Circuit Description Of Tel/Liu And Hook Sw Pwb (Gr/Gh/My) - Sharp FACSIMILE FO-2950M Service Manual

[3-1] Circuit description of TEL/LIU and
Hook SW PWB (GR/GH/MY)
1. TEL/LIU block operation description
(1) Block diagram
SHUNT
LINE
TAX
SURGE
PULSE
PROTECTION
FILTER
EARTH
CURRENT
SHUNT
DETECTOR
1
EXT.
0
TEL
EARTH
E-RLY
(2) Circuit description
The TEL/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 circuit
(3) Block description
1) Surge Protection circuit
This circuit protects the circuit from the surge voltage occurring on
the telephone line.
• The VA2 protects the circuit from the 300V or higher line surge
voltages.
• The AR1 and AR2 protect the circuit from the 500V or higher verti-
cal surge voltage.
LIU PWB
1
0
DI
DETECTOR
1
0
DB
1
0
0
1
CONTROL PWB
CML
GAINC
0
1 TXOUT
RXIN 0
EXTSIG 1
MPXA
CI
MPXB
MPXC
HS
TELMUTE2
TELMUTE
RHS
0
1
RCVOL RCVOL
1
0
VOLA
+5V
VOLB
+24V
DG
VOLC
Fig. 23
2) On-hook status detection circuit
The on-hook status detection circuit detects the status of the hook
switch (RHS) of built-in telephone, and the status of the hook of a
telephone externally connected.
• The status of on-hook switch (RHS) is determined from the logical
level of RHS signal. (RHS is in the hook SW PWB)
RHS LOW: ON-HOOK
RHS HIGH: OFF-HOOK
This circuit comprises the photo-coupler PC4, resistors R7 and R8.
When the line is looped and enters the on-hook mode, the LED of
photo-coupler PC4 emits light and the light receiving element turns
on. The status signal HS is input to the pin119 of (SH7041) (IC5:
control PWB).
HS LOW: LINE LOOP
HS HIGH: LINE UNLOOP
3) Dial pulse generation circuit
The pulse dial generation circuit comprises the photo-coupler
PC1,PC2 and the CML relay.
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 – 21
FO-2950MGR/GH/MY/TH
SIGTX
SIGRX
TELOUT
SPOUT
BZOUT
PA0
PA1
PA2
PA3
PA4
PA5
BZCONT PA23
PE8
PE9
0
1 BZOUT PE10
PE11
PE12
PE13
PF1
PF2
PF3
MODEM
(FM214)
G.A.
(LR38784)
SH2
(7041)