UBS Axcera 325A 500-Watt VHF Low-band Television Transmitter User Manual Chapter 5

UBS-Axcera 500-Watt VHF Low-band Television Transmitter Chapter 5

Chapter 5

500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-1Chapter 5Detailed Alignment ProceduresThe 325A transmitter was aligned at thefactory and should not require additionalalignments to achieve normal operation.This transmitter operates using thebaseband audio and video inputs or, ifthe (optional) 4.5-MHz composite inputkit is purchased, either a singlecomposite video + 4.5-MHz input orseparate baseband video and audioinputs.Check that the RF output at J2 of (A9-A5)the coupler is terminated into a dummyload of at least 500 watts. Whileperforming the alignment, refer to theTest Data Sheet for the transmitter andcompare the final readings from thefactory with the readings on each of thetrays. They should be very similar. If areading is off by a significant amount, theproblem is likely to be in that tray.Switch on the main AC and the VHFexciter circuit breakers on the ACdistribution panel behind the rear cabinetdoor.5.1 (A4) VHF Low-Band Exciter Tray(1070820; Appendix C) withBaseband Video and Audio InputsThe (A4) low-band VHF exciter tray(1070820) has adjustments for videolevels, audio modulation levels, and otherrelated parameters.Connect an NTSC baseband video testsignal input (1 Vpk-pk) to the transmittervideo input jack J2 on the (A12) remoteinterface panel. Jacks J1 and J2 on theVHF exciter tray are loop-throughconnected and the unused jack can beused as a video source for anothertransmitter by removing jumper W4 onjack J3 on (A5) the sync tip clampmodulator board (1265-1302). Connect abaseband audio input (+10 dBm) to thebalanced audio input terminal block TB1-1 (+), TB1-2 (-), and TB1-3 (ground). Ifstereo/composite audio is provided,connect it to BNC jack J6, the compositeaudio input jack on the remote interfacepanel. Jacks J3 and J13 on the rear of theexciter panel are loop-through connectedand the unused jack can be used as anaudio source for another transmitter byremoving jumper W1 on jack J15 on theaural IF synthesizer.Look at the front panel meter on the VHFexciter tray. In the Video position, themeter indicates active video from 0 to 1Vpk-pk. The normal video input level is 1Vpk-pk on the meter. If this reading isnot at the proper level, the overall videolevel can be changed by adjusting videolevel control R12 on the sync tip clamp/modulator board.Switch the meter to the Audio position toshow the audio deviation (modulationlevel) of the signal from 0 to 100 kHz.The aural IF synthesizer board wasfactory set for a ±25 kHz deviation with abalanced audio input of +10 dBm. If thereading is at not the correct level, adjustbalanced audio gain pot R13 on the auralIF synthesizer board, as needed, to attainthe ±25 kHz deviation. The aural IFsynthesizer board was factory set for a±75 kHz deviation with a compositeaudio input of 1 Vpk-pk. If this reading isnot correct, adjust composite audio gainpot R17 on the aural IF synthesizerboard, as needed, for the ±75 kHzdeviation.5.2 (A4) VHF low-Band Exciter Tray(1070820; Appendix C) with the 4.5-MHz Composite Input KitWith the 4.5-MHz composite input kit,the (A4) VHF exciter tray is able tooperate using either the separate videoand audio baseband inputs or the single4.5-MHz composite input.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-2The 4.5-MHz composite input kit includesa composite 4.5-MHz filter board (1227-1244) and a 4.5-MHz bandpass filterboard (1265-1307).To align the VHF exciter using basebandvideo and audio, refer to the alignmentinstructions described in Section 5.1 ofthis chapter. Select the baseband inputoperation by a applying a basebandselect, using a jumper or closed contacts,connected between J7-6 and J7-7 on therear of the tray.To operate the transmitter using the 4.5-MHz composite input, remove thebaseband select command from J7-6 andJ7-7 on the rear of the tray.Connect a multiburst test signal from anenvelope delay measurement set to theinput of the rear interface panel at J2. On(A24) the composite 4.5-MHz filter board(1227-1244), connect an oscilloscopebetween J7, the center pin, and pin 1 or3, which are ground. Adjust C21, ifnecessary, for the best frequencyresponse. Adjust R32 for a signal level of1 Vpk-pk on the oscilloscope. The output,as measured at J6 and J7 of the board,should be video only with a minimum4.5-MHz aural subcarrier.On the (A25) 4.5-MHz bandpass filterboard (1265-1307), adjust the filter withL2, C3, L4, and C7 for a frequencyresponse of no greater than ±.3 dB from4.4 to 4.6 MHz. Adjust C19 for an overallpeak-to-peak variation of less than ±.3dB from 4.4 MHz to 4.6 MHz. Recheckthe frequency response; it may havechanged with the adjustment of theenvelope delay.5.3 (A4) VHF Exciter Tray (1070820;Appendix C) with either Baseband orthe 4.5-MHz Composite InputThe IF section of the (A4) VHF excitertray includes adjustments for automaticlevel control (ALC), linearity (amplitudepredistortion), and phase (phase changevs. level) predistortion for correction ofthe nonlinearities of the RF amplifiertrays. The upconverter section alsoincludes adjustments to the localoscillator chain tuning and the localoscillator center frequency tuning. Bothof these were completed at the factoryand should not require adjustments atthis time.Move the Operate/Standby switch on theVHF exciter tray to Standby. The setup ofthe RF output includes an adjustment tothe drive level of the two VHF amplifiertrays, the adjustment of the linearity andphase predistortion (which compensatefor any nonlinear responses of theamplifier trays), and the gain andphasing adjustments of the two VHFamplifier trays.Verify that all of the red LEDs on the ALCboard are extinguished. The following listdescribes the meaning of each LED whenthey are illuminated:• DS1 (Input Fault) – Indicates that anabnormally low or no IF is present atthe input of the board• DS2 (ALC Fault) – Indicates that theALC circuit is unable to maintain thesignal level requested by the ALCreference. This is normally due toexcessive attenuation in the linearitysignal path or the IF phase correctorsignal path or because jumper W3 onJ6 is in the Manual ALC Gain position.