BK Technologies GMH599 VHF Mobile Transceiver User Manual Section 5

RELM WIRELESS CORP. - BK RADIO VHF Mobile Transceiver Users Manual Section 5

Users Manual Section 5

BK RADIOPage 5-1SECTION VMAINTENANCE5.1 INTRODUCTIONThis section contains test and alignment procedures for an operational GMH Series mobile radio. Thissection also contains disassembly and alignment procedures. An understanding of the theory of operationis recommended before maintenance is attempted.5.1.1 Test Equipment RequiredRF Signal Generator HP8640 or equivalentDistortion Analyzer HP334A or equivalentPower Meter HP435B with 30 dB pad or equivalentService Monitor Cushman 4000 Radio Communications Test Set or equivalentDigital Multimeter IBM PC or equivalent, with 256K memory and an RS-232 serial portLAA0705 or LAA 0725 programming cableGMH Editor SoftwareGMH Tool/Cable Kit LAA 0621, P/N 050-03152-0000This tool kit consists of the following parts:Heatsink 076-01475-0000Spanner Tool 076-01451-0000GMH RF Test Cable 155-02773-00002 X 6 Test Cable 155-02705-0000Flex Cables (2 included) 155-02705-00002 X 5 Test Cable (2 included) 155-02622-0000Audio Cable 155-02269-00005.2 OVERHAUL5.2.1 Visual InspectionThis section contains instructions to assist in determining, by inspection, the condition of GMH assemblies.Defects resulting from wear, physical damage, deterioration, or other causes can be found by theseinspection procedures. To aid inspection, detailed procedures are arranged in alphabetical order.5.2.1.1 Capacitors, FixedInspect capacitors for case damage, body damage, and cracked, broken, or charred insulation. Check forloose, broken, or corroded terminal studs, lugs, or leads. Inspect for loose, broken, or improperly solderedconnections. On chip caps be especially alert for hairline cracks in the body and broken terminations.
Maintenance GMH Series VHF RadioPage 5-2BK RADIO5.2.1.2 Capacitors, VariableInspect trimmers for chipped and cracked bodies, damaged dielectrics, and damaged contacts.5.2.1.3 ChassisInspect the chassis for deformation, dents, punctures, badly worn surfaces, damaged connectors,damaged fastener devices, loose or missing hardware, component corrosion, and damage to the finish.5.2.1.4 ConnectorsInspect connectors for broken parts and other irregularities. Inspect for cracked or broken insulation andfor contacts that are broken, deformed, or out of alignment. Also, check for corroded or damaged platingon contacts and for loose, improperly soldered, broken, or corroded terminal connections.5.2.1.5 Covers and ShieldsInspect covers and shields for punctures, deep dents, and badly worn surfaces. Also, check for damagedfastener devices, corrosion, and damage to finish.5.2.1.6 Flex CircuitsInspect flex circuits for punctures and badly worn surfaces. Check for broken traces, especially near thesolder contact points.5.2.1.7 FuseInspect for blown fuse and check for loose solder joints.5.2.1.8 InsulatorsInspect insulators for evidence of damage, such as broken or chipped edges, burned areas, and presenceof foreign matter.5.2.1.9 JacksInspect all jacks for corrosion, rust, deformations, loose or broken parts, cracked insulation, bad contacts,or other irregularities.5.2.1.10 Resistors, FixedInspect the fixed resistors for cracked, broken, blistered, or charred bodies and loose, broken, orimproperly soldered connections. On chip resistors, be especially alert for hairline cracks in the body andbroken terminations.5.2.1.11 RF CoilsInspect all RF coils for broken leads, loose mountings, and loose, improperly soldered, or broken terminalconnections. Check for crushed, scratched, cut, or charred windings. Inspect the windings, leads,terminals, and connections for corrosion or physical damage. Check for physical damage to forms andtuning slug adjustment screws.5.2.1.12 Terminal Connections, Soldered1. Inspect for cold-soldered or resin joints. These joints present a porous or dull, rough appearance.Check for strength of bond using the points of a tool.2. Examine the terminals for excess solder, protrusions from the joint, pieces adhering to adjacentinsulation, and particles lodged between joints, conductors, or other components.3. Inspect for insufficient solder and unsoldered strands of wire protruding from conductor at the terminal.Check for insulation that is stripped back too far from the terminal.