Data Sheet Chapter 4

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3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures

835-3, Rev. 0 4-1

Chapter 4

Detailed Alignment Procedures

Before beginning the alignment procedures, make sure that the RF output at J2 of (A11)

the output coupler assembly of the transmitter is terminated into a dummy load of at

least 3000 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. The readings should be very similar. If one of the readings is off by a

significant degree, the problem is likely to be in that tray.

Switch on the UHF exciter tray circuit breaker on the AC distribution panel in the UHF

exciter cabinet and the circuit breaker on the rear of the UHF exciter tray for the 3-kW

transmitter. Also switch on the main and the individual amplifier circuit breakers on the

AC distribution panel in the amplifier array cabinet.

4.1 (A1) UHF Exciter Tray (1227-1200; Appendix A)

The (A1) UHF exciter tray has been factory tuned and should need no alignment to

achieve normal operation. Apply the baseband video and audio inputs of the test signals,

as needed, to the A/V input and remote interface panel on the rear of the transmitter.

4.1.1 (Optional) (A6) Delay Equalizer Board (1227-1204; Appendix B)

This board may not be present in the tray.

This board has been factory tuned and should not be re-tuned without the proper

equipment. To align this board:

•

Connect a sinX/X test signal into jack J1-2 on the delay equalizer board.

•

Monitor the video output of the board at video sample jack J2 with a video

measuring set VM700 adjusted to measure group delay.

•

Tune the four stages of the board using the variable inductors (L1-L4) and

potentiometers (R7, R12, R17, and R22) until the signal attains the FCC group delay

curve. The stages are arranged in order of increasing frequency. Adjust R29, if

needed, for the desired video gain.

4.1.2 (A7) IF Carrier Oven Oscillator Board (1191-1404; Appendix B)

While monitoring J3 with a spectrum analyzer, observe the 45.75 MHz visual IF (+5

dBm).

Connect a frequency counter to J3 and adjust C17 for 45.750000 MHz.

Connect a frequency counter to J1 and check for 50 kHz; this is the aural phase lock

loop reference.

4.1.3 (A5) Clamp/Modulator Board (1227-1208; Appendix B)

Determine if jumper W1 on jack J3 is present. Jumper W1 terminates the video input

into 75 ohms. Remove jumper W1 if video loop-through is required on the rear chassis

jacks J1 and J2.

Summary of content (13 pages)

PAGE 1
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Chapter 4 Detailed Alignment Procedures Before beginning the alignment procedures, make sure that the RF output at J2 of (A11) the output coupler assembly of the transmitter is terminated into a dummy load of at least 3000 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. The readings should be very similar.
PAGE 2
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Set the controls R28, the white clip; R31, the sync clip; and R43, the sync stretch, to their fully counter-clockwise (CCW) position. Note: On Revision 6 or later boards, controls R43, sync stretch cut-in, and R176, sync stretch magnitude, should be set fully CCW. Place jumper W5 on jack J11 to the clamp disable position (towards the top of the board) and jumper W2 on jack J4 to the AGC disable (out) position.
PAGE 3
-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Monitor pin 13 of IC U6 (or TP7 on Revision 1 or later boards) with an oscilloscope. The waveform shown in Figure 4-3 should be observed. Figure 4-3. Waveform Observed When Pin 13 of IC U6 is Monitored Restore the input video level to 1 Vpk-pk. Monitor TP5 (on Revision 1 or later boards) or pin 2 of J7 (terminated into 75 ohms) with an oscilloscope. Adjust pot R105, manual bias, for a blanking level of -0.8 VDC.
PAGE 4
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures splitter/coupler to the RF input of a television demodulator. Connect the direct port to a spectrum analyzer. • Connect the 75-ohm video output of the demodulator to the video input of a waveform monitor. For ICPM measurements, also connect the quadrature output of the demodulator to the horizontal input of the waveform monitor using a 250-kHz low pass filter (an oscilloscope can be used in place of a waveform monitor).
PAGE 5
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures spectrum analyzer set at 1 dB/division. Tune R153 and C77, the IF frequency response adjustments, for a flat-frequency response (±0.5 dB). While still monitoring J20 with a spectrum analyzer, re-adjust R150, visual IF gain, for a 0-dbm visual output level. Adjust R127, A/V ratio, for a –13-dB aural-to-visual ratio or to the required A/V ratio. Reconnect the plug to J20.
PAGE 6
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures matching the level of the generator to the exciter. R17 adjusts the composite audio gain. Check the distortion level on the distortion analyzer (S/B =< 0.5%). 4.1.
PAGE 7
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Input a multiburst video test signal. Remove the plug from J8 and connect a spectrum analyzer. Tune C63 for a flat-frequency response of ±0.5 dB. Reconnect the plug to J8. Move the Operate/Standby switch on the front panel to the Operate position. Place jumper W3 on jack J6 in the Manual mode (towards the top of the board) and adjust R87 for 0.8 volts at J19, pin 2, or at TP4.
PAGE 8
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures If a spectrum analyzer is not available, connect a digital voltmeter (DVM) to TP1 on the x8 multiplier board. Tune capacitors C6 and C18 for maximum voltage and then also tune L2 and L4 for maximum voltage output at TP1. Connect the sample output of the channel oscillator, J2 to a suitable counter and tune C11, coarse adjust, and C9, fine adjust, to the crystal frequency.
PAGE 9
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures The board is adjusted to give a peak-detected output indication to the front panel meter for combined output, visual-only output, aural-only output, and reflected output. The board should need no adjustments to achieve normal operation. 4.1.12 (A3-A2) +12-VDC(4A)/-12-VDC(1A) Power Supply Board (1227-1203; Appendix B) This board is mounted in (A3) the +12V(4A)/-12V(1A) power supply assembly (12271210). The board has no adjustments.
PAGE 10
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Connect a sweep test signal to J1, the RF input jack of the 600-watt amplifier tray and monitor the output of the board at J2 with a padded-input spectrum analyzer. Tune capacitors C104 and C204 for peak output and then tune C106 and C206 for peak output power with a flat-frequency response at J2. 4.2.5 (A4-A2) Coupler Board Assembly (1227-1316; Appendix B) There are no adjustments to this board. 4.2.
PAGE 11
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures the (A5-A3) amplifier board, tune capacitors C104 and C204 for peak output power. Tune C106 and C206 for peak output power with a flat-frequency response and minimum current. To adjust the idling currents, no RF applied to the tray, of the devices on the (A5-A4) amplifier board, connect a voltage meter across E1 and E2 on the amplifier protection board. Move switch S1 to the I10 position and adjust R106 for a reading of 2.5 mV.
PAGE 12
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures Move switch S1 on the amplifier control board to the Manual position. Insert a visualonly with sync-only test signal to the system and verify 40 IRE units of sync. Adjust R16 for 360 watts on the wattmeter. Monitor a sample of the output with a spectrum analyzer set to the zero-span position and adjust the spectrum analyzer so that the level of the output is at the top line of the gaticule.
PAGE 13
3-kW UHF Transmitter Chapter 4, Detailed Alignment Procedures For major tuning, such as changing the notch depth or moving the notch frequency more than 1 MHz, the outer conductor and the center conductor of the trap section must both be moved. This requires an RF sweep generator to accomplish. Apply the sweep signal to the input of the trap filter and monitor the output.