Panasonic KX-FLB758RU, Service Manual

Page: 227 / 292

8.6. NCU SECTION
8.6.1. GENERAL
This section is the interface between the telephone line and external telephone. It is composed of an EXT. TEL line relay (RY202),
bell detection circuit, TAM interface circuit, line amplifier and side tone circuits and a multiplexer.
8.6.2. EXT. TEL. LINE RELAY (RY202)
1. Circuit Operation
Normally, this relay switches to the external telephone side (break) and switches to the open side (make) while OFF-HOOK.
{ IC610 (149) High Level
→
CN616 (18) High Level}
→
CN202 (11) High Level
→
Q210 ON
→
RY202 ON
→
(make)
{ IC610 (145) Low Level
→
CN616 (22) Low Level}
→
CN202 (7) Low Level
→
PC203 ON
→
Q211 ON
→
(make)
8.6.3. BELL DETECTION CIRCUIT
1. Circuit Operation
The signal waveform for each point is indicated below. The signal (low level section) input to pin T1 of ASIC IC604 on the digital
board is read by ASIC and judged as a bell.
TEL LINE
→
PC204 (1, 2 - 4)
→
IC604 (T1)
8.6.4. LINE AMPLIFIER AND SIDE TONE CIRCUIT
1. Circuit Operation
The reception signal output from the line transformer T201 is input to pin (6) of IC207 via R269, C285 and then the signal is
amplified at pin (7) of IC207 and sent to the reception system at 1.7dB.
The transmission signal goes through C243, R276 and enters IC207-pin (2), where the signal is amplified to about 27.2dB.
Then, it is output from pin (1) of IC207 and transmitted to T201 via C312, R293 and R292. Without a side tone circuit, the
transmission signal will return to the reception amplifier via R269, C285. When the side tone circuit is active, the signal output
from IC207 pin (1) passes through C312, R291, C306, R284, R280, C300, R267 and C288 and goes into the amplifier IC207
pin (5). This circuit is used to cancel the transmission return signal.
The TX signal is output to the circuit analog the route from the IC207 1 pin
→
C312
→
R293
→
R292
→
T201
→
TEL LINE.
However, if balance is lost in the bridge, a voltage occurs between the IC207 6 pin and 5 pin and a side tone results, because
the balance cannot be maintained completely at all frequencies in the audio range some side tone always occur.
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