Sharp FO-DC635U, Service Manual

Page: 82 / 166

FO-DC635U
5 – 16
[4] Circuit description of LIU PWB
1. LIU block operation description
1.1. Block diagram
1.2. Circuit description
The LIU PWB is composed of the following 9 blocks.
1. Surge protection circuit
2. On-hook status detection circuit
3. Dial pulse generation circuit
4. CML relay
5. Matching transformer
6. Hybrid circuit
7. Signal selection
8. CI detection circuit
9. Power supply and bias circuits
1.3. Block description
1.3.1 Surge Protection circuit
This circuit protects the circuit from the surge voltage occurring on the
telephone line.
• The AR1 protect the circuit from the 390V or higher line surge volt-
ages.
• The AR2 and AR3 protect the circuit from the 500V or higher verti-
cal surge voltages.
• The ZD6 and ZD7 control the voltage generated on the secondary
side of matching transformer to 2V.
1.3.2 On-hook status detection circuit
The on-hook status detection circuit detects the Status of the push
speaker key, and the status of the hook of a telephone externally con-
nected.
• External telephone hook status detection circuit (XEXHS0)
This circuit comprises the photo-coupler PC3, resistors R13 and
R12, Zener diodes ZD1 and ZD2. When an external telephone is
connected and enters the on-hook mode, the LED of photo-coupler
PC3 emits light and the light receiving element turns on. The status
signal XEXHS0 is input to the pin10 of (SUB ASIC) (IC21: Control
PWB).
XEXHS0 LOW: EXT. TEL OFF-HOOK
XEXHS0 HIGH: EXT. TEL ON-HOOK
1.3.3 Dial pulse generation circuit
The pulse dial generation circuit comprises CML.
CML ON: Make
CML OFF: Break
1.3.4 CML relay
The CML relay switches over connection to the matching transformer
T1 while the FAX is being used.
1.3.5 Matching transformer
The matching transformer performs electrical insulation from the tele-
phone line and impedance matching for transmitting the FAX signal.
1.3.6 Hybrid circuit
The hybrid circuit performs 2-wire-to-4-wire conversion using the IC 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.
SURGE
PROTECTION
CIRCUIT
HS
DETECTOR
DIODE
BRIDGE
ACTIVE
CHOKE
HI
LO
CI
DETECTOR
+5V
TXA2
MODEM
(FM336PLUS)
RIN
SPKR
TXA1
VREF
RXIN RXIN
TXOUT
CONTROL LIU
HI
HI
HI
XCI0 XCI0
XEXHS0 XEXHS0
CML0 CML0 CML0
XESHS0
XCI0
MONSEL
BZOUTEN
SUB ASIC
BZCONT
SPMUTE
VOLC
VOLB
VOLA
HI
LO
LO
LO
LO
TXOUT
+5V
+24VA
DG
LINE
EXT.TEL
SPEAKER
Fig. 8