Circuit Description Of Liu Pwb - Sharp AM-900 Service Manual

AM-900U

[3] Circuit description of LIU PWB

1. LIU block operation description
1.1. Block diagram
SURGE
PROTECTION
LINE
HI
CIRCUIT
LO
HS
DETECTOR
EXT.TEL
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
DIODE
ACTIVE
BRIDGE
CHOKE
CI
DETECTOR
+5V
CONTROL
LIU
TXA2
TXOUT TXOUT
TXA1
HI
RXIN RXIN
LO
VREF
CML CML
EXHS EXHS
CI CI
HI
LO
SPEAKER
+5V
+24VA
DG
Fig. 17
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 protects the circuit from the 390V or higher line surge volt-
ages.
• The VA2 and AR3 protect the circuit from the 500V or higher verti-
cal surge voltage.
• 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 hook of a
telephone externally connected.
• External telephone hook status detection circuit (EXHS)
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 EXHS is input to OA-2000 by way of IC10 (Logic IC).
EXHS LOW : EXT. TEL OFF-HOOK
EXHS HIGH: EXT. TEL ON-HOOK
1.3.3 Dial pulse generation circuit
The pulse dial generation circuit comprises CML.
CML ON: Make
CML OFF: Breaks
5 – 19
MODEM
(FM336PLUS)
RIN
CI
CML
IC10
PB_DATA
LOGIC IC
BZOUT
SPMUTE
VOLC
VOLB
VOLA
OA-2000