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-34. Inspect for corrosion at the terminal.5.2.2 CleaningA. Using a clean, lint-free cloth lightly moistened with soap and water only, remove the foreign matterfrom the equipment case and unit front panel. Wipe dry using a clean, dry, lint-free cloth.B. Using a hand controlled dry air jet (not more than 15psi), blow the dust from inaccessible areas. Careshould be taken to prevent damage by the air blast.C. Clean the receptacles and plugs with a hand controlled dry air jet (not more than 25psi), and a clean,lint-free cloth lightly moistened with soap and water only. Wipe dry with a clean, dry, lint-free cloth.5.2.3 RepairThis section describes the procedure along with any special techniques for replacing damaged or defectivecomponents.5.2.3.1 ConnectorsWhen replacing a connector, refer to the appropriate PC board assembly drawing and follow thenotes to insure correct mounting and mating of each connector.5.2.3.2 CrystalThe use of any other than a BK Radio crystal is considered an unauthorized modification.5.2.3.3 DiodesUse long nose pliers as a heat sink under normal soldering conditions. Note the diode polaritybefore removal.5.2.3.4 Integrated CircuitsRefer to Appendix A for removal and replacement instructions.5.2.3.5 Wiring/Coaxial CableWhen repairing a wire that has broken from its terminal, remove all old solder and pieces of wirefrom the terminal, restrip the wire to the necessary length and resolder the wire to the terminal.Replace a damaged wire or coax with one of the same type, size, and length.5.3 DISASSEMBLY & ASSEMBLYThe GMH radio has three major assemblies which contain the following circuit boards:• Control Head Assembly (at the front of the radio)Switch BoardControl BoardOption Board• Core Assembly (in the extruded housing)Systems BoardRX/TX Board
Maintenance GMH Series VHF RadioPage 5-4BK RADIOVCO BoardPA Flex Circuit• PA Assembly (in the heatsink casting)Filter BoardPA BoardAccessory Board5.3.1 Unit Disassembly1. Remove the four corner screws from the back of the heatsink casting.2. Carefully pull the heatsink casting from the core assembly.3. Disconnect the two flex cables and two coaxial cables from the PA board.NOTE: The black-end coax is the RX input to the receiver. The white-end coax is the exciterinput to the PA.4. Place both thumbs on the back of the core assembly and push it out of the extruded housing.5.3.2 The Control Head Assembly1. Remove the PA Assembly.2. Remove the 6 flat head screws around the Control Head housing.3. Carefully pull the Control Head from the core assembly.4. Disconnect the two flex cables from the Option board.5. Remove the 2 flat-head screws from the audio amplifier and 2 pan-head screws that hold the unittogether.6. Remove the audio heat sink.7. Remove the Option board.5.3.2.1 Control Board1. Pull off the two rotary knobs from the front of the radio and remove rubber resistance washerbehind volume knobs.2. Using the spanner tool, remove the nuts holding the rotary switches in place.NOTE: The spanner tool (076-01475-0000) has been modified for use with GMH radios. Olderspanner tools may not have sufficient depth.NOTE: These 2 nuts do not have the same thread pitch and are not interchangeable. Duringreassembly, replace each nut in its original location.3. Carefully remove the Control board by prying it up.NOTE: During reassembly, check that the microphone jack is seated properly in the housingbefore tightening the nuts on the rotary switches.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-55.3.2.2 Display Board1. Remove the Control board.2. Remove the flat spring clip by prying the end with the tab toward the top of the housing.3. Lift out the Display board.NOTE: During reassembly, check that the flex cable does not block the display by foldingbetween the display and window.5.3.2.3 Pushbutton Labels1. Pull out a pushbutton from the front of the radio.2. Remove the old label by pushing from the back.3. Insert the new label.4. Insert the pushbutton in its original location.5.3.2.4 Reconnect Control Head1. Attach flex cables before placing Control Head onto Core assembly.2. Check that coax cables are not on the lid of the harmonic filter.3. Attach Control Head to Core Assembly with 6 flat-head screws.NOTE: Use care to prevent breaking the 6 Control Head tabs when tightening the flat-headscrews.5.3.3 The Core Assembly5.3.3.1 VCO and RX/TX Boards1. Remove the sheet metal shield from the RX(TX board by prying it up with a flat-blade screwdriver.2. Remove the 4 screws from the VCO cover and lift off the VCO cover.3. Carefully remove the VCO by lifting it from the center or prying it up evenly around the sides.4. Remove the 7 screws from the RX/TX board, pull both coax leads (one at a time) through theferrite bead, and carefully lift out the RX/TX board.5.3.3.2 VCO and RX/TX Reassembly1. Insert the RX/TX board into the g, pulling the two coaxial cables through the hole in the housing.Pull the coaxes snug and make certain they clear the 12-pin connector between the RX/TX boardand the System board. Thread each coax through the ferrite bead.NOTE: Check that connectors are properly aligned with the System board before seatingthem firmly.2. Insert the VCO, using care to not bend the pins.3. Attach the VCO cover and snug down the screws.4. Fasten the RX/TX board to the housing with the seven screws.5. Insert the flex cables.5.3.3.3 System Board1. Remove the 4 screws and lift off the Synthesizer cover.