• DS3 (Video Loss) – Indicates a loss ofvideo at the input of the board• DS4 (Mute) – Indicates that a visualMute command is present (not usedin this configuration)• DS5 (Modulator Enable) – Indicatesthat the modulator IF output hasbeen selected (this is only used if areceiver tray is present in thesystem). DS5 is always on with noreceiver.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-3The ALC is muted when the transmitter isin Standby. To monitor the ALC, turn offthe two amplifier on/off circuit breakerson the AC input assembly in the rear ofthe cabinet and switch the transmitter toOperate. Adjust the power adjust gainpot on the front panel of the VHF excitertray to obtain +0.8 VDC on the frontpanel meter in the ALC position. On theALC board (1265-1305), move jumperW3 on J6 to the Manual position,between pins 2 and 3, and adjust R87 onthe ALC board for +0.8 VDC on the frontpanel meter in the ALC position. Movejumper W3 back to Auto (between pins 1and 2); this is the normal operatingposition. The detected IF signal level atJ19-2 of the ALC board is connected tothe transmitter control board thatdistributes the level to the two VHFamplifier trays where it is used as areference for the automatic gain control(AGC) in each amplifier tray.5.4 IF Phase Corrector AdjustmentAs shipped, the exciter was preset toinclude linearity (gain vs. level) andphase (phase vs. level) predistortion. Thepredistortion was adjusted toapproximately compensate thecorresponding non-linear distortions ofthe amplifier trays and should not requireadditional adjustments.Locate (A9) the IF phase corrector board(1227-1250) mounted in the VHF exciter.The amplitude correction portion of theboard is not utilized in this configuration.As a result, jumper W3 on J10 should bein the Disable position, to +6.8 VDC, andR35 and R31 should be fully counter-clockwise (CCW). R68 is the rangeadjustment and should be set in themiddle of the range. The phasecorrection Enable/Disable jumper W2 onJ9 should be in the Enable position, toground.Switch the input video test source toselect an NTSC 3.58-MHz modulatedstaircase or ramp test waveform. Set upthe station demodulator and monitoringequipment to monitor the differentialphase or intermodulation products of theRF output signal. There are threecorrector stages on the IF phasecorrector board, each with a magnitudeand a threshold adjustment that areadjusted, as needed, to correct for anydifferential phase or intermodulationproblems. Adjust the R3 threshold for thecut-in point of the correction and the R7magnitude for the amount of thecorrection that is needed.Jumper W1 on J8 is set to give thedesired polarity of the correction shapedby the threshold R11 and the magnitudeR15 adjustments. After setting thepolarity, adjust the R11 threshold for thecut-in point of the correction and the R15magnitude for the amount of thecorrection that is needed. Finally, adjustthe R19 threshold for the cut-in point ofthe correction and the R23 magnitude forthe amount of the correction that isneeded.Note: Adjusting these pots changesall visual parameters and should bedone cautiously.5.5 Linearity Corrector AdjustmentThe IF linearity correction functionconsists of three non-linear cascadedstages, each having adjustablemagnitude and threshold, or cut-inpoints, on the ALC board. The thresholdadjustment determines at what IF signallevel the corresponding corrector stagebegins to increase gain. The magnitudeadjustment determines the amount ofgain change for the part of the signal thatexceeds the corresponding thresholdpoint. Refer to the VHF exciter traycontrol locations drawing, ALC board(1265-1305), to find the adjustments forthe first through third linearity correctorstages. Because the stages are cascaded,the order of correction is important. Thefirst stage should cut in near white level,with the cut-in point of the next stagetoward black, and with the last stageprimarily stretching sync.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-4To adjust the linearity correctors fromscratch, ensure that the transmitter isoperating at full power with the desiredA/V ratio. Check that jumper W1 on J4 ofthe ALC board is enabled between pins 1and 2. Make sure that the ALC voltage isset to +0.8 VDC as monitored on thefront panel meter in the ALC position.Insert a modulated ramp video testsignal into the transmitter. Demodulatethe output signal of the transmitter andobserve the waveform on a waveformmonitor while also looking at the signalon a spectrum analyzer. On the IF ALCboard (1265-1306), preset pots R34,R37, and R40 (threshold) fully CCW andthe magnitude adjustments R13, R18,and R23 fully clockwise (CW). On the IFphase corrector board (1227-1250),preset pots R7, R15, R23, and R35 fullyCW and R3, R11, R19, and R31 fullyCCW.Set the waveform monitor to differentialstep filter and the volts/division scale to.1 volts. Center the display toapproximately the blanking level.Gradually adjust pots R3, R11, and R19clockwise on the IF phase correctorboard, as needed, to minimize theobserved thickness of theintermodulation as