Sharp FO-2970M Service Manual - Page 59

(Example: Fax signal send) 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. 24 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 [4] Circuit description of power supply PWB 1. Noise filter circuit The filter part removes noises generated from the power unit to avoid noise release outside and prevent external noises from entering. Excessive surge such as thunder is prevented by varistor Z1. 2. Rectified smoothing circuit The rectified smoothing circuit rectifies AC input at diodes D10, 11, 12, and 13, and then smoothens it at capacitor C5 to supply DC voltage to the switching part. 3. Switching part This circuit adopts the ringing choke converter system of self-excited type. By repeating ON/OFF of MOS FETQ1, this system converts DC voltage supplied from the rectified smoothing part into high-frequency pulse, stores energy in the primary winding of transformer T1 during ON period, releases energy to the secondary winding during OFF period, and supplies power. Frequency changes according to output load; As load increases, ON period becomes longer. Constant voltage is controlled by applying feedback to the control circuit via photo coupler from 24 V output. The overcurrent protective circuit detects prolonged ON period caused by excessive output load, lengthens Q1 OFF period by using the control circuit, and restricts energy stored in the primary winding of transformer T1. Increase of the secondary output voltage 24 V is led to the overcurrent condition by turning on power zener diode D202 between 24 V output and GND. Thus overvoltage is protected by operating the overcurrent protective circuit of the control circuit. 4. 24 V circuit To supply output, transformer T1 output is rectified and smoothened with the use of diode D101 and capacitor C101. Voltage is controlled by Volume VR101. 5. +5 V circuit Transformer T1 output is rectified and smoothened with the use of diode D301 and capacitor C301 to stabilize +5 V output by using 3-terminal regulator IC301. 6. Heater circuit To maintain the optimal temperature, the heater lamp is controlled by HLON signal from the control panel. This HLON signal is to switch ON/OFF the heater lamp. If this signal is input LOW, PC2 is switched ON, resulting TRIACK TRA1 ON. Accordingly, AC power is supplied to the heater lamp to switch the heater lamp ON. 7. Zero cross circuit When AC input reaches the zero cross point (0 V), PC3 is switched ON. When Q501 is switched ON, the zero cross signal is output to the control panel. 5 - 23