Minolta XG7 - Principle of Metering and Circuit Description

7.
Principle
of
Metering
and
Description
of
Circuit
A. Outline
®@ Function
The electric circuit of XG has various functions as follows:
1)
Automatic
exposure
control
and
shutter
speed
indication
by
LED.
2) Manual exposure control.
3)
Shutter
sec,
time
auto
changeover
and
blinking
indication
by
exclusive
strobe.
4) Bulb exposure control.
5) Touch switch.
6) Magnetic release.
7)
Electronic
self-timer
and
blinking
interval
variable
indicator
circuit.
8) Voltage detection (inel. B.C)
9) Winder control.
®@ Composition
The circuit of XG is complicated having a lot of functions, and therefore solid-state con-
struction is employed as much as possible for the circuit. It basically consists of two
monosyllic ICs with 20 pins each for metering, operation, control (IC 1), and for indication
(IC 2), one monosyllic IC with 10 pins mainly for self-timer (IC 3), and hybrid IC (HIC)
for the purpose of connection between magnetic release and each circuit. To cope with
such complicated wiring, a flexible circuit board is employed.
The switches are also transistorized as much as possible. Trigger switch, the representa-
tive one of mechanical switches, is of OFF type to minimize the influence of chattering.
For the brushes of sliding resistors such as ASA resistor and diaphragm resistor, the
number of brushes is increased in order to reduce noise. Also, 12 LED's are built into
the indication board as indication elements.
B. General description
®@ Metering system
XG
is
a
single-lens
reflex
camera
using
TTL
metering
system.
And
the
amount
of
light
to
CdS
is
changed
and
shut
off
due
to
camera
operations
(such
as
auto
exposure
control
and
mirror
operation).
It
is
therefore
necessary
for
the
camera
to
memorize
the
result
of
TTL
metering
(expo-
sure time in XG) just before release.
® Memory system
XG
employs
such
a
system
that
both
in
auto
and
manual
the
exposure
time
is
converted
into
an
electrical
amount
(voltage)
and
it
is
charged
into
a
condenser
and
then
the
charge
voltage
is
memorized
by
the
camera.
Also,
the
system
is
to
memorize
logarithmic
values
because
the
metering
range
and
film
sensitivity
interlocking
range
are
wide;
interchange-
ability
is
required
for
lenses
and
parts,
making
the
circuits
complicated;
and
the
range
of
memorized
values
(exposure
time)
is
also
very wide.
® Metering switch
In
order
to
prevent
wasting
of
the
battery
due
to
forgetting
to
turn
off
the
metering
switch,
the
metering
switch
is
interlocked
with
the
release
button.
A
touch
switch
is
installed
to
improve
the
maneuverability
during
metering.
(A
touch
switch
uses
a
part
(finger)
of
the
user's
body
as
a
switch
contact.
So,
when
the
user
wears
gloves,
the
finger
will
not
serve
as
a
contact.
In
that
case,
pressing
the
shutter
button
by
one
step
more
will
turn
on
the
metering
switch.