Maintenance GMH Series VHF RadioPage 5-6BK RADIO2. Remove the 3 screws from the System board.3. Disconnect the front flex cable.4. Lift out the System Board by lifting the front of the board first, then pulling the back flex through thecasting.5.3.3.4 System Board Reassembly1. Insert the System board in the housing, carefully threading the back flex through the casting.2. Attach the Synthesizer cover with the 4 screws. Insure that the rubber tubing is in the cavity abovethe large rectangular capacitor C119.NOTE: When reassembling the Synthesizer and VCO, snug down the screws on the covers ormalfunctions may occur.3. Attach the System board with the 3 screws.4. Insert the other flex cable.5.3.4 The PA Assembly1. Using a 7/16" deepwell socket, remove the hex nut from the antenna connector on the back of theheatsink housing.2. Remove the 13 screws from the PA board.3. Remove the PA board from the heatsink housing by pushing the power connector and antennaconnector.4. Remove the Accessory board by removing the one screw on the back.NOTE: During reassembly, press the heatsink housing evenly against the core assembly whileinserting the 4 screws. Tighten screws at diagonal corners to seat the heatsinkhousing firmly. Install the remaining screws.5.3.5 Unit AssemblyTo assemble the unit, complete the disassembly procedure in reverse order.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-75.5 ALIGNMENT PROCEDURESThe GMH radio uses all electronic tuning with no manual adjustments. Use the GMH Editor computersoftware when alignment or adjustment is required. You will need an IBM or compatible computer with adisk drive, an RS-232 serial port, and DOS 3.3 or later. You will also need the GMH Editor softwarediskette (LAA0737) and an RS-232 interface cable (LAA0725), available from BK Radio.Refer to the GMH Editor Software Manual for details on using the program. When performing a completealignment with the GMH Editor program, follow these steps in order:1. Radio Set-up2. Read Radio3. Tune Radio4. Synthesizer Calibration5. Crystal Color6. Transmit Power Adjust7. Freq Adjust8. Deviation Adjust9. Reference Mod10. Tune Receiver11. Discriminator Adjust12. Squelch Adjust5.5.1 RADIO SET-UPMount the radio in a suitable fixture supplied with 1 3.8VDC power from a negative ground externalpower supply. Connect the interface cable to the radio's microphone connector and the computer'sserial port. Set the radio's manual controls as follows:Channel Selector: Channel 1On/Off Volume: On, Volume minimumScan: OffPriority: OffStart the GMH Editor computer program by typing EMEDIT and pressing Enter. After the titlescreen appears, press any key on the computer keyboard to proceed to the Edit Screen.5.5.2 READ RADIOFrom the Edit Screen, press F9 (Read Radio) to upload the settings currently in the radio into theGMH Editor program. The radio displays PROG during upload.5.5.3 TUNE RADIO
Maintenance GMH Series VHF RadioPage 5-8BK RADIOFrom the Edit Screen, press F6 (Menu). Press the down arrow key to select "Tune Radio" thenpress Enter. The program reads the EEPROM map of the core microprocessor Ii (system board).This takes about 5 seconds, during which the radio displays PROG.5.5.4 SYNTHESIZER CALIBRATIONSelect "Synthesizer Calibrations" from the "Tune GMH" menu then press Enter. The radio willautomatically perform any necessary adjustment. This takes about 15 seconds, during which theradio displays PROG.Synthesizer calibration affects the way the voltage from the pretune DIA converter applied tothe VCO STEER line changes with frequency. This adjustment may be necessary if VCOcomponents are replaced. Synthesizer calibration should be performed before any otheradjustments are made.During the automatic calibration, the core microprocessor Ii (system board) will direct thesynthesizer to acquire lock at various frequencies in both receive and transmit mode. For eachtest frequency the microprocessor will vary the voltage from the DIA converter network 1102(system board) used to assist the synthesizer in obtaining frequency lock. The LOCK DETECTsignal is monitored to determine when lock has occurred. The DIA voltages that result inproper operation will be recorded in the microprocessor non-volatile memory.5.5.5 CRYSTAL COLORSelect "Crystal Color" from the "Tune GMH" menu then press Enter. A list of crystal colorsappears. Select the correct color from the