Technical Service Bulletin 030523

Technical Service Bulletin 030523 Setup of IOX Processing

Procedure 030523: Setup of IOX Video Processing Applicability All IOX transmitters. Prerequisites Correct setup of NTSC test equipment (030626). Equipment Required Consult Service Bulletin 030626. SMB to BNC adapter. Comments Video setup for type 45 323 526 exciter. Adaptation of procedures in B2 and B3 section of exciter manual.

This bulletin contains a procedure to check the set-up of the video processing stages of the IOX exciter. It is always a good idea to verify correct video set-up before proceeding to adjust any other exciter controls.

To check B2 module frequency response The B2 module is essentially an internal video proc amp that should ideally pass a flat video signal from input to output when all precorrections are disabled. 1. Connect 75 ohm cable from video sample port J12 at rear of exciter to unused input of waveform monitor. Terminate video feed with 75 ohm video termination on other (loop- thru) port of waveform monitor. 2. Apply multiburst test signal to transmitter. 3. Disable and corrections by setting exciter B2 module switches B2 - 13 and B2 - 23 in the OFF positions. 4. Observe frequency response of B2 module on waveform monitor. If frequency response is flat, skip to step 9. 5. If frequency response is not flat, double-check frequency flatness of video generator and test cables by connecting input and output test cables end-to-end with a BNC barrel (exciter bypassed). If frequency response is not flat, check for use of incorrect cable type (50 ohm vs. 75 ohm) or possibly a poor connection. Correct errors in test and measurement set up and return to step 5. 6. If test equipment / test cable frequency response is flat, but B2 module response is not, proceed to fine tune B2 module frequency response by means of internal trimmer adjustments: a. While monitoring test point B2 - TP20 with an oscilloscope, adjust trimmer capacitor B2 - C11 (inside B2 module), as necessary, to compensate for any minor frequency roll off effects.

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b. While monitoring test point B2 - TP21 with an oscilloscope, adjust trimmer capacitor B2 - C167 (inside B2 module), as necessary, to compensate for any minor frequency rolloff effects.

7. If trimmer capacitor C11 is unable to achieve flat multiburst video response at TP21, proceed to verify flatness of receiver group delay corrector by setting exciter switches B2GD - S1 = OFF (left) and B2GD - S2 = OFF (left) (located behind holes in side cover of exciter B2 module…module must be removed from the exciter to gain access to switches). If frequency response problem is corrected by deactivating receiver group delay corrector, contact Comark for further details. Otherwise, reactivate corrector. 8. If video response problem remains, deactivate group delay corrector by placing jumper B2-GD TB2 in