Panasonic KX-T7730X Service Manual

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Panasonic KX-T7730X Service Manual Manual pdf 16 page image

7.6. SPEAKERPHONE CIRCUITFunction:This circuit controls the automatic switching of the transmitted and received signals to and from the telephone line, when the unitis used in the hands-free mode.Circuit Operation:The speakerphone can only provide a one-way communication path at a given time, but cannot do both simultaneously.Therefore, a switching circuit is necessary to control the flow of the outgoing and incoming signals.In other words, it can either transmit an outgoing signal or receive an incoming signal at a given time, but cannot do bothsimuitaneousiy. Therefore, a switching circuit is necessary to control the flow of the outgoing and incoming signals.This switching circuit is contained in IC3 and consists of a Voice Detector, Tx Attenuator, Rx Attenuator, Comparator and AttenuatorControl. The circuit analyzes whether the Tx (transmit) or Rx (receive) signal is louder, and then it processes the signals so that thelouder signal is given precedence.The Voice Detector provides a DC input to the Attenuator Control corresponding to the Tx signal. The Comparator receives a Txand Rx signal, and supplies DC input to the Attenuator Control corresponding to the Rx signal.The Attenuator Control provides a control signal to the Tx and Rx Attenuator to switch the appropriate signals ON and OFF.The Attenuator Control also detects the level of the volume control to automatically adjust the volume for changing ambientconditions.1) Control Signal PathControl signal for transmission and reception are input to IC3 via the following path:(Transmission Control Signal Path)MIC(CN2) → IC3 Pin 9 → IC3 Pin 10 → IC3 Pin 3 → IC3 Pin 4 → IC3 Pin 5(Reception Control Signal Path)Telephone Line → Q3 → Q4 → IC3 Pin 72) Transmission/Reception SwitchingThe comparison result between Tx and Rx output is a DC level at IC3 Pin 26.Tx level is high .................... Pin 21-Pin 26=6mVRx level is high .................... Pin 21-Pin 26=150mVThe comparator output is connected to the attenuator control inside IC3.3) Voice DetectorThe output of the mic amp (Pin 10 of IC3) is supplied to Pin 14 of IC3 as a control signal for the voice detector.4) Attenuator ControlThe attenuator control detects the setting of the volume control through Pin 25 of IC3 and automatically adjusts the volume forchanging ambient conditions.5) Transmission Signal PathThe input signal from the microphone is sent through the circuit via the following path:MIC(CN2) → C64 → R206 → C59 → IC3 Pin 9 → IC3 Pin 10 → R45 → C48 → IC3 Pin 3 → IC3 Pin 4 → C41 → R41 → Q3 →C4 → T1 → C3 → Q1 → D1 → JK1 → Telephone Line6) Reception Signal PathSignals received from the telephone line are output to the speaker via the following path:Telephone Line → D1 → Q1 → C3 → T1 → C4 → Q3 → R10 → C8 → R12 → Q4 → C42 → R42 → C46 → IC3 Pin 29 → IC3Pin 28 → R57 → C62 → C61 → IC3 Pin 20 → IC3 Pin 16 → C63 → CN3 Speaker7) Busy Tone Detector CircuitThe busy tone detection for automatic redealing is executed as follows.Telephone Line → D1 → Q1 → C3 → T1 → C91 → R91 → IC6 → D3 → R99 → Q20 → IC1 Pin 6416KX-T7730X