sending base current through T5. When
T5 turns on, C5 discharges through the
coil of M2 to arrest the diaphragm
closure.
The shutter-speed setting, as you've
seen, just changes the voltage input at
one lead of the comparator. So, to check
the shutter-speed resistor RTV, you can
measure the pin 11 voltage in the S mode.
You should see the voltage go more
positive as you set faster shutter speeds.
Since the voltage goes more positive, the
diaphragm doesn't have to stop down as
far in bringing the pin 8 voltage to the pin
11 voltage.
A t the X, B, and 0 settings, the shutter-
speed brush moves to the ground land
shown just below the RTV resistor in Fig.
63. Pin 11 of IC1 now connects to
ground. As a result, the voltage at pin 8
never reaches the voltage at pin 11. And
the diaphragm-closing lever always has
its full movement.
Y ou'll notice that the diaphragm-
closing system works independently of
the readout. In the first part of this article,
we mentioned that the MC lens w o n 't
give you the proper indication in the LED
display; it lacks the tab which couples to
the minimum-aperture resistor. However,
you can still use the MC lenses w ith the
camera. Although you w o n 't get the
diaphragm readout, the lens nonetheless
stops down to the proper aperture in the
S mode. The diaphragm-control magnet
still arrests the diaphragm closure.
W ith th e fro n t-p la te /m irro r-c a g e
assembly removed, you can see the
diaphragm -control
magnet,
Fig.
62.
Locate the ratchet gear in Fig. 62. W hen
C5 discharges through the coil of M2, the
armature moves into engagement w ith
the ratchet gear. Stopping the rotation of
the ratchet gear also stops the movement
of the diaphragm-closing lever.
In the A and M modes, the diaphragm-
closing lever must again have its full
movement. The diaphragm then closes to
whatever f/s to p you've selected. W hen
you select either A or M, switch S11 (part
of the mode switch) connects pin 11 of
IC1 to ground through the trigger switch
S4. Consequently, the pin 8 voltage never
reaches the pin 11 voltage.
OPERATION IN THE A MODE
In the A mode, the shutter-speed
resistor RTV has nothing to say about the
operation. Switch S9, Fig. 63 (another
contact on the mode switch) disconnects
the shutter-speed resistor from the cir
cuit. Then, regardless of your shutter-
speed setting — 1 second through 1/1000
second — you get an automatically con
trolled shutter speed.
The charge across the memory ca
pacitor C2 now controls the exposure
time. Fig. 64 shows the circuit inside IC1
that charges the memory capacitor and
controls the current to the timing
capacitor C3. As long as the memory
switch S6 remains closed, the charge
across the memory capacitor can con
tinue changing.
W hen the diaphragm reaches the
proper aperture size, the memory switch
opens. Now the mirror starts to rise. Even
though the SPC can no longer see the
light coming through the lens, the circuit
has a memorized reference voltage — the
voltage stored in the memory capacitor.
