Data Sheet Chapter 5

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500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures

425A, Rev. 0 5-1

Chapter 5

Detailed Alignment Procedures

The 425A transmitter was aligned at the

factory and should not require additional

alignments to achieve normal operation.

This transmitter operates using the

baseband audio and video inputs or, if

the (optional) 4.5-MHz composite input

kit is purchased, either a single

composite video + 4.5-MHz input or

separate baseband video and audio

inputs.

Check that the RF output at J2 of (A9-A5)

the coupler is terminated into a dummy

load of at least 500 watts. While

performing the alignment, refer to the

Test Data Sheet for the transmitter and

compare the final readings from the

factory with the readings on each of the

trays. They should be very similar. If a

reading is off by a significant amount, the

problem is likely to be in that tray.

Switch on the main AC and the VHF

exciter circuit breakers on the AC

distribution panel behind the rear cabinet

door.

5.1 (A4) High-Band VHF Exciter Tray

(1070901; Appendix C) with

Baseband Video and Audio Inputs

The (A4) high-band VHF exciter tray

(1070901)

has adjustments for video

levels, audio modulation levels, and other

related parameters.

Connect an NTSC baseband video test

signal input (1 Vpk-pk) to the transmitter

video input jack J2 on the (A12) remote

interface panel. Jacks J1 and J2 on the

VHF exciter tray are loop-through

connected and the unused jack can be

used as a video source for another

transmitter by removing jumper W4 on

jack J3 on (A5) the sync tip clamp

modulator board (1265-1302). Connect a

baseband audio input (+10 dBm) to the

balanced audio input terminal block TB1-

1 (+), TB1-2 (-), and TB1-3 (ground). If

stereo/composite audio is provided,

connect it to BNC jack J6, the composite

audio input jack on the remote interface

panel. Jacks J3 and J13 on the rear of the

exciter panel are loop-through connected

and the unused jack can be used as an

audio source for another transmitter by

removing jumper W1 on jack J15 on the

aural IF synthesizer.

Look at the front panel meter on the VHF

exciter tray. In the Video position, the

meter indicates active video from 0 to 1

Vpk-pk. The normal video input level is 1

Vpk-pk on the meter. If this reading is

not at the proper level, the overall video

level can be changed by adjusting video

level control R12 on the sync tip clamp/

modulator board.

Switch the meter to the Audio position to

show the audio deviation (modulation

level) of the signal from 0 to 100 kHz.

The aural IF synthesizer board was

factory set for a ±25 kHz deviation with a

balanced audio input of +10 dBm. If the

reading is at not the correct level, adjust

balanced audio gain pot R13 on the aural

IF synthesizer board, as needed, to attain

the ±25 kHz deviation. The aural IF

synthesizer board was factory set for a

±75 kHz deviation with a composite

audio input of 1 Vpk-pk. If this reading is

not correct, adjust composite audio gain

pot R17 on the aural IF synthesizer

board, as needed, for the ±75 kHz

deviation.

5.2 (A4) VHF Exciter Tray (1070901;

Appendix C) with the 4.5-MHz

Composite Input Kit

With the 4.5-MHz composite input kit,

the (A4)

VHF exciter tray (1070901)

able to operate using either the separate

video and audio baseband inputs or the

single 4.5-MHz composite input. The 4.5-

MHz composite input kit includes a

Summary of content (17 pages)

