Service manual

FO-4700U
FO-47UC
5 – 17
3) Block description
1. Surge Protection circuit
This circuit protects the circuit from the surge voltage occurring on
the telephone line.
• The AR1 pretects the circuit from the 390V or higher line surge
voltages.
• The VA1 and VA2 protect the circuit from the 470V or higher vertical
surge voltages.
• The ZD6 and ZD7 control the voltage generated on the secondary
side of matching transfomer to 2V.
• The VA3 protect the circuit from 100V or higher line surge voltage.
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
connected.
• External telephone hook status detection circut (EXHS)
This circut 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 receiv-
ing element tums on. The status signal EXHS is input to the pin118 of
(G/A-A)(IC17:control PWB).
EXHS LOW : EXT. TEL OFF-HOOK
EXHS HIGH: EXT. TEL ON-HOOK
SPMUTE
+5V
SURGE
PROTECTION
CIRCUIT
HS DETECTOR
LINE
EXT.
TEL
1
0
CI DETECTOIN
DB
ACTIVE
CHOKE
1
0
TXOUT
1
0
RXIN
EXTSIG
VREF
TXMUTE
MODEM
(R144AFXL)
CPU
(SH7021)
/EXHS
CML
TXOUT
RXIN
PB3
PB0
PB1
PB8
PB9
1
0
GA(A)
(LZ9FJ59)
VOLC
VOLB
VOLA
PB7
PB4
PB2
PA11
XEXHS
XCI
MPXC
MPXB
CDCMSK
BZCONT
BZOUT
+24VA
DG
DPON
CI
GAINC
CONTROL PWB
LIU PWB
[3] Circuit description of LIU PWB
(1) LIU block operational description
1) Block diagram
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 puse generation circuit
4. CML relay
5. Matching transformer
6. Hybrid circuit
7. Signal selection
8. CI detection circuit
9. Power supply and bias circuit
Fig. 5