Olympus OM-4 Handbuch zur Fehlersuche

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C & C ASSOCIATES

ELECTRONIC TROUBLESHOOTING THE OLYMPUS OM-4

1. CIRCUIT DESCRIPTION

Power Circuit

As with other OM-series cameras, battery positive

connects to the chassis and to the flex circuit through

screws holding the circuit in place. In this guide, battery

positive is designated as Vcc and connections to the

chassis are shown as the "earth" symbol,

A black wire connects battery negative to the Shutter

Terminal Board and to the main flex near the rows of

variable resistors, From there, conductors fan out to pins

22, 51, IC-101; pins 6, 26, 62, IC-102; and pin 13, IC-103.

The Display Flex and Reset Board receive battery

negative through a black wire soldered to the Main Flex

near IC-101,

When batteries are installed, a quartz oscillator

involving two capacitors and IC-101 is activated.

Pressing the release button one-half stroke closes switch

SW-I11 to connect pins 15 and 18, IC-101. IC-101 then

signals IC-102 to power up the metering circuit and the

LCD for about 2 minutes. Power signals are from pins 53,

54, IC-101 to pins 24, 23, IC-102. Pressing the release

button renews the 2-minute power latch, A 1.8v constant

voltage, Vref

is generated at pin 75, IC-102.

A diode is connected to the battery terminals in reverse

direction to protect the circuit if batteries are installed

backward.

Battery requirement: two S-76 silver oxide or two

LR44 alkaline-manganese.

Metering Circuit

There are four ICs involved in metering, exposure

control, and display. IC-101 contains a microprocessor

(CPU) and understands only binary signals as inputs and

outputs. It keeps track of mode, calculates exposures,

and owns the clock. IC-102 is a bipolar IC which contains

comparators for A / D conversion, watches over variable

resistors,

controls

communicates with IC-101,

IC-103, under the floor panel of the mirror box,

supports "average" and

amplifiers and temperature compensation circuits. It also

takes care of integration for automatic exposure in

ambient and flash modes. IC-104 is a decoder/driver for

the comprehensive LCD.

Prior to exposure a sub-mirror behind the reflex

mirror reflects a portion of light going to the viewfinder

to the photodiodes on IC-103, A BV voltage, changing

18mv/Ev with light, is output from pin 16, IC-103 to pin

troubleshooting

m a g n e t s

a n d

LEDs,

"spot"

photodiodes with

7, IC-102, At the same time, IC-102 watches aperture

setting (AV-VR at pin 74), ASA (SV-VR at pin 67), and

shutter speed setting (TV-VR at pin 73). Although

variable resistors are used

digital conversion process which changes all exposure

information to binary codes for the CPU.

The code is sent back as series data from pin 38, IC-102

to pin 29, IC-101 so the CPU knows ASA, aperture

setting, and shutter speed setting. How the CPU

interprets the data depends upon operating mode

selected, IC-101 will calculate an exposure, then send a

coded message to IC-104 which drives an LCD display to

indicate exposure and other information, A quartz clock

operating at 32.8 KHz keeps the operation orderly and

the whole process is repeated every 100ms to keep data up

to date,

IC-101 sends digital-to-analog data along eight lines

(DAC-0 to DAC-7). During exposure in Manual and

Spot modes, IC-103 will need analog voltage to set

exposure times.

Details such as different voltage levels between average

and spot SPDs are taken care of by voltage boosting

circuits. Because of a wide exposure range, integration

capacitors are switched in and out of circuit according to

ASA setting,

Release Circuit

Pressing the release button full stroke mechanically

unlatches the mirror as with other OM-series cameras, As

the mirror rises, switch SW-101 closes to connect Vcc

(B+) through two diodes to bases of transistors Q-103 and

Q-904. As the transistors turn on, pin 49, IC-101 is set low

and pin 58, IC-102 is set high. In this manner the ICs are

told the mirror is on its way up and they had better gel

ready for an exposure. IC-101 stores exposure data and

and

LEDs are temporarily disabled to conserve power.

IC-101 also sets an exposure program according to mode.

IC-102 tests battery voltage to be sure it is above 2.65v,

then pin 61 goes low. Pin 61 is connected to the base of

transistor Q-201 (PNP) on the shutter board, Q-201

switches on causing capacitor C-201 to discharge through

the coil of combination magnet M-l. M-l repels its

keeper which unlatches the opening curtain, IC-102 then

waits for switch SW-103, the Trigger Switch (Timing

Switch), to signal the first curtain is moving before

initiating exposure control.

they are part of an analog-to-