Sharp FO-1660, Service Manual

Page: 64 / 116

[4] Circuit description of power supply PWB
1. Block diagram
Fig. 8
The power unit intakes input of AC 120V, 50/60 Hz and supplies
output of +24V, +7.4V and +5V as shown in the block diagram. (See
Fig. 8.)
2-1. Filter, rectifying and smoothening circuit
In the filter section, the noise generated from the power unit is elimi-
nated from being discharged to the external, and external noise is
prevented from entering. Thunder or other excessive surge is pre-
vented by the varistor Z1.
In the rectifying and smoothening section, AC input is rectified by the
diodes D10, 11, 12 and 13, and is smoothened by the capacitor C5.
Thus, DC voltage is supplied to the switching section. Moreover, the
thermistor NTC1 prevents surge current when the power is turned on.
2-2. Switching section
The circuit uses the ringing choke converter of a self-excited type.
Since MOS.
FETQ1 is repeatedly turned on and off in this system, the DC voltage
supplied from the rectifying and smoothening section is converted
into the high frequency pulses. While Q1 is on, energy is accumu-
lated in the primary winding of the transformer T1, and while Q1 is off,
energy is discharged to the secondary side. Thus, the power is sup-
plied.
Moreover, the frequency is varied depending on the load of the out-
put. As the load becomes the heavier, the frequency becomes the
smaller to extend the ON period.
The constant voltage is controlled by applying the feedback to the
control circuit through the photo coupler PC1 from +24V output. The
overcurrent-protective circuit detects that the ON period becomes the
wider as the output load becomes the heavier. For the control, the
OFF period is extended by the control circuit to squeeze the energy
which is accumulated in the primary winding of the transformer T1.
For protection against overvoltage, the rise of the output voltage of
+24V on the secondary side is brought into the overcurrent state
through the continuity of the power zener diode D104 b24V
output and GND. Thus, the overcurrent-protective circuit of the con-
trol circuit is activated for the protection.
2-3. +24V circuit
Output is supplied by rectifying and smoothening the output of the
transformer T1 with the diode D101 and capacitor C101.
2-4. +5V circuit
+5 V output is generated by rectifying and smoothening the output of
the transformer T1 with the diode D102 and capacitor C102 and
stabilizing it with the 3-terminal regulator IC103.
AC IN
120V
50/60Hz
F1
4.0A 125V
FILTER/
RECTITYING
SMOOTHING
CIRCUIT
SWITCHING
CIRCUIT
TRANSFORMER
+7.4V
CIRCUIT
+5V
CIRCUIT
+7.4V
+5V
GND
+24V
+24V
CIRCUIT
PHOTO
COUPLER
CONTROL
CIRCUIT
[5] Circuit description of CCD PWB
The CCD board picks up optical information from the document, con-
verts it into an electrical (analog) signal and transfers it to the control
board.
(1) Block diagram
Fig. 9
(2) Description of blocks
1. CCD
The TCD1208GL is a highly sensitive charged coupled image sensor
that consists of 2160 picture elements.
Receiving four drive signal (
φ
T,
φ
2,
φ
1,
φ
R) from the control board, the
tranferred photoelectric analog signal OS is impedance converted,
and the signal VO, is supplied to the control board.
2. Waveforms
1.
φ
1,
φ
2 (=
φ
1) signals within the control board.
Fig. 10
T
1
2
R
+5V
GND
VO
CCD
TCD1208AP
OUTPUT CIRCUIT
(emitter follower)
OS
Φ
Φ
Φ
Φ
Approx. 5V
0V
Approx. 5V
0V
Approx. 5V
0V
Approx. 100mV
OS
øR
ø1 (=ø2)
øT
1.74
µ
SEC
2. OS
φ
UX-600MU/MC
FO-1660U/C
5 – 12