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H-IC
Figure 51
CHECKING H-IC
You might suspect H-IC, Fig. 48, if
nothing works electronically — the LEDs
w o n 't turn on and the shutter w o n 't
release. Also suspect H-IC if you're get
ting battery drain w itho u t depressing the
release button part way. The customer
then complains that the batteries do n 't
last.
H-IC supplies the power to the other
tw o IC's. W hen you push the release bu t
ton far enough to close the metering
switch S5, transistor T1 turns on, Fig. 49.
The + 3 V signal then appears at pin 6 of
H-IC, the collector of T1.
So, w ith the release button partially
depressed, you should measure close to
3V at pin 6 of H-IC (actually, the power-
supply voltage minus the drop across T1).
You should also measure 3V at pin 1 of
IC1, Fig. 50.
If you d o n 't get the 3V signal at pin 6 of
H-IC, check the voltage at pin 5. Notice in
Fig. 49 that pin 5 connects directly to the
positive side of the supply. Consequently,
you should measure the full power-supply
voltage at pin 5 — even w itho u t depress
ing the release button. If not, check the
battery connections and the pink wire
that runs from the lower circuit board to
pin 5 of H-IC, Fig. 48.
Notice in Fig. 49 that the full power-
supply voltage always appears at the two
release capacitors — C5 for the diaphragm-
control system and C6 for the mirror-release
system. You should then measure 3V
across each capacitor regardless of the
release-button position. The power-supply
voltage also connects directly to the emitter
of T12, the transistor in series with the shut
ter's electromagnet.
However, at the collector of T12 — pin
10 of H-IC — you should measure 0V.
The voltage at pin 10 should switch to
nearly 3V w ith the shutter open. Tran
sistor T12 then turns on, conducting cur
rent through the coil of M3.
Fig. 51 shows the remaining pin
voltages of H-IC. The voltage at pin 7 pro
vides the cathode bias for the SPC. This
voltage is the sum of the voltage drops
across tw o transistors connected as
diodes — T2 and T7, Fig. 49. A forward-
biased diode provides an effective voltage
regulator; its voltage drop remains con
stant at around 0.6V, despite changes in
current. So, w ith tw o diodes in series, the
voltage at pin 7 should remain constant at
approximately 1.2V. The filter capacitor
C1 helps maintain the constant reference
voltage.
The 1.2V reference connects to the
cathode of the SPC, Fig. 49 and Fig. 50.
Here, the voltage should remain constant
— despite changes in the light level. But
the voltage at the anode of the SPC
should go more positive as the light level
increases. The anode voltage provides
the gate bias for the FET, Fig. 49.
In effect, the FET takes the linear cur
rent changes through the SPC and con
verts them to linear voltage changes
which drive the amplifier. A silicon
photodiode does put out its own voltage.
But the voltage changes across a silicon
diode aren't linear w ith respect to light-
level changes; rather, the voltage change
is the log of the light-level change. The
FET then performs logarithmic expan
sion. Voltage changes at the output of
the FET match the light-level changes in
linear fashion.
The op amp inside IC1 now takes the in
put from the FET and provides an
amplified output at pin 19, Fig. 49. Notice
that the memory capacitor C2 connects to
pin 19 through the closed memory switch
S6. W ith the memory switch closed, the
memory capacitor then charges to the pin
19 voltage. The voltage at pin 19 goes
more positive as the light level increases.
You should now be able to visualize the
symptoms w hich could result from a
defect in H-IC. Suppose, for example,
that the LEDs don 't turn on when you
depress the release button part way. You
then check the voltage at pin 6 of H-IC. If
you measure 0V at pin 6 — but you get 3V
at pin 5 — suspect the metering switch
S5 and transistor T1, Fig. 49.
Earlier, we described one way to check
the metering switch at the top of the
camera. You can also check the metering
switch at H-IC, Fig. 49. Shorting pin 3 to
ground should turn on the LEDs. If so,
you know that H-IC must be o.k.; the
problem must be w ith the metering
switch or the wiring. But, if the LEDs still
w o n 't turn on, you'll probably have to
replace H-IC.
W e also mentioned another possibility
— the camera constantly draws current,
causing excessive battery drain. Try
measuring the voltage at pin 6 w itho u t
depressing the release button. If you
measure close to 3V, transistor T1 may
have an internal short. Or the power-
holding circuit — T11 and D2, Fig. 49 —
may be keeping T1 turned on. Either way,
replace H-IC.
