ACTUAL CIRCUIT FOR THE
COMBINATION MAGNETS
Fig. 18 shows the actual circuit for the
combination magnets M1 and M2. The
tw o IC's which drive the switching tran
sistors mount to the flex circuit inside the
camera. Notice in Fig. 18 that a com
parator output of IC1 controls the poten
tial on the emitter of T13; switch S7 con
nects to the T13 base (in the partial
schematics used earlier for explanation,
we showed the comparator controlling
the base of T13).
W ith S7 closed, the base of T13 con
nects to ground. Opening S7 then con
nects the base of T13 to a positive voltage
— around 1.3V, measured at the w hite
wire, Fig. 18. T13 now has the voltage it
needs at the base. However, the tran
sistor can't turn on until the emitter
voltage goes less positive than the base
voltage.
The comparator inside IC1, Fig. 18,
decides when it's time to turn on T13. IC1
actually makes its exposure calculation as
the diaphragm stops down. A t the " S "
mode, IC1 compares the selected shutter-
speed setting at one input w ith the light
level and the film-speed setting at the
other input. To stop the diaphragm at the
proper f/s to p , the comparator inside IC1
switches low. The emitter of T13 now
goes less positive than the base. As a
result, T13 turns on and allows C5 to
discharge through the coil of M2.
You can see how the comparator
makes its f/s to p calculation in Fig. 19.
The shutter-speed setting provides the in
put voltage to one input of the com
parator; the combination of the film-
speed setting and the photocell output
provides the voltage at the other input. If
input #1 in Fig. 19 is less positive than in
put #2, the comparator output remains
high to keep T13 turned off.
However, as the diaphragm stops
down, the output of the photocell
decreases. The voltage at input #2 then
goes less positive. W hen the tw o inputs
see the same voltage, the comparator
knows that the diaphragm has reached
the proper f/s to p . Now the comparator
switches states — its output switches low
and turns on T13.
Earlier, we mentioned that S7 must
open before T13 can conduct. If S7 re
mains closed, the base of T13 stays at
ground potential. Remember, S7 opens
when the mirror-release combination
magnet M1 repels its armature. Opening
S7 both applies the base bias to T13 and
disconnects IC2 from ground.
IC2 serves as the driver for the LEDs.
So, when S7 breaks the IC2 ground con
nection, the viewfinder LEDs turn off.
Also, IC2 provides the shutter-release
lock for the mirror-release combination
magnet M1, Fig. 18. If the battery voltage
drops too low, the comparator inside IC2
w o n 't sw itch on transistor T6. As a result,
the mirror-release combination magnet
can't release the mirror.
VARIATIONS IN THE LOWER
CIRCUIT BOARD
As mentioned previously, the lower cir
cuit board. Fig. 20, comes as a complete
unit. A lthough we described techniques
for checking the individual components,
you can test the complete circuits w ith
one shot — just short the bases of the
control transistors to ground.
