Service Guide OKIFAX 5750/5950Chapter A Board DescriptionsPage: 285A3.4.3 FNS circuit diagram1 Lightning arresters (AR1, 2)The nominal operating voltage is 500 V. When connecting the ground of the arrestor to the chassis, tighten ARG on the PCB with a screw. At this time, the PCBis grounded through the power cable. The TB1 arrestor ground terminal can also be used to connect to the earth directly.3 Diode bridge (DB1)This circuit rectifies the loop current so that the DC circuit characteristics are not affected by a polarity change over the line.4 DC circuits (Q3, R540, R541, C13, R9, R209)These circuits provide DC characteristics according to the line requirements.5 Impedance matching network (R523, R536, C503, R623, R636, C603)This circuit matches the impedance between the line and equipment to reduce reflection of transmitted signals.It provides impedance (return loss) characteristics to meet the line requirement using the S5-1 and 5-2.6 Receiving sensitivity (R537, R539, R637, R639)The receiving sensitivity at line hunting is determined by R539 or R639 and the MF tone receiving sensitivity at parallel pickup is determined by R537 or R639.The receiving sensitivity set using DIP switch S5-3 to S5-6.7 CML (RL1)This circuit selectively switches the line between the telephone or facsimile.8 SR (RL2)This circuit connects the line with the telephone. During facsimile transmission, it disconnects the telephone.9 DP (IC5)This circuit generates pulse dial signals.0 MUTE (IC7)During pulse dialing, this circuit closes to reduce the DC loop resistance.A PP (RL6)When it detects MF or CNG tones without seizuring a line, it disconnects NT (5) to increase the input impedance and also sets the receiving sensitivity.
