Service manual

FO-4700U
FO-47UC
5 – 20
[5] Circuit description of power supply PWB
In the AC power supply unit, in order to gain DC voltage(+24V/+5V), AC
input is processed directly by rectification smoothing; the voltage is trans-
formed through a transformer by a switching method and, after that, the
AC input is processed again by rectification smoothing.
The block diagrams of those operations are shown and the details of
those circuits are described below:
1. Noise filter circuit
This circuit reduces the normal mode noise and common mode noise
that come from an AC line. The normal mode noise is the noise weighted
to an output line and is attenuated by L1, C1 and C2. The common
noise is the noise generated between the AC line and ground (GND)
and flows into the ground through C3 and C4.
Fig. 8
C1 C2
C3
C4
L1
2. Rush current prevention circuit
When the power turns on, if this circuit cannot be utilized, considerably
excessive rush current flows into a condenser because the AC input is
processed directly by rectification smoothing. This may damage a switch
contact point. The powerthermistor TH1 between the rectifier DB2 and
smoothing capacitor C6 limits the amount of the inrush current.
The resistance of TH1 decreases as its temperature rises and nearly 0-
ohm at high temperature.
3. Primary rectification smoothing circuit
It is full wave rectification circuit, which converts the AC voltage of 50/
60Hz into DC voltage. The solid line and dotted line shown in the under-
mentioned figure indicate a charging route of C6.
Fig. 9
DB2
C6
[4] Circuit description of operation PWB
1. Block diagram
Fig. 7
2. Operational description
1) Panel PWB
The panel PWB includes the ten key scan circuit and the LED lighting
circuit.The LS145 is controlled through LD0~LD3 (4 bits) and ten key
detection is performed. The HC174 is controlled through LD0~LD5 (6
bits) to provide LED lighting information.
2) COG (LMG2022-TPR)
The COG uses the one-chip LCD driver IC to display 20 digits x 2 lines.
The LCD display density is controlled with an extemal resistor.
ULN2003
SEN1
SEN2
SEN3
SEN4
SEN5
SEN6
LS145
HC174
+5V
LD0 ~ LD7
LCD DRIVER
LCD
COG (LMG2022-TPR)
PANEL PWB
ULN2003