PAGE 1
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures Chapter 5 Detailed Alignment Procedures The 425A transmitter was aligned at the factory and should not require additional alignments to achieve normal operation. 1 (+), TB1-2 (-), and TB1-3 (ground). If stereo/composite audio is provided, connect it to BNC jack J6, the composite audio input jack on the remote interface panel.
PAGE 2
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures composite 4.5-MHz filter board (12271244) and a 4.5-MHz bandpass filter board (1265-1307). amplifier trays. The upconverter section also includes adjustments to the local oscillator chain tuning and the local oscillator center frequency tuning. Both of these were completed at the factory and should not require adjustments at this time.
PAGE 3
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures the two amplifier on/off circuit breakers on the AC input assembly in the rear of the cabinet and switch the transmitter to Operate. Adjust the power adjust gain pot on the front panel of the VHF exciter tray to obtain +0.8 VDC on the front panel meter in the ALC position. On the ALC board (1265-1305), move jumper W3 on J6 to the Manual position, between pins 2 and 3, and adjust R87 on the ALC board for +0.
PAGE 4
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures To adjust the linearity correctors from scratch, ensure that the transmitter is operating at full power with the desired A/V ratio. Check that jumper W1 on J4 of the ALC board is enabled between pins 1 and 2. Make sure that the ALC voltage is set to +0.8 VDC as monitored on the front panel meter in the ALC position. distortion as displayed on the waveform monitor should be no more than 1 IRE.
PAGE 5
00-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures procedure and replace the 2-inch length of cable with a 4-inch length of cable. Once a peak has been reached, move the phase control that is fully CCW up two turns and repeak using the phase control on the other tray. This allows both trays to have some range of adjustment. Adjust R28, visual calibration, on (A19) the visual/aural metering board (12651309) for 100% on the front panel meter in the % Visual Output position.
PAGE 6
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures 5.9 (A11) 2-Way Combiner Assembly (1219-1006; Appendix C) assembly, which is the combined output of the VHF amplifier trays. The filter is made of aluminum waveguide and has five resonant cavities. The filter has five bolts for tuning adjustments. The bandpass filter also utilizes two integral traps at -4.5 MHz and +9 MHz from FV. Refer to the bandpass filter drawing for the location of the adjustments.
PAGE 7
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures board (1142-1601). The output power level should drop to 20% because of the VSWR cutback and DS4 should be illuminated. The front panel Module Status LED should not be lit. pot R10 for +24 VDC at pin 0 of the regulator IC U1. To set the bias, remove the RF drive from the board, measure the voltage across R6 and R7 (two 1Ω resistors in parallel on the high-band driver board), and adjust R4 for .30 volts (using Ohms’ Law: [E=I x R] : [E=.
PAGE 8
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures C19 for peak output with the best linearity across the spectrum. pot R5 for 1 amp on the meter for a visual, or visual + aural, amplifier or .5 amps on the meter for an aural amplifier. Adjust R9 for 2 amps on the meter for a visual, or visual + aural, amplifier or 1 amp for an aural amplifier. 5.11.7 (A3-A3) 4-Way Splitter Board (1219-1101; Appendix D) There are no tuning adjustments for (A3A3) the 4-way splitter board (12191101).
PAGE 9
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures 600-watt amplifier, to the Manual position. Turn the transmitter to the Operate position. Readjust R5 for 100% on the meter to achieve a 300 watts peak of sync and sound output. However, if the system requires less output power per amplifier tray, adjust each tray by the same amount to give the desired total output power. 2. Connect a sync and black test signal to the video input jack of the remote interface panel. 3.
PAGE 10
500-Watt VHF Transmitter by the baseband video and audio inputs with baseband has been selected by the 4.5-MHz composite input kit, the composite 4.5-MHz filter board and the 4.5-MHz bandpass filter board are not used. The tray has been factory tuned and should not need any alignments to achieve normal operation. To align the tray for the 4.5-MHz composite input, apply the 4.
PAGE 11
500-Watt VHF Transmitter 1. Adjust the filter with L2, C3, L4, and C7 for a frequency response of no greater than ±0.3 dB from 4.4 to 4.6 MHz. 2. Adjust C19 for an overall peak-topeak variation of less than ±0.3 dB from 4.4 MHz to 4.6 MHz. 3. Recheck the frequency response; it may have changed with the adjustment of the envelope delay. If necessary, retune the board. 5.12.5 (A7) IF Carrier Oven Oscillator Board (1191-1404; Appendix D) To align this board: 1.
PAGE 12
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures Figure 5-1. Waveform 7. The following test setup is for the adjustment of the depth of modulation and ICPM at IF: A. B. C. Remove the cable that is on J18 and connect the doublesideband, 45.75-MHz visual IF signal from J18 to a 10-dB splitter/coupler. Connect the coupled port of the splitter/coupler to the RF input of a television demodulator. Connect the direct port to a spectrum analyzer.
PAGE 13
500-Watt VHF Transmitter 13. On a spectrum analyzer, adjust pot R70 for a level of approximately -10 dBm at J18. 14. Remove the input video test signal. Place the front panel meter in the video position and, while monitoring the meter, adjust pot R144, zero adjust, for a reading of zero. 15. 16. 17. 18. 19. Replace the input video test signal (the 5-step staircase).
PAGE 14
500-Watt VHF Transmitter Chapter 5, Detailed Alignment Procedures Time/Division – 50 msec Trigger – Free run A. Adjust L5 for approximately +3.5 VDC at TP2. B. The green LED DS1 should be illuminated, indicating a locked condition. If not, retune L5 for a locked condition. 4. Adjust R13, balanced audio gain, on the aural IF synthesizer board for ±25-kHz deviation. 5. Check the distortion on the aural distortion analyzer (THD=< 0.5%). 6. Disconnect the 600Ω balanced audio input to the tray.
PAGE 15
500-Watt VHF Transmitter 3. Jumper W3 on J6 should be in the Manual position. Monitor jack J3 with a spectrum analyzer. 4. With a multiburst video signal present, tune C4 for a flatfrequency response of ±0.5 dB. 5. Chapter 5, Detailed Alignment Procedures raise/lower kit is present, then adjust switch S1 on the board to maximum voltage at TP4. Adjust R74, the range adjust, for 1 volt at TP4. 5. Adjust the front panel power adjust control A20 for 0.5 VDC at TP4.
PAGE 16
500-Watt VHF Transmitter R23, the magnitude controls, should be set fully CW. Controls R34, R37, and R40 are the linearity cut-in adjustments. Chapter 5, Detailed Alignment Procedures 1206) is used, the fine-frequency adjust C9 is not located on the VCXO board. Use R9 on the FSK with EEPROM board. 5.12.11 (A14-A1) Channel Oscillator Board, Dual Oven (1145-1201; Appendix D) 3. This board is mounted in (A14) the channel oscillator assembly (1145-1202). To align the board: 5.12.
PAGE 17
500-Watt VHF Transmitter the LO frequency and minimum out-of-band products. Adjust C13 and C17 for the best frequency response for the LO frequency. 2. Adjust C3 and C6 to determine the center frequency. Use C2 and C7 to locate the upper and lower channel-edge shaping. C4 is used to determine the channel bandwidth. 5.12.14 (A11-A3) High-Band VHF Filter/Amplifier Board (1064252; Appendix D) The filter/amplifier board has been factory swept and adjusted for a 6-MHz bandwidth.