Ncu Section - Panasonic KX-FC971CX-S Service Manual

KX-FC971CX-S / KX-FGA521CX-S
8.7.

NCU SECTION

8.7.1. GENERAL
It is composed of bell detection circuit, pulse dial circuit, line amplifier, sidetone circuits.
8.7.2. BELL DETECTION CIRCUIT
1. Circuit Operation
The signal waveform for each point is indicated below. The signal (low level section) input to pin 10 of BBIC IC4 on the digital
board.
TEL LINE → PC504 (1, 2 → 4) → IC4 (10)
8.7.3. ON/OFF HOOK CIRCUIT
Normally (ON-HOOK condition), LINE RELAY (RL500) is OFF. While OFF-HOOK, RL500 turns ON. This LINE RELAY is
controlled by pin 83 of IC4 through the Q505.
ON-HOOK:
IC4 (83) Low Level → Q505 OFF → RL500 OFF
OFF-HOOK:
IC4 (83) High Level → Q505 ON → RL500 ON
8.7.4. PULSE DIAL CIRCUIT
Make state:
IC4 (83) High Level → Q505 ON → RL500 ON
Break state:
IC4 (83) Low Level → Q505 OFF → RL500 OFF
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