Data Sheet Chapter 4

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100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures

420A, Rev. 0 4-1

Chapter 4

Detailed Alignment Procedures

If this transmitter contains the (optional)

4.5 MHz composite input kit, the

baseband video input is used when the

baseband video is connected to J2 and a

baseband select is connected to J18 pins

6 and 7 on the rear of the tray.

Check that the RF output at J15 of the

transmitter is terminated into a dummy

load of at least 100 watts. While

performing the alignment, refer to the

Test Data Sheet for the transmitter and

compare the final readings from the

factory with the meter readings on the

tray. They should be very similar. If a

reading is more than 10% different, a

problem with the tray may exist. Switch

on the on/off AC circuit breaker on the

rear of the tray.

4.1 Alignment of the 420A

Transmitter

4.1.1 Baseband Section

This tray has adjustments for video and

audio modulation levels and other related

parameters.

Connect an NTSC composite video test

signal input (1 Vpk-pk) or an (optional)

4.5 MHz composite input to the

transmitter video input jack J2 on the

rear of the tray.

Note: J2 is a loop-through connected

to J1. It can be used as a baseband

video source if the jumper W1 on J3

of the sync tip clamp/modulator

board is removed.

Connect the baseband audio, if it is

balanced audio (+10 dBm), to the

terminal block (TB1) or connect the

composite audio (stereo) (1 Vpk-pk) to

the BNC jack (J13).

Note: J13 is a loop-through

connected to J3. It can be used as a

composite audio source if the jumper

W4 on J12 of the aural IF

synthesizer board is removed.

The baseband audio is not used with the

(optional) 4.5 MHz composite input.

Observe the front panel meter on the

tray. In the video position, the meter

indicates active video from ≅ 0.3V to

1.0V. With an input video of 1 Vpk-pk,

the display should indicate 100 IRE units

at white. If the reading is at not the

proper level, the overall video level can

be changed by adjusting the video level

control (R15) on (A5) the sync tip

clamp/modulator board (1265-1302).

Switch the meter to the audio position

that will indicate the audio deviation

(modulation level) of the signal; the

meter indicates from 0 to 100 kHz. The

aural IF synthesizer board was factory

set for ±25 kHz deviation with a balanced

audio input of +10 dBm. If the reading is

not the correct level, adjust the 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

±75 kHz deviation with a composite

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

not correct, adjust the composite audio

gain pot R17 on the aural IF synthesizer

board, as needed, for the ±75 kHz

deviation.

Summary of content (13 pages)

PAGE 1
00 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures Chapter 4 Detailed Alignment Procedures If this transmitter contains the (optional) 4.5 MHz composite input kit, the baseband video input is used when the baseband video is connected to J2 and a baseband select is connected to J18 pins 6 and 7 on the rear of the tray. Note: J13 is a loop-through connected to J3. It can be used as a composite audio source if the jumper W4 on J12 of the aural IF synthesizer board is removed.
PAGE 2
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures 4.1.2 IF/Upconverter Section • The upconverter section of the tray includes adjustments for automatic level control (ALC), linearity (amplitude predistortion), and incidental phase (phase change vs. level) pre-distortion for correction of the non-linearities of the RF amplifier section of the tray.
PAGE 3
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures corresponding non-linear distortions of the amplifier that is driven to place the sync level near saturation. stretch the signal above the white region. Back off on the corresponding magnitude control R13 as required. Next, advance the second threshold control R37 to stretch the signal above the black range and then back off on the magnitude control R18 as required. Adjust the third threshold pot R40 to stretch sync.
PAGE 4
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures R3 threshold for the cut-in point of the correction and the R7 magnitude for the amount of the correction that is needed. The jumper W1 on J8 is set to give the desired polarity of the correction shaped by the threshold R11 and magnitude R15 adjustments. After setting the polarity, adjust the R11 threshold for the cut-in point of the correction and the R15 magnitude for the amount of the correction that is needed.
PAGE 5
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures 4.2 Board Level Alignment Procedures 4.2.2 Delay Equalizer Board (12271204) 4.2.1 (Optional) 4.5 MHz Composite Input Kit The jumper W1 on J5 of the sync tip clamp/modulator board, if present, must be in the Enable position between pins 2 and 3. If the (optional) 4.5 MHz composite input kit is purchased, the tray is capable of operating by using either the 4.5 MHz composite input or the baseband audio and video inputs.
PAGE 6
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures level of 1 Vpk-pk on the oscilloscope. 1. Determine if jumper W4 on jack J3 is present. Jumper W4 terminates the video input into 75Ω. Remove jumper W4 if a video loop-through is required on the rear chassis at jacks J1 and J2. 2. Set the controls R20, the white clip, R24, the sync clip, and R45, the sync stretch cut-in, to their full counter clockwise (CCW) position.
PAGE 7
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures Figure 4-1. 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 double sideband 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 8
100 Watt High Band VHF Transmitter 13. On a spectrum analyzer, adjust the 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. Chapter 4, Detailed Alignment Procedures the sync clip, just below the point where sync clipping begins to occur.
PAGE 9
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures Trigger – Free run 4. 5. 6. 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. 75Ω stereo audio input (400 Hz at 1 Vpk-pk) to the composite audio input jack J3 on the rear of the tray. Follow the procedure in the stereo generator instruction manual for matching the level of the generator to the exciter.
PAGE 10
0 Watt High Band 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 flat frequency response of ±0.5 dB. 5. Chapter 4, Detailed Alignment Procedures remote power 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 11
100 Watt High Band VHF Transmitter correctors. Controls R13, R18, and R23, the magnitude controls, should be set full CW. Controls R34, R37, and R40 are the linearity cutin adjustments. 4.2.11 (A14-A1) Channel Oscillator Board (1145-1201) This board is mounted in (A14) the channel oscillator assembly (1145-1202). To align the board: 1. Connect the main output of the channel oscillator (J1) to a spectrum analyzer, tuned to the crystal frequency, and peak the tuning capacitors C6 and C18 for maximum output.
PAGE 12
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures 4.2.14 (A11-A2) Filter/Amplifier Board, High Output (1064150) amps on the meter and then adjust R9 for 1.5 amps total current on the meter. To set the bias for the final amplifier board (A23-A4), the RF drive will need to be removed. To monitor the current adjustment, read the voltage drop across the 3-watt resistor, R7, mounted on the overcurrent protection board (12731130).
PAGE 13
100 Watt High Band VHF Transmitter Chapter 4, Detailed Alignment Procedures all three amplifier boards for peak power and frequency response. 4.2.19 (A3) +12 VDC (4A)/-12 VDC (1A) Power Supply Board (12651312) 4.2.17 (A29) Overcurrent Protection Board (1273-1130) There are no adjustments to this board. 4.2.