Oki OKIOFFICE 84 Service Manual page 428

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, ...)
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 S4-3 and S4-4.
6 Receiving sensitivity (R537, R539, R637, R639)
The receiving sensitivity at line hunting is determined by R539 to R639 depending on the line impedance. Similarly,
the MF tone receiving sensitivity at parallel pickup is determined by R537 or R637. The receiving sensitivity is set
using the DIP switches S4-5 to S4-8.
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)
If this circuits 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. If 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 incomingRI.
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 (S4-3 to S4-4)
These circuits set the impedance according to the line requirement.
S4-3: 220 ohm + 820 ohm//115 nF
S4-4: 600 ohm
19 Ring impedance switches (S1-4)
These switches set the ring impedance according to the line requirement.
20 Ring sensitivity switch (S4-4, S4-2)
This switch sets the ring sensitivity according to the line requirement.