Sharp FO-2970M Service Manual - Page 57

[3] Circuit description of TEL/LIU and Hook SW PWB 1. TEL/LIU block operation description (1) Block diagram LIU PWB HS CML CONTROL PWB GAIN -C LINE EXT. HS DETECTOR CML H L CI DETECTOR TEL MUTE (H:MUTE) RX TX TXOUT RXIN 0:CID 1:RX TEL IN TELOUT 0:MID 1:HIGH SPEAKER SP MUTE (H:MUTE) BZCONT 0:LINE 1:BZ CI +5VA +24VA DG HOOK-SW PWB Fig. 23 FO-2970MU SIGTX SIXGRX IC8 MODEM(FM336) SPKR BZCONT BZOUT MIC MUTE RCVOL VOL C VOL B VOL A SP MUTE TEL MUTE CI RHS IC15 G.A LR38784A IC5 SH2 (7041) GAIN-C CML HS (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 AR1 protects the circuit from the 390V or higher line surge voltages. • The VA1 and VA2 protect the circuit from the 470V or higher vertical surge voltage. 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 • External telephone hook status detection circuit (HS) This circuit comprises the photo-coupler PC1, resistors R1 and R2, Zener diodes ZD1 and ZD2. When an external telephone is connected and enters the on-hook mode, the LED of photo-coupler PC1 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: EXT. TEL OFF-HOOK HS HIGH:EXT. TEL ON-HOOK 3) Dial pulse generation circuit The pulse dial generation circuit comprises 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