list and press Enter. The message "Write Crystal Colorto EEPROM (Y/)?" appears in the lower left corner of the computer screen. Press Y to store thenew color value, or N to cancel.The reference oscillator crystal must be compensated for temperature variations. One of fourtypes, specified by a color dot on the crystal, is used in the GMH. If the reference oscillatorcrystal is replaced with one of a different color code, the programmed compensation valuemust be changed.Different crystal temperature compensation curve parameters are stored in the radio coremicroprocessor Ii (system board) for each color dot. In normal operating mode, themicroprocessor observes the temperature using RT1OO. This information, along with thestored curve parameters, is then used to develop a crystal temperature compensation value.This value is used to slightly modify the reference oscillator frequency using DIA network RN1and varactors CR102 and CR103 (system board).5.5.6 TRANSMITTER POWERNOTE: Transmitter should not be keyed for extended periods while setting TX power.Prolonged transmitting will cause the thermal protection circuitry of the radio todecrease the TX power.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-9Connect a suitable 50~ power meter to the antenna output of the radio. Select "Transmitterpower" from the "Tune GMH" menu and press Enter. Select "Start Transmit Power Test" andpress Enter. "OK to Transmit?" appears in the lower left corner of the computer screen. Press Yto transmit. You can only adjust transmit power settings while transmitting.Press the Tab or arrow keys on the computer to move the cursor to the 2-character hexadecimalvalue in the right column of the menu. Type in a different value, or press Page Up or Page Down tochange the value. For each of the three frequencies, adjust the hexadecimal control value toobtain an output power of approximately 50 Watts.To save the new settings in the radio, select "Store Transmit Power" and press Enter. "Writepower values to radio?" appears in the lower left corner of the computer screen. Press Y to savethe new values in the radio.This setting is used to help maintain a constant transmitter output power of 50 Watts acrossthe RF frequency band. If components on the PA board are replaced, this adjustment may benecessary.Adjustment values at several RF frequencies are stored in non-volatile memory bymicroprocessor Ii (system board). The power is set by the voltage from DIA network RN2(system board) which feeds the power control circuit.NOTE: To adjust the GMH transmit power to a setting below 50 Watts, select "RF PowerCode" from the "Edit Globals" menu.5.5.7 FREQUENCY ADJUSTConnect a suitable attenuator and frequency counter to the antenna output of the radio. Theattenuator must be capable of handling the full power output of the radio and protecting the inputof the frequency counter. A 30dB attenuator capable of SOW is recommended.In the "Tune GMH" menu, select "Freq Adjust" and press Enter. "Start Freq Adjust" appears in themiddle of the computer screen. When you press Enter, the message "Ok to transmit?" appears inthe lower left corner of the computer screen. When you press Y (for Yes) the radio beginstransmitting and the Control value is outlined. The message "Transmitting!! Press <ESC> to stop"appears in the lower left corner of the computer screen.With the cursor under the 2-character hexadecimal value in the right column of the menu, type in adifferent value, or press Page Up or Page Down to change the value. Adjust the control value upor down until the transmitter frequency is within +~00 Hz of the correct value. When the correctfrequency is obtained, press the Esc key on the computer.The GMH radio's exact transmit and receive frequencies are derived from the referenceoscillator. The reference oscillator frequency may require adjustment due to crystal aging or ifcomponents in the reference oscillator circuit are replaced. The above "Crystal Color Dot'setting must be correct before performing this adjustment.Changing the reference oscillator frequency setting causes the voltage applied to varactorsCR102 and CR103 (system board) to be varied, in turn altering the oscillation frequency ofreference crystal circuit slightly. the microprocessor Ii (system board) controls the voltageusing /A network RN1 (system board). The final setting is stored in non-volatile memory.
