EFJohnson 761X, 764X service manual Second if System IC1 UHF

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UHF (7640) CIRCUIT DESCRIPTION

Figure 5-4 Second IF System IC1 (UHF)

Figure 5-5 Audio Switching and Amplifier Circuit (UHF)

and applied to the limiter amplifier and quadrature detector. The quadrature detector demodulates the audio signal contained in the second IF signal. Com- ponents connected to pins 10 and 11 including X1 are part of the quadrature detector. The audio signal is fed out on pin 9 (IC1) and applied to the AF amplifier circuit.

5.5.5 AUDIO AMPLIFIER CIRCUIT

The AF (audio frequency) amplifier circuit pro- vides drive to the speaker (see block diagram in Figure 5-5). The audio signal from IC1, pin 9 is applied to AF amplifier IC7b and then fed to high-pass filter IC6c/ IC6d. 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 IC6d, pin 14 is applied to pin 8 of AF switching circuit IC11. The switched output signal on IC11, pin 9 is then applied to pin 16 of level controller IC12. This device allows the CPU to control the speaker volume level. The vol- ume controlled output on pin 15 is then applied to pin 2 of AF switching circuit IC11. The switched output on pin 1 is applied to pin 4 of de-emphasis circuit IC4a. The signal is then amplified by IC9 to provide drive to the speaker.

5.5.6 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	5B-8

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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 Factory Returns 75xx AccessoriesAccessory Toll-Free 800Internet Home Replacement PartsThis page intentionally left blank General VHF and 7640 UHF Mobile SpecificationsGeneral Information Transceiver Installation Installation and DisassemblyCable Installation Installing Mounting BracketInstalling Microphone Hanger Power Cable InstallationAccessory Cable Installation Power Cable FusesChanging 20-Ampere Power Cable Fuses Using AN External SpeakerTransceiver Disassembly Removing PC BoardRemove shield E 2 2-TONE Programming 2-TONE/5-TONE Module InstallationInstallation Procedure Setting 5-Tone DeviationInstallation and Disassembly Operation FeaturesControls and Display Transceiver Back PanelBasic Operation Direct Selection The F1-F4 Receiving and Placing Calls Function Key Label Description Section No TIME-OUT TimerProgrammable Functions Option Key Programmable FunctionsChannel Select Monitor Mode SelectOutput Power Select Call Guard Tone SelectChannel Scan Priority Channel Sampling Programming Software Programming SetupSection Programming Programming CablesFile Menu Starting the ProgramSpecial Keys and Functions Menu FlowModel Menu Screen MenuProgram Menu Print MenuSetup Menu Memory Channel Screen Description LMR ModelsParameter Description Bank Select Rx/Tx TimerCh Atr FrequencyTone Auto ResetLockout ScanKey Description Key and Display Assign Screen DescriptionKey Description Label Bank/Free RF PowerDtmf Autodial Screen Description SelectionContinuous Tone Screen Description Scan Function Screen DescriptionParameter Description Auto CH Stop Timer2-Tone Code Channel Screen Description ResumeStun EXO TimerTimer Auto Tx Auto TxCommon 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 CircuitModulation Circuit Power Amplifier ModuleVHF Transmitter Circuit Microphone Amplifier CircuitAPC Circuit VHF PLL CircuitPLL Circuit VHF IC PIN Descriptions Line DescriptionPin Port Description Pin Port Description Name VHF I/O Expander IC5 Main UnitPin VHF Port Expander IC11 Main UnitUHF 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 UnitEquipment Grade and Range Adjustment ProcedureTest Setup Suggested Test EquipmentTest Cable Test ChannelsCOMPUTER-AIDED Tuning VHF Test FrequenciesUHF Test Frequencies PLL Adjustment VHF Models VHF AdjustmentsVHF Adjustments Preliminary SetupVHF Adjustments CONT’D Output Power AdjustFM Deviation Adjust Squelch Adjust Dtcs Waveform AdjustReceiver Adjustments VHF Models Bandpass Filter 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 Front Unit Schematic Diagrams and Component LayoutsVHF 7610 Block Diagram Main 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