Toshiba e-STUDIO 550, Service Manual

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e-STUDIO550/650/810 RADF 16 - 62 JUNE 2002 © TOSHIBA TEC
16. 11. Circuits
(1) Input circuits for read sensor, large original exit sensor, small original reverse sensor and original
length sensor
Since the read sensor, large original exit sensor, small original reverse sensor and original length sensor
have almost the same circuitries, only the input circuit for the read sensor is described as followed.
This circuit consists of the voltage-current conversion circuit, which adjusts the amount of light of the
infrared LED, and the voltage comparison circuit, which compares the voltage output from a
phototransistor (PTr) with a reference voltage and converts the output voltage into digital signals.
The read sensor is a mirror reflection sensor, and consists of a pair of emission elements (infrared
LEDs) and a light receiving element (PTr). Both elements have a light axis running in the same direc-
tion. There is a mirror on the extended light axis of these elements. When there is no original between
the elements and the mirror, infrared light emitted from LED is reflected by PTr in high reflectance. If
there is an original, light emission to the mirror and reflected light from the mirror are blocked by the
original, and the reflected light entering PTr becomes extremely small.
When a large amount of light is entering PTr (this means there is no original) the optical current flowing
in the PTr is increased, and the voltage of IC6-8pin becomes higher due to a drop in voltage caused by
R109. IC6 is a comparator which compares the reference voltage input to 9pin (non-reverse input
terminal) with the signal voltage of 8pin (reverse input terminal). If the voltage of 8pin is higher, the
output level (IC6-14pin) is "L". On the contrary, when the amount of the incoming light is small (this
means there is an original), the voltage of 14pin is low and the voltage of IC6-14pin becomes "H". R56,
C56 and C57 are noise filters. This circuit has an automatic sensitivity adjustment function to control the
unevenness of the sensor sensitivity. Uneven sensor sensitivity is corrected by controlling the machine
to have a constant PTr voltage when there is no original. The intensity of the infrared light emitted from
the LEDs is changed to adjust the PTr voltage.
When the sensor sensitivity adjustment mode is selected, the PTr output voltage (analog value) is
measured by the A-D input terminals of CPU. The output voltage of the D-A converter is changed to
keep the PTr output voltage to a certain level, and the LED current is adjusted by a voltage-current
conversion circuit consisting of IC11, R65, Q6, etc. Each machine; has a different D-A output voltage
value, which is stored in the volatile memory.