Maintenance GMH Series VHF RadioPage 5-10BK RADIO5.5.8 DEVIATION ADJUSTConnect a suitable attenuator and service monitor to the antenna output of the radio. Connect themodulation output of the service monitor to the Mic High input of the radio. Adjust the servicemonitor modulation output to a level of 0.15 VRMS and a frequency of I KHz. Select "DeviationAdjust" from the "Tune GMH" menu and press Enter. Insure that "Start Deviation Adjustment" ishighlighted and press Enter. "Ok to transmit?" appears in the lower left corner of the computerscreen. When you press Y the radio begins transmitting and the present tuning values appear.Press the Tab or arrow keys to move the cursor to the 2-character hexadecimal control value ofthe first transmit frequency. Type in a new value or press the Page Up or Page Down keys to setTX FM deviation to 4.5 kHz. Repeat for each of the four frequencies, then select "Store DeviationAdjustment" and press Enter. "Write Dev Comp values to radio?" appears in the lower left cornerof the computer screen. Press Y to store the new values in the radio, or N to cancel changes.This adjustment controls the maximum FM deviation of the transmitter. Since the deviationsensitivity changes with carrier frequency, the adjustment must be performed at severallocations in the band. If components in the VCO or transmit audio amplifier or filter arechanged this adjustment may be necessary to maintain an FM deviation below 5 kHz.The level of the transmit audio is controlled using a digital potentiometer 19 (system board).The microprocessor Ii (system board) sets values for the attenuation as the adjustment isperformed. The final setting is stored in non-volatile memory.5.5.9 REFERENCE MODULATIONNOTE: For radios using Digital Code Guard only."Start Freq Adjust" appears in the middle of the computer screen. When you press Enter, themessage "Ok to transmit?" appears in the lower left corner of the computer screen.Connect a suitable attenuator and service monitor to the antenna output of the radio. Adjust theservice monitor modulation output to a level of 0 VRMS. In the "Tune GMH" menu, select"Reference Mod" and press Enter "Start Ref Mod adjust" appears in the middle of the computerscreen. When you press Enter, the message "Ok to transmit?" appears in the lower left corner ofthe computer screen. When you press Y the radio begins transmitting and displays "PROG." Onthe computer screen, the hexadecimal tuning value is outlined.With the cursor under the 2-character hexadecimal value in the right column of the menu, type in adifferent value, or press Page Up or Page Down to change the value. Observe the Digital CodeGuard signal on the service monitor display. Adjust the control value up or down to obtain theflattest waveform with as little overshoot as possible. When adjustment is complete, press the Esckey on the computer.Reference modulation is used to allow transmission of low frequency Digital Code Guardsignals. The reference modulation sensitivity must be adjusted to match the transmit audiodeviation. This adjustment should be made after any necessary "Transmit Audio Deviation","Reference Oscillator Crystal Parameters" or "Reference Oscillator Frequency" adjustments.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-11Unlike normal voice and audible tones, the low frequency signals present in Digital CodeGuard cannot be applied as VCO modulation only. They must be presented to the referenceoscillator using the DIA network RN1 and varactors CR102 and CR103 (system ). The sub-audible information passes through the core microprocessor Ii (system board) where thevariable attenuation is effected by software. The Digital Code Guard signals are generated bythe control processor Ii (control board) and sampled by microprocessor Ii 5 stem board) onthe LF AUDIO (pin 13) line using an AID converter. The final setting is stored in non-volatilememory.5.5.10 TUNE RECEIVERConnect an RF signal generator to the radio antenna input. Set the generator FMmodulation to a frequency of 1 kHz and a deviation of 3 kHz. Connect a SINADmeter to the radio audio output. Select "Tune Receiver" from the "Tune GMH"menu and press Enter. Insure that "Start Rx Front End Alignment" is selected andpress Enter. The radio displays "PROG". Press the Tab key to select analignment frequency and perform the following procedure for each of the threelisted frequencies.Set the RF signal generator carrier frequency to the alignment frequency. Adjustthe generator output level to obtain a SINAD of approximately 12 dB. Press theTab or arrow keys to move the cursor to the hexadecimal tuning value in the"Common" column. Type in a new value or press Page Up or Page Down tochange the value and obtain the maximum SINAD reading. (During the procedure,anytime the SINAD exceeds 18 dB, the generator output level should bedecreased to return to 12 dB SINAD.)Press the Tab or arrow keys to move the cursor to the hexadecimal tuning valuein the "VARS" column. Type in a new value or press Page Up or Page Down tochange the value and obtain the maximum SINAD reading. Once the SINAD hasbeen maximized using the "VARS" value, repeat the procedure for VAR4, VAR3,VAR2, then VAR1. Press the Tab or arrow keys to select "Store Front EndAlignment" and press Enter. The message "Write radio core EEPROM?" appearsin the lower left corner of the computer screen. Press Y to store the new settingsin the radio, or N to cancel changes.The final settings of VAR1, VAR2, VAR3, VAR4, and VARS should be greaterthan 04. If this is not the case, decrease the "Common" value and repeat theadjustment of the VAR settings. When finished, press the Esc key to return to themain screen.Bandpass filters in the receiver front end are varied for different receivefrequencies to optimize radio performance. The tuning is customized using thereceiver front end tuning adjustment. If components in the preselectors, RFamplifier, or mixer are replaced this adjustment may be necessary to maintainspecified RF sensitivity.
Maintenance GMH Series VHF RadioPage 5-12BK RADIOFour varactors are used in the receiver front end bandpass filter. The controlvoltages for these varactors change to accommodate different receiverfrequencies. The proper values are determined by microprocessor Ii (systemboard) using data stored in non-volatile memory. All four varactors share acommon tuning voltage generated by DIA network I3 and RNA (RX/TXboard). Each varactor also receives an individual tuning voltage from one ofthe D/A converters of 12 (RX[TX board) that corrects for minor differences inthe varactors and other components of each filter. During the tuning process,all the individual varactors are set to a fixed value while the common voltagefrom 13 and RN1 is adjusted for best receiver sensitivity. The individualvoltages from 12 are then adjusted to optimize the tuning of each varactor.5.5.11 DISCRIMINATOR ADJUSTConnect an RF signal generator to the radio antenna input. Set the generator FMmodulation to a frequency of 1 kHz and a deviation of 3 kHz. Connect an ACvoltmeter to the Discriminator Audio output from the Accessory Connector.In the "Tune GMH" menu select "Discriminator Adjust" and press Enter. The radiounmutes and displays "PROG". "Start Discr Adjust" appears in the middle of thecomputer screen. When you press Enter to begin, the frequency and hexadecimaltuning value are outlined. Set the RF signal generator carrier frequency to thealignment frequency. With the cursor under the 2-character hexadecimal value inthe right column of the menu, type in a different value, or press Page Up or PageDown to adjust the control value to obtain a voltage reading of 0.707 V RMS onthe voltmeter. When finished, select "Store Discr value" and press Enter.This setting controls the amplitude of the receiver audio signal from the FMdiscriminator. If components in the discriminator are replaced, this adjustmentmay be necessary.The level of the discriminator audio is controlled using a digital potentiometer19 (system board). The microprocessor Ii (system board) sets values for theattenuation as the adjustment is performed. The final setting is stored in non-volatile memory.5.5.12 SQUELCH ADJUSTConnect an RF signal generator to the radio antenna input. Set the generator FM modulation to afrequency of 1 kHz and a deviation of 3 kHz. Connect a SINAD meter to the radio audio output.In the "Tune GMH" menu select "Squelch Adjust" and press Enter. The radio unmutes and displays"PROG". "Start squelch adjust appears in the middle of the computer screen. When you pressEnter, the radio is automatically tuned to the indicated frequency (which can be changed beforethe test starts). Adjust the RF level to obtain 12dB Sinad. With "Store squelch values" highlighted,press Enter to save the changes in the radio.
GMH Series VHF Radio MaintenanceBK RADIOPage 5-13The GMH is designed to turn the audio on at approximately a 12 dB SINAD level and off atabout a 6 dB level. If components in the IF filter or squelch filter are replaced, adjustment ofthe squelch levels may be necessary. The "Discriminator Audio Level", "Receiver Front EndTuning", and "Second Local Oscillator Frequency" adjustments must be correct beforeperforming the Squelch adjustment.The channel noise is filtered by the squelch filter and applied to the SQUELCH AID converterinput of the microprocessor Ii (system board). The noise signal is processed digitally accordingto values stored in non-volatile memory.

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