seen on the display.Adjust pots R34, R37, and R40 clockwiseon the IF ALC board, as needed, to givecorrection at sync or at low luminancelevels as viewed at the left-most edge ofthe waveform monitor.The intermodulation beat productsbetween the colorburst and the auralcarrier at 920 kHz above visual carriershould also be observed on the spectrumanalyzer while performing the precedingadjustments. The frequency will vary fordifferent video systems. When theadjustments are performed properly, theintermodulation products on thespectrum analyzer should be at least -52dB down, with a red field input, frompeak visual carrier. The intermodulationdistortion as displayed on the waveformmonitor should be no more than 1 IRE.Pot R31 on the IF phase corrector boardis used for any extra intermodulationcorrection that may be needed.Note: Any adjustments to the abovepots affects other visual parametersand some slight adjustments of all ofthe pots may be needed to meet allspecifications simultaneously.If the transmitter is being driven veryhard, it may not be possible to getenough sync stretch while maintaining aflat differential gain. In this case, somevideo sync stretch may be used from thesync tip clamp/modulator board; thesync stretch adjustment is R48.Switch the transmitter to Standby.5.6 Phase and Gain Adjustment ofthe VHF Amplifier TraysThe following procedure was completedat the factory and should only befollowed if one of the VHF amplifier traysis replaced.Preset the phase and gain potentiometeron each VHF amplifier tray fully CCW.Switch the transmitter to Operate andadjust the gain pot on each tray for 25%Output Power. Adjust the phase controlCW on the left VHF amplifier tray. If the% Visual Output Power goes up, continueto adjust the phase control until eitherthe peak is reached or the end-of-travelis reached. If the % Output Power goesdown, reset the phase control on the VHFamplifier tray fully CCW and repeat theabove procedure with the phase controlof the other amplifier tray.If the end-of-travel is reached on thephase adjust, reset the phase controlCCW and add a 2-inch length of cable tothe input of the affected VHF amplifiertray at J1. Readjust the phase of thattray until a peak is reached or until theend-of-travel is achieved.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-5If the end-of-travel is reached, repeat theabove procedure and replace the 2-inchlength of cable with a 4-inch length ofcable. Once a peak has been reached,move the phase control that is fully CCWup two turns and repeak using the phasecontrol on the other tray. This allowsboth trays to have some range ofadjustment.Adjust the gain of both VHF amplifiertrays for 90% Tray Output Power.Readjust each phase control to peak thecombined output; the phase should onlyhave been slightly affected. Although itmay take a few turns to notice a change,there should be a definite peak that isachieved while adjusting the phase ofeach tray. Raise or lower the outputpower of each tray to achieve 100%Output Power. The output power of eachtray should be 90% to 100%.5.7 Calibration of the ForwardOutput Power Level of theTransmitterNote: Only perform the followingprocedure if the power calibration issuspect.Switch the transmitter to Standby andpreset R51, the aural null pot on thevisual/aural metering board (1265-1309), fully CCW. Adjust R48, the nulloffset pot on the visual/aural meteringboard, for 0% Visual Output. Perform thefollowing adjustments with no auralpresent. This is accomplished byremoving jumper cable W1, the aural IFloop-through, that is connected to J16 on(A5) the sync tip clamp/modulator board(1265-1302). Connect a sync and blacktest signal to the video input jack of theVHF exciter tray. Switch the transmitterto Operate.Set up the transmitter for the appropriateaverage output power level: sync + black0 IRE setup/wattmeter=298 watts; sync+ black 7.5 IRE setup/wattmeter=273watts.Note: The transmitter must have 40IRE units of sync.Adjust R28, visual calibration, on (A19)the visual/aural metering board (1265-1309) for 100% on the front panel meterin the % Visual Output position.With the spectrum analyzer set to thezero span mode, obtain a peak referenceon the screen. Reconnect jumper cableW1 to J16 on (A5) the sync tipclamp/modulator board. While in theVisual Output Power position, adjust L3for a minimum visual power reading.Turn the power adjust pot on the frontpanel until the original peak referencelevel is attained. Peak L1 and C8 for amaximum aural power reading and thenalso adjust R20 for a 100% Aural Powerreading. Switch the transmitter to theVisual Output Power position and adjustR51, the aural null pot, for 100% VisualPower.5.8 Calibration of the ReflectedOutput Level of the TransmitterOn the meter, in the Visual Powerposition, turn the power adjust pot to20%. Check that the jumper is in Manualon the VHF filter/amplifier board(1064251). Reverse the cables on A9-A5,J3 and J4, and adjust R39 on thevisual/aural metering board (1265-1309)for a 20% reading in the Reflected Powerposition. At this 20% reference powerreading, the VSWR LED mounted on thefront panel of the exciter should beilluminated. If this LED is not lit, adjustR22 on the transmitter control board inthe VHF exciter tray until the VSWR LEDjust turns on. Turn the power adjust potslightly CCW and the LED should go out.Turn the pot CW until the LED just turnson. The reflected output power is nowcalibrated.Switch the transmitter to Standby. Movethe cables on A9-A5, J3 and J4, to theiroriginal positions.