EFJohnson 761X, 764X service manual Audio Amplifier Circuit, Receiver Mute Circuit

Page 45

VHF (7610) CIRCUIT DESCRIPTION

Figure 5-1 Second IF System IC3 (VHF)

Figure 5-2 Audio Switching and Amplifier Circuit (VHF)

are part of the quadrature detector. The audio signal is fed out on pin 9 (IC3) and applied to the AF amplifier circuit.

5.1.5 AUDIO AMPLIFIER CIRCUIT

The AF (audio frequency) amplifier circuit pro- vides drive to the speaker (a block diagram is shown in Figure 5-2). The audio signal from IC3, pin 9 is applied to AF amplifier IC7A and then fed to pin 1 of AF switching circuit IC4. The switched output signal on IC4, pin 2 is then applied to high-pass filter IC8A/ B. The cut-off frequency of this filter is controlled by the “AFHPF” line. When “AFHPF” is high, the cut-off frequency is shifted higher to attenuate the Call Guard (CTCSS or DTCS) signals.

The filtered output signal on IC8, pin 7 is applied to limiter IC7b and buffer IC7c. De-emphasis is pro-

vided by R145 and C182. The audio signal is then applied to pin 16 of level controller IC5. This device allows the CPU to control the speaker volume level. The level controlled signal is fed out of IC5 on pin 15 and applied to low-pass filter IC6b. It is then applied to AF switching circuit IC4 on pin 8, and the switched output is on pin 9. This signal is buffered by IC6d and then amplified by AF amplifier IC10 to provide drive to the speaker.

5.1.6 RECEIVER MUTE CIRCUIT

Noise Squelch

The noise squelch circuit turns off the audio sig- nal when no RF signal is being received. The squelch circuit is controlled by the amount of noise present in the audio signal.

