Oki OKIOFFICE 84 Service Manual page 426

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.
10 MUTE (IC7)
During pulse dialing, this circuit closes to reduce the DC loop resistance.
11 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.
12 MUTE (RL3)
During pulse dialing, this circuit opens to prevent pulse distortion caused by capacitor C11. When it detects MF or
CNG tones without seizuring a line, it opens to increase the impedance.
13 Pickup RC (R590, C31)
These circuits insert a high-impedance resistor and capacitor serially to prevent the line impedance from dropping by
the line transformer T1.
14 Ring detectors (IC1)
These circuits detect a ring signal arriving to the line. If the input ring signal exceeds a specific voltage, the circuits
output a signal of RI having the same frequency as the incoming RI.
15 Line transformer (T1)
This circuit processes send/receive signals required for facsimile transmission, dial tone receive signals required for
automatic dialing, and MF tone send and remote receive signals. It separates between the line and equipment in
terms of DC and also keeps a balance between the line and the ground.
16 Off-hook detectors (IC2, RL7)
These circuits detect the off-hook state of the telephone connected to the TEL1, TEL2, through LINE terminals. IC2
uses a high detection sensitivity than of RL7. In TEL/FAX mode, the higher sensitive IC2 is used to detect the
off-hook state of the telephone while the main equipment is hunting a line. Usually, IC2 is short-circuited by the CML
relay (7) in the standby state and RL7 is used for off-hook detection.
17 Impedance switches (S5-1 to 5-2)
These circuits set the impedance according to the line requirement.
S5-1: 370 ohm + 620 ohm//310 nF (UK)
S5-2: 600 ohm (F)
18 DC resistance switches (CN26 and CN36)
These switches set the DC resistance according to the line requirement.