Sharp FO-780A, Service Manual

Page: 55 / 102

FO-780A
5 – 9
2. Noise filter
The noise filter comprises the RF choke coil, L6, L7 and L8.
3. Dial pulse generation circuit
The pulse dial generation circuit comprises the photo-coupler PC2,
PC3, polarity guard REC1, and resistor R1.
The photo-coupler PC3 shunts the line current using the DP signal
before transmitting the dial signal, then turns off the CML relay.
After the pulse dial signal is transmitted by turning on/off the DP sig-
nal, the CML relay is turned on again.
4. CML relay
The CML relay switches over connection to the matching transformer
T1 while the FAX or built-in telephone is being used.
5. Matching transformer
The matching transformer performs electrical insulation from the tel-
ephone line and impedance matching for transmitting the TEL/FAX
signal.
6. Hybrid circuit
The hybrid circuit performs 2-wire-to-4-wire conversion using the IC102
of operational amplifier, transmits the voice transmission signal to the
line, and feeds back the voice signal to the voice reception circuit as
the side tone.
7. Signal selection
The following signals are used to control the transmission line of TEL/
LIU signal. For details, refer to the signal selector matrix table.
8. Sensor circuit
For the recording paper sensor (PE), when there is recording paper,
the photo transistor in the light receiving side is ON and the detection
level is LOW. When there is no recording paper, the photo transistor
in the light receiving side is OFF and the detection level is HIGH.
9. CI detection circuit
The CI detection circuit detects the CI signals. A CI signal, which is
provided to the photo-coupler PC6 through the C3 (0.56
µ
F), R3 (22
K), and ZD2 when the ring signal is inputted from the telephone line.
10. Power supply and bias circuits
The voltages of +5V and +24VA are supplied from the control PWB
unit.
Signal Name Description
CML Line connecting relay and DP generating relay
(The circuit is located
H: Line make
in the TEL/LIU PWB.)
L: Line break
Speaker tone mute control signal
SP MUTE H: Muting (Power down mode)
L: Muting cancel (Normal operation)
Handset reception mute control signal
TEL MUTE H: Muting
L: Muting cancel
Speaker volume control signal,
VOL A VOL A VOL B matrix
VOL B
(The circuit is located
in the control PWB.)
TXCONT
Handset transfer mute control signal
(The circuit is located
H: Signal sending, when transmitting
in the control PWB.)
L: During reception, transmission mute,
(during standby)
GAIN-C Reception gain switching signal
(The circuit is located
H: When connected to line, 1: 1 gain
in the control PWB.)
L: When not connected to line, HIGH gain
BZCONT Speaker output signal switching
(The circuit is located
H: Buzzer signal output (during stand by)
in the control PWB.)
L: When monitoring line signal
[Control signals from output port]
VOL A VOL B
RING.
Buzzer DTMF
Receiving
H H — — —
L H
Low
—
Low
H L Middle Fixed Middle
L L High
—
High