Oki OKIOFFICE 84 Service Manual page 424

1 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
PCB is grounded through the power cable. The TB1 arrestor ground terminal can also be used to connect to the
earth directly.
2 Loop current detector (IC4) -- Optional
When a line is seizured, this circuit detects a DC loop current to notify the fact. For detection (OH1), it outputs the
low level to the nominal input current of 10 mA or more.
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, and R309)
These circuits provide DC characteristics according to the line requirements depending on the DIP SW (S3) position.
5 Impedance matching network (R523, R536, C503 ... R823, R836, C803)
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 connector keys (CN15 to
CN45).
6 Receiving sensitivity (R537, R539...R837, R839)
The receiving sensitivity at line hunting is determined by R539 to R839 depending on the line impedance. Similarly,
the MF tone receiving sensitivity at parallel pickup is determined by R537 to R837. The receiving sensitivity is set
using connector keys (CN15 to CN45).
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 Impedance matching Net work (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 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 terminal. IC2
uses a high detection sensitivity than of RL7. In TEL/FAX mode, the higher sensitive IC2 is used to detect the