November 1998
Part No. 001-7600-001	5A-2

Image 45

Contents 761x VHF 764x UHF Conventional Mobile Conventional FM TWO-WAY Radio Table of Contents Table of Contents CONT’D Schematic Diagrams and Component Layouts General Information Accessory 75xx AccessoriesToll-Free 800 Factory ReturnsInternet Home Replacement PartsThis page intentionally left blank General VHF and 7640 UHF Mobile SpecificationsGeneral Information Transceiver Installation Installation and DisassemblyInstalling Microphone Hanger Installing Mounting BracketPower Cable Installation Cable InstallationChanging 20-Ampere Power Cable Fuses Power Cable FusesUsing AN External Speaker Accessory Cable InstallationTransceiver Disassembly Removing PC BoardRemove shield E Installation Procedure 2-TONE/5-TONE Module InstallationSetting 5-Tone Deviation 2 2-TONE ProgrammingInstallation and Disassembly Operation FeaturesControls and Display Transceiver Back PanelBasic Operation Direct Selection The F1-F4 Receiving and Placing Calls Programmable Functions TIME-OUT TimerOption Key Programmable Functions Function Key Label Description Section NoOutput Power Select Monitor Mode SelectCall Guard Tone Select Channel SelectChannel Scan Priority Channel Sampling Section Programming Programming SetupProgramming Cables Programming SoftwareSpecial Keys and Functions Starting the ProgramMenu Flow File MenuProgram Menu Screen MenuPrint Menu Model MenuSetup Menu Memory Channel Screen Description LMR ModelsParameter Description Bank Select Ch Atr TimerFrequency Rx/TxLockout Auto ResetScan ToneKey Description Key and Display Assign Screen DescriptionKey Description Label Dtmf Autodial Screen Description RF PowerSelection Bank/FreeContinuous Tone Screen Description Scan Function Screen DescriptionParameter Description 2-Tone Code Channel Screen Description Stop TimerResume Auto CHTimer Auto Tx EXO TimerAuto Tx StunCommon Screen Description Expert Screen Description Unique PMR Screens Unique PMR European ScreensCloning Programming ONE Transceiver with Another Memory Channel Screen PMR Models This page intentionally left blank VHF Receiver Circuit VHF 7610 Circuit DescriptionReceiver Mute Circuit Audio Amplifier CircuitVHF Transmitter Circuit Power Amplifier ModuleMicrophone Amplifier Circuit Modulation CircuitAPC Circuit VHF PLL CircuitPLL Circuit VHF IC PIN Descriptions Line DescriptionPin Port Description Pin VHF I/O Expander IC5 Main UnitVHF Port Expander IC11 Main Unit Pin Port Description NameUHF Receiver Circuit UHF 7640 Circuit DescriptionSecond if System IC1 UHF UHF Transmitter Circuit UHF PLL Circuit PLL Circuit UHFUHF CPU IC1 Port Allocation UHF Power SupplyUHF Port Expander IC13 Main Unit UHF I/O Expander IC12 Main UnitTest Setup Adjustment ProcedureSuggested Test Equipment Equipment Grade and RangeTest Cable Test ChannelsCOMPUTER-AIDED Tuning VHF Test FrequenciesUHF Test Frequencies VHF Adjustments VHF AdjustmentsPreliminary Setup PLL Adjustment VHF ModelsVHF Adjustments CONT’D Output Power AdjustFM Deviation Adjust Receiver Adjustments VHF Models Dtcs Waveform AdjustBandpass Filter Adjust Squelch AdjustBeep Tone Level Beep Tone Level AdjustmentUHF Adjustments PLL Adjustment UHF ModelsTransmitter Adjustments UHF Models UHF Adjustment Points UHF Adjustments CONT’DUHF Adjustments CONT’D Receiver Adjustments UHF Models Æçìíýõ9+ôý0$,1ý81,7 Parts ListÍäç HUDPLFý&ìçíåý-%ýì+ýìíë.ð7ð$ Íëëðêäíçðíçí Íäæ Ìää HUDPLFý&ìçíåý-%ýì&ýìíé.7ð1 Íëëðêäíçðçìé Ëíí Ëäí HUDPLFý&ìçíåý-%ýìýìíê.ð7ð$ Íëëðêäíçðíçê Ëäì RLOý0/ìçíå$ýì5í.ð7 Íëëðêäíçðçèç 03ì Ýìäëëý9&2ýFDVH 5íéé 5HVý5-ê*-ýìííý9ýõìíýRKPô Íëëðêäíçðìêå 5íéè 5ìèé 5HVý5-ê*-ýìíéý9ýõìííýNýRKPô Æçéíýõ8+ôý0$,1ý81,7 Íêì HUDPLFý&ìçíåý-%ýì+ýìíë.ð7ð$ Íëëðêäíçðíçí Íêë Íäé HUDPLFý+0çí6-ý&+ýíêíýèíí9ý Õéäíðèìëý0+ô Ìèì 5HIý1Rï Êëç Íëëðêäíçðäèë 4éì 7UDQVLVWRUý7&êçê.ý7ìéæ 5íæé 5ìçå 5HVý5-ê*-ýìíéý9ýõìííýNýRKPô 5217ý81,7 Õ9+ý$1ý8+ý02/6ô =HQHUý0$åíèìð0ýõ7ô Æçìíýõ9+ôý&+$66,6ý3$576 Front Unit VHF 7610 Exploded ViewUHF 7640 Exploded View VHF 7610 Block Diagram Schematic Diagrams and Component LayoutsMain Unit Front UnitVHF 7610 Transistor Basing Diagrams Foldout → VHF 7610 Main Board TOP View VHF Main Unit VHF 7610 Main Board SchematicUHF 7640 Block Diagram UHF 7640 Main Board Bottom View J4 T o O p tion al Un it UHF 7640 Main Board TOP ViewUHF 7640 Main Board Schematic VHF/UHF Front Unit Board Bottom View VHF/UHF Front Unit Board TOP ViewVHF/UHF Front Unit Schematic UHF 7640 Transistor Basing Diagrams