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-6Switch the transmitter to Operate andadjust the front panel power pot for a100% Visual Power reading.5.9 (A8) 2-Way Combiner Assembly(1198-1010 CH. 2-4 or 1222-1002CH. 5-6; Appendix C)There are no adjustments to (A8) theVHF combiner assembly.Note: The bandpass filter is factoryswept and should not be tunedwithout the proper equipment. Donot attempt to tune the filterswithout a sweep generator or,preferably, a network analyzer. Iftuning is required, consult theAxcera Field Support Departmentbefore attempting to make anyadjustments.5.10 (A9) Bandpass FilterAssemblies (1076291 through1076293 & 1077130; Appendix C)The input to the (A9) bandpass filterassemblies is the output of the VHFcombiner assembly, which is thecombined output of the VHF amplifiertrays. The filter is made of aluminumwaveguide and has five resonant cavities.The filter has five bolts for tuningadjustments. The bandpass filter alsoutilizes two integral traps at -4.5 MHzand +9 MHz from FV. Refer to thebandpass filter drawing for the location ofthe adjustments.To tune the filter, connect a sweep signalto the input of the filter and adjust thefive tuning bolts for a 6-MHz bandwidthand a flat-frequency response across thedesired band.Note: The bandpass ripple should be≤≤≤≤0.25 dB. The 6-MHz band shouldalso have a minimum of 20 dB returnloss across the pass band.Table 5-1 contains typical values for thebandpass filter.Table 5-1. Bandpass Filter Typical ValuesFREQUENCY INSERTION LOSS (DB) RETURN LOSS (DB)FV-4.5 ≥ 35FV-0.5 ≥ 20FV≤ 0.8 ≥ 20Fa≤ 0.8 ≥ 20FV+8.08 ≥ 15FV-9 ≥ 302FV≥ 305.11 (A6 and A7) Low-Band VHFAmplifier Tray (1198-1600; AppendixC)The (A6 and A7) low-band VHF amplifiertray (1198-1600) has been adjusted atthe factory to meet all specifications,including phase adjustment to match themultiple trays in an amplifier array whenthey are combined. The tray should notneed to be adjusted to attain normaloperation. Any adjustments to the boardsin this tray should be performed in theManual Gain position, with S1 on (A13)the AGC control board (1142-1601) inManual. The idling current for theamplifier boards are adjusted with no RFdrive applied. S1 should be in the AutoAGC position for the normal operation ofthe transmitter.Connect a dummy load with a rating of aleast 500 watts to J2, the RF output jackof the tray.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-75.11.1 (A5) AGC Control Board(1142-1601; Appendix D)Using a calibrated wattmeter, check thatthe tray is operating at the rated power.Remove the sample forward powerconnection J4 from the (A13) AGC controlboard (1142-1601). The output powerlevel should drop to 20% because of theVSWR cutback and DS4 should beilluminated. The front panel ModuleStatus LED should not be lit.Reconnect J4 and adjust R59 to begincutting back on the output power levelwhen the reflected level increases above20%.In the Power Supply Voltage position, thefront panel meter is calibrated to +48VDC using R86 on the AGC control board.5.11.2 (A1-A1) Phase Shifter Board(1198-1602; Appendix D)There are no adjustments to (A1-A1) thephase shifter board (1198-1602). Thefront panel has adjustments for phasethat are made during the amplifier arraysetup procedure.5.11.3 (A1-A2) VHF Filter/AmplifierBoard (1198-1606; Appendix D)The (A1-A2) VHF filter/amplifier board(1198-1606) has approximately 15 dB ofgain. Tune the channel filter capacitorsC29 and C20 (loading), C26 and C23(center frequency), and C24 (coupling) atJ6 on the board for the best response.Set voltage adjust pot R19 for +24 VDCat the anode of CR5.The idling current, no RF drive applied, ofthe device Q1 is set for 250 mA. To setthe current, remove the RF drive,measure the voltage across R16 (a 1Ωresistor on the filter/amplifier board) andadjust R13 for .25 volts (using Ohms’Law: [E=I x R] : [E=250 mA x 1 Ω] :E=250 mV).5.11.4 (A2-A1) Low-Band VHFAmplifier Board (1198-1605;Appendix D)The (A2-A1) VHF low-band amplifierboard (1198-1605) has 20 dB of gain andis biased for 3 amps of idling current, noRF drive applied. Adjust voltage adjustpot R10 for +24 VDC at pin 0 of theregulator IC U1. To set the bias, removethe RF drive from the board, measure thevoltage across R6 and R7 (two 1Ωresistors in parallel on the high-banddriver board), and adjust R4 for 1.5 volts(using Ohms’ Law: [E=I x R] : [E=3amps x .5 Ω] : E=1.5 volts).Connect a spectrum analyzer to outputjack J2 on the board and adjust C15 forpeak output.5.11.5 (A2-A2) Overdrive ProtectionBoard (1198-1601; Appendix D)The level of the RF input and output ofthe (A2-A2) overdrive protection board(1198-1601) should be +35 dBm duringnormal operation.To set up the overdrive circuit, check thatthe output power level of the transmitteris at 100% and adjust R11 on the boardfor a reading of .4 VDC at TP1. Increasethe output power level of the transmitterto 110%, sync only, and adjust R12 untilthe output power begins to drop off.Return the output power level of thetransmitter to 100%.5.11.6 (A2-A3) 3-Way Splitter Board(1198-1607 or 1198-1608; AppendixD)There are no tuning adjustments for (A2-A3) the 3-way splitter board. The boardtakes the +35 dBm input and splits it intothree equal +30 dBm inputs.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-85.11.7 (A3-A1, A3-A2, and A3-A3)Low-Band Amplifier Board (1198-1624 or 1198-1631; Appendix D)Each of the (A3-A1, A3-A2, and A3-A4)low-band amplifier boards have 20 dB ofgain and are biased at 2 amps of idlingcurrent when they are used as a visual,or visual + aural, amplifier, no RF driveapplied. To set the bias for the finalamplifier board (A3-A1), remove the RFdrive and switch the front panel currentmeter to the I1 position. Preset biasadjust pot R4 CCW. Slowly tune biasadjust pot R4 for 2 amps on the meterfor a visual, or visual + aural, amplifier.To set the bias for the final amplifierboard (A3-A2), remove the RF drive andswitch the front panel current meter tothe I2 position. Preset bias adjust pot R4CCW. Slowly adjust bias adjust pot R4 for2 amps on the meter for a visual, orvisual + aural, amplifier.To set the bias for the final amplifierboard (A3-A3), remove the RF drive andswitch the front panel current meter tothe I3 position. Preset bias adjust pots R4CCW. Slowly adjust bias adjust pot R4 for2 amps on the meter for a visual, orvisual + aural, amplifier.Connect a spectrum analyzer to outputjack J2 on each of the boards and adjustC1 on each board for peak output.5.11.8 (A4-A1) 3-Way CombinerBoard (1198-1625 or 1198-1626;Appendix D)There are no adjustments to the (A4-A1)3-way combiner board. The three +50dBm inputs are combined to produce the500 watts peak of sync output at J4 ofthe combiner board.5.11.9 Calibration of the Visual PlusAural Output Power and VSWRCutbackCheck that a dummy load of at least 500watts is connected to the output of thetray that is to be calibrated. Place switchS1 on the AGC control board in theManual position before beginning thesetup.To adjust the visual output power levels:1. Remove the J16 cable from (A5) thesync tip clamp/modulator board(1265-1302) in the exciter tray. SetManual AGC switch S1, on the (A13)AGC control board (1142-1601) in the600-watt amplifier, to the Manualposition. Turn the transmitter to theOperate position.2. Connect a sync and black test signalto the video input jack of the remoteinterface panel.3. Adjust manual gain pot R5 on theAGC control board for:• Sync + black 0 IRE setup;wattmeter=300 watts• Sync + black 7.5 IRE setup;wattmeter=270 wattsNote: The transmitter must have 40IRE units of sync.4. Obtain a zero span reference of thevisual-only carrier on a spectrumanalyzer. Replace the J16 connectoron the sync tip clamp/modulatorboard in the exciter tray. Adjust R5on the AGC control board until thesame visual reference is obtained.Adjust R44 on the AGC control boardfor 100% Forward Power.Lower the forward power reading to 80%on the front panel meter using R5, themanual gain adjust on the AGC controlboard. Adjust R65, the AGC fault adjuston the AGC control board, until the greenModule LED DS3 on the front panel justbegins to light. Use R5 to readjust theforward power to 100%.Switch off the tray and reverse the J8and J5 cables on the 3-way combinerenclosure.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-9Switch on the tray and adjust the frontpanel meter, in the Reflected OutputPower position, to a 100% reading usingR53, the reflected power meter adjust onthe AGC control board. Adjust thereflected output power to a 20% readingusing R5 on the AGC control board.Adjust R59, the VSWR cutback adjust onthe AGC control board, until the redVSWR Cutback LED DS4 on the frontpanel lights. This sets up the VSWRcutback circuitry.Readjust R5 for 100% on the meter toachieve a 500 watts peak of sync output+ 50 watts aural power. However, if thesystem requires less output power peramplifier tray, adjust each tray by thesame amount to give the desired totaloutput power.Switch off the tray and return the J5 andJ8 cables on the 3-way combinerassembly back to their original positions.If the tray was originally operating below100% output power, the AGC fault adjustwas set for 20% below the operational %Output Power of the tray. See the TestData Sheet for the transmitter for theactual readings for the tray. Place S1 onthe AGC control board in the AGCposition. This is the normal operatingposition after the setup is completed.There is a spare 1-amp and 10-amp fuseon the top, right-hand side of the tray.These are replacements for fuses on thecurrent metering board.The VHF amplifier tray is aligned,calibrated, and ready for normaloperation.5.12 Board Level AlignmentProcedures5.12.1 (Optional) 4.5-MHzComposite Input KitIf the (optional) 4.5-MHz composite inputkit is purchased, the tray is capable ofoperating by using either the 4.5-MHzcomposite input or the baseband audioand video inputs. The kit adds the (A24)composite 4.5-MHz filter board (1227-1244; Appendix D) and the (A25) 4.5-MHz bandpass filter board (1265-1307;Appendix D) to the transmitter. When the4.5-MHz intercarrier signal generated bythe 4.5-MHz composite input has beenselected by the 4.5-MHz composite inputkit, the 4.5-MHz generated by the auralIF synthesizer board is not used. Whenthe 4.5-MHz intercarrier signal generatedby the baseband video and audio inputswith baseband has been selected by the4.5-MHz composite input kit, thecomposite 4.5-MHz filter board and the4.5-MHz bandpass filter board are notused.The tray has been factory tuned andshould not need any alignments toachieve normal operation. To align thetray for the 4.5-MHz composite input,apply the 4.5-MHz composite input, withthe test signals used as needed, to thevideo input jack (J1 or J2 [loop-throughconnections]) on the rear of the tray.Select the 4.5-MHz composite input byremoving the baseband select from J18-6and J18-7 on the rear of the tray.To align the exciter using baseband videoand audio inputs, apply the basebandvideo, with the test signals used asneeded, to the video input jack (J1 or J2[loop-through connections]) and thebaseband audio to the proper basebandaudio input on the rear of the tray. Forbalanced audio input, connect TB1-1(+),TB1-2(-), and TB1-3 (GND). Forcomposite/stereo audio, connect thecomposite audio input jack (J3 or J13[loop-through connections]) and connecta baseband select from J18-6 and J18-7on the rear of the tray.5.12.2 Delay Equalizer Board (1227-1204; Appendix D)The jumper W1 on J5 of the sync tipclamp/modulator board, if present, mustbe in the Enable position between pins 2and 3.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-10Note: This board has been factorytuned and should not be retunedwithout the proper equipment.To tune this board:1. Connect a sinX/X test signal intojack J1-2 on the delay equalizerboard.2. Monitor the video output of theboard, at the video sample jack J2,with a video measuring set, such asthe VM700, adjusted to measuregroup delay.3. Tune the four stages of the boardusing the variable inductors (L1-L4)and potentiometers (R7, R12, R17,and R22) until the signal attains theFCC group delay curve. The stagesare arranged in order of increasingfrequency. Adjust R29, as needed,to attain the same level out of theboard as into the board.5.12.3 (A24) Composite 4.5-MHzFilter Board (1227-1244;Appendix D)This board is part of the 4.5-MHz inputkit and will only function properly with a4.5-MHz composite input signal and the4.5-MHz composite input selected. Toalign this board:1. Connect the test signal from anenvelope delay measurement set tothe video input of the tray at J1 orJ2.2. Connect an oscilloscope to jack J7,video out, between the J7 centerpin and pin 1 or 3 (ground). AdjustC21, frequency response, if needed,for the best frequency response.Adjust R32, video gain, for a signallevel of 1 Vpk-pk on theoscilloscope.The output at J6 and J7 on the boardshould be video only, without the 4.5-MHz aural subcarrier.5.12.4 (A25) (Optional) 4.5-MHzBandpass Filter Board (1265-1307;Appendix D)This board is part of the 4.5-MHz inputkit and will only function properly with a4.5-MHz composite input signal and the4.5-MHz composite input selected. Toalign this board:1. Adjust the filter with L2, C3, L4,and C7 for a frequency response ofno greater than ±0.3 dB from 4.4to 4.6 MHz.2. Adjust C19 for an overall peak-to-peak variation of less than ±0.3 dBfrom 4.4 MHz to 4.6 MHz.3. Recheck the frequency response; itmay have changed with theadjustment of the envelope delay.If necessary, retune the board.5.12.5 (A7) IF Carrier OvenOscillator Board (1191-1404;Appendix D)To align this board:1. While monitoring J3 with aspectrum analyzer, observe the45.75-MHz visual IF (typical +5dBm).2. Connect a frequency counter to J3and adjust C17 for 45.750000 MHz.3. Connect a frequency counter to J1and check for 50 kHz, which is theaural phase lock loop reference.5.12.6 (A5) Sync TipClamp/Modulator Board (1265-1302; Appendix D)To align this board:1. Determine if jumper W4 on jack J3is present. Jumper W4 terminatesthe video input into 75Ω. Removejumper W4 if a video loop-throughis required on the rear chassis atjacks J1 and J2.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-112. Set the controls R20, the white clip,R24, the sync clip, and R45, thesync stretch cut-in, to their fullCCW position. Set R48, the syncmagnitude, fully CW and place thejumper W7 on jack J4 to theClamp-Off, Disable, position.3. Connect a 5-step staircase videotest signal to the input of thetransmitter.4. Monitor TP2 with an oscilloscope.Adjust R12, the video gain pot, for1 Vpk-pk.5. Change the video input test signalto a multiburst test pattern. Whilemonitoring TP2, adjust C8 and R32for a flat-frequency response.Change the input video test signalback to the 5-step staircase.6. Monitor TP2 with an oscilloscope.Adjust pot R41, manual offset, for ablanking level of -0.8 VDC. Thewaveform shown in Figure 5-1should be observed. Move thejumper W2 on J4 to the ClampEnable position. Adjust pot R152,depth of modulation, for a blankinglevel of -0.8 VDC.Note: This waveform represents thetheoretical level for propermodulation depth. Step 9 belowdescribes how to set the modulationdepth through the use of a televisiondemodulator or a zero-spannedspectrum analyzer tuned to thevisual IF frequency.Figure 5-1. Waveform7. The following test setup is for theadjustment of the depth ofmodulation and ICPM at IF:A. Remove the cable that is onJ18 and connect the double-sideband, 45.75-MHz visual IFsignal from J18 to a 10-dBsplitter/coupler. Connect thecoupled port of thesplitter/coupler to the RF inputof a television demodulator.Connect the direct port to aspectrum analyzer.B. Connect the 75Ω video outputof the demodulator to the videoinput of a waveform monitor.For ICPM measurements, alsoconnect the quadrature outputof the demodulator to thehorizontal input of thewaveform monitor using a 250-kHz, low-pass filter. (Anoscilloscope can be used inplace of a waveformmonitor).C. Set the controls of thedemodulator to the following:
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-12Detector mode – ContSound trap – InZero carrier – OnAuto – SyncAudio source – SplitDe-emphasis – In8. Move jumper W7 on J4 to theClamp Disable position. Readjustpot R41, manual offset, for thecorrect depth of modulation byobserving the demodulatedwaveform on the waveform monitoror on the spectrum analyzer set tozero span.9. Check the demodulated video for aproper sync-to-video ratio (sync is28.6% of the total white videosignal). If sync stretch is needed,adjust R45, sync stretch cut-in,until sync stretch occurs. AdjustR48, sync stretch magnitude, forthe proper amount of stretch.Readjust R41, manual offset, ifneeded, for the correct depth ofmodulation.10. Move jumper W7 on J4 to theClamp Enable position. Readjust potR152, depth of modulation, for thecorrect depth of modulation.11. Set the waveform monitor todisplay ICPM. Preset R53 fully CCW,adjust C78 for the greatest effect atwhite on the ICPM display, and thenadjust R53 for minimum ICPM.12. Recheck the depth of modulationand, if necessary, adjust R152,depth of modulation.13. On a spectrum analyzer, adjust potR70 for a level of approximately -10dBm at J18.14. Remove the input video test signal.Place the front panel meter in thevideo position and, while monitoringthe meter, adjust pot R144, zeroadjust, for a reading of zero.15. Replace the input video testsignal (the 5-step staircase).Turn the front panel meter tothe video position and adjustR20 on the transmitter controlboard for a reading of 1 volt(10 on the 0 to 10 scale). Thisboard does not have syncmetering.16. Reconnect the plug to J18 andmove the spectrum analyzer testcable to the 41.25 IF output jackJ16. Tune C59 and L17 to L20 tomaximize the 41.25-MHz aural IFsignal and minimize the out-of-band products. Adjust pot R97 for-20 dBm at J16.17. Reconnect the plug to J16 andmove the spectrum analyzer testcable to IF output jack J20. PresetR62, the visual IF gain pot, to themiddle of the range. Insert amultiburst test signal into thetransmitter and observe the visualfrequency response with thespectrum analyzer set at 1dB/division. Tune R63 and C30, theIF frequency response adjustments,for a flat-frequency response (±0.5dB).18. While still monitoring J20 with aspectrum analyzer, readjust R62,visual IF gain, for a 0 dBm visualoutput level. Adjust R85, A/V ratio,for a minus 10 dB aural-to-visualratio or to the desired A/V ratio.Reconnect the plug to J20.19. Using an input video test signal (the5-step staircase) with 100 IREwhite level, monitor TP2 with anoscilloscope. Set control R24, thesync clip, just below the pointwhere sync clipping begins to occur.Similarly, set R20, the white clip, tojust below the point at which thewhite video begins to clip.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-135.12.7 (A4) Aural IF SynthesizerBoard, 4.5 MHz (1265-1303;Appendix D)1. To set up the test equipment forthis board:A. Connect the 600Ω balancedaudio output from an audiooscillator to the balanced audioinput terminals of the tray atTB1-1 (+), TB1-2 (-), and TB1-3 (ground) on the rear chassis.B. Connect the combined IFoutput at J21 (IF sample) onthe clamp modulator board tothe input of an IF splitter.Connect one output of thesplitter to the videodemodulator and the otheroutput to the spectrumanalyzer.C. At the front of thedemodulator, connect a shortcable from the RF-out jack tothe IF-in jack.D. Connect a cable from the 600Ωaudio output jack of thedemodulator to the input of anaudio distortion analyzer.2. Set the output frequency of theaudio oscillator to 400 Hz and theoutput level to +10 dBm.3. Center the aural carrier on thespectrum analyzer with thespectrum analyzer set to thefollowing:Frequency/Division – 10 kHzResolution bandwidth – 3 kHzTime/Division – 50 msecTrigger – Free runA. Adjust L5 for approximately+3.5 VDC at TP2.B. The green LED DS1 should beilluminated, indicating a lockedcondition. If not, retune L5 fora locked condition.4. Adjust R13, balanced audio gain, onthe aural IF synthesizer board for±25-kHz deviation.5. Check the distortion on the auraldistortion analyzer (THD=< 0.5%).6. Disconnect the 600Ω balancedaudio input to the tray. Connect a75Ω stereo audio input (400 Hz at 1Vpk-pk) to composite audio inputjack J3 on the rear of the tray.Follow the procedure in the stereogenerator instruction manual formatching the level of the generatorto the exciter. Use R17 to adjustthe composite audio gain.7. Check the distortion level on thedistortion analyzer (THD)=< 0.5%)5.12.8 (A8) ALC Board (1265-1305;Appendix D) (Part 1 of 2)Table 5-2 describes the functions of eachLED on the ALC board (A8).Table 5-2. ALC Board LEDsLED FUNCTIONDS1 (Red LED) Indicates that an abnormally low IF signallevel is present at IF input connector J1DS2 (Red LED)Indicates that the ALC circuit is unable tomaintain the level requested by the ALCreference due to excessive attenuation inthe linearity or the IF phase corrector signalpath or because jumper W3 on J6 is in
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-14manual gainDS3 (Red LED) Indicates a video loss faultDS4 (Red LED) Indicates that a Mute command is presentDS5 (Green LED) Indicates that the output from themodulator is selected as the input to theboard1. To align the ALC board, preset thefollowing controls on the tray:A. ALC Board (1265-1305)Connect jumper W1 on J4 todisable, between pins 2 and 3 (todisable linearity correctors).Connect jumper W3 on J6 tomanual, between pins 2 and 3 (formanual gain control).Adjust R87, manual gain pot, tomid-range.B. IF Phase Corrector Board (1227-1250)Move W2 on J9 to phasecorrection: enable. Move W3 onJ10 to amplitude correction:disable.2. The combined IF output of the synctip clamp modulator board is cabledto jack J32 of the ALC board.Remove J32 from the board, andlook to see if DS1, Input Fault, isilluminated. Reconnect J32 andmake sure that DS1 isextinguished.3. Jumper W3 on J6 should be in theManual position. Monitor jack J3with a spectrum analyzer.4. With a multiburst video signalpresent, tune C4 for a flat-frequency response of ±0.5 dB.5. Before proceeding with the secondpart of the ALC board alignment,check to see that the IF phasecorrector board (1227-1250) isfunctioning properly.5.12.9 (A9) IF Phase Corrector Board(1227-1250; Appendix D)See Section 5.4 of this chapter for thesystem alignment procedures for the IFphase corrector board. The signal levelinto the board should be approximatelythe same as the output of the board.The IF input jack of the IF phasecorrector board is fed from the J3 IF O/Pjack of (A8) the ALC board.The IF output jack of the IF phasecorrector board is fed to the J7 IF I/Pjack of the ALC board (A8).5.12.10 (A8) ALC Board, NTSC(1265-1305; Appendix D) (Part 2 of2)To align this board:1. Input a multiburst video test signal.Connect a spectrum analyzer toJ11. Tune C63 for a flat-frequencyresponse of ±0.5 dB.2. Move the Operate/Standby switchon the front panel to the Operateposition.3. Place jumper W3 on jack J6 in theManual mode and adjust R87 for0.5 volts at TP4.4. Place jumper W3 on J6 in the Automode and adjust the front panelpower adjust control A20 fully CW.If the (optional) remote powerraise/lower kit is present, thenadjust switch S1 on the board tomaximum voltage at TP4. AdjustR74, the range adjust, for 1 volt atTP4.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-155. Adjust the front panel power adjustcontrol A20 for 0.5 VDC at TP4. Ifthe (optional) remote powerraise/lower kit is present, moveswitch S1 on the board to mid-range and then adjust (A20) thefront panel power adjust control for0.8 VDC at TP4.6. Disconnect the plug that is on J12(IF output) and monitor J12 with aspectrum analyzer. Verify an outputof approximately 0 dBm. Ifnecessary, adjust R99 to increasethe output level. If less of an outputlevel is needed, move jumpers J27and J28 to pins 2 and 3 and thenadjust R99. Reconnect J12.7. Move W2 on J5 to the CutbackEnable position. Remove the inputvideo signal and verify that theoutput of the transmitter drops to25%. Adjust R71, the cutback level,if necessary. Restore the inputvideo.Note: The following step affects theresponse of the entire transmitter.8. Connect a video sweep signal to theinput of the tray. Monitor theoutput of the system with aspectrum analyzer. Adjust C71 withR103 and C72 with R106, asneeded, to flatten the response.C71 and C72 adjust for thefrequency of the correction notchbeing applied to the visual responseof the transmitter. R103 and R106are used to adjust the depth andwidth of the correction notch.9. Refer to Section 5.5 of this chapterfor the system alignmentprocedures for the linearitycorrectors. Controls R13, R18, andR23, the magnitude controls,should be set fully CW. ControlsR34, R37, and R40 are the linearitycut-in adjustments.5.12.11 (A14-A1) Channel OscillatorBoard, Dual Oven (1145-1201;Appendix D)This board is mounted in (A14) thechannel oscillator assembly (1145-1202).To align the board:1. Connect the main output of thechannel oscillator (J1) to aspectrum analyzer, tuned to thecrystal frequency, and peak tuningcapacitors C6 and C18 formaximum output. Tune L2 and L4for maximum output. The outputlevel should be about +5 dBm. Thechannel oscillator should maintainan oven temperature of 50° C.If a spectrum analyzer is notavailable, connect a digitalvoltmeter (DVM) to TP1 on the x4multiplier board. Tune capacitors C6and C18 for maximum voltage,then also tune L2 and L4 for amaximum voltage output at TP1.2. Connect the sample output of thechannel oscillator (J2) to a suitablecounter and tune C11, coarseadjust, and C9, fine adjust, to thecrystal frequency.Note: Do not repeak C6, C18, L2, orL4. This may change the outputlevel.Note: While adjusting C9 and C11 tothe crystal frequency, the peakvoltage monitored at TP1 of the x4multiplier board should notdecrease. If a decrease does occur,there may be a problem with thecrystal. Contact Axcera FieldSupport for further instructions.Note: If the VCXO board (1145-1204) in the VCXO assembly (1145-1206) is used, the fine-frequencyadjust C9 is not located on the VCXOboard. Use R9 on the FSK withEEPROM board.
500-Watt VHF Low Band Transmitter Chapter 5, Detailed Alignment Procedures325A, Rev. 0 5-163. Reconnect the main output (J1) ofthe channel oscillator to the input(J1) of the x2 multiplier.5.12.12 (A11-A1) x2 Multiplier Board(1172-1111; Appendix D)While monitoring the board with a DCvoltmeter, maximize each test pointvoltage by tuning the broadbandmultipliers in the following sequence:1. Monitor TP1 with a DVM and tuneC4 for maximum voltage. MonitorTP2 with a DVM and tune C6 formaximum voltage. Repeak C4 andC6 for maximum voltage.2. Connect a spectrum analyzer,tuned to two times the crystalfrequency, to the x2 multiplieroutput jack J2. While trying tokeep the out-of-band products toa minimum, monitor the outputand peak the tuning capacitors formaximum output.The output of the x2 multiplier connectsto (A11-A1) the filter/mixer board.5.12.13 (A11-A2) VHF Filter/MixerBoard (1153-1101; Appendix D)To align the board:1. Monitor J4, the LO output of theboard, with a spectrum analyzerand adjust C12 and C18 formaximum output (+14 dBm) atthe LO frequency and minimumout-of-band products. Adjust C13and C17 for the best frequencyresponse for the LO frequency.2. Adjust C3 and C6 to determinethe center frequency. Use C2 andC7 to locate the upper and lowerchannel-edge shaping. C4 is usedto determine the channelbandwidth.5.12.14 (A11-A3) Low-Band VHFFilter/Amplifier Board (1064251;Appendix D)The filter/amplifier board has beenfactory swept and adjusted for a 6-MHzbandwidth.Note: This board should not be tunedwithout the proper equipment.The filtered output connects to J1 of theboard and is amplified by U1 to anominal +8 dBm visual and -2 dBm aurallevel by adjusting R9. The output at J2 isfed to J4 on the A11 enclosure and fromthere to J15 on the rear of the tray.To align the board, use a multiburst orsweep video signal inserted into theexciter tray.Reconnect the cable from J6 to J1 on thefilter/amplifier board. Monitor J2, the RFoutput of the board, and peak C17 forthe maximum signal level. Tune manualgain adjust R9 for a +8 dBm peak visualoutput.This completes the detailed alignmentprocedures for the 325A transmitter. If aproblem occurred during the alignment,refer to the detailed alignment procedurefor that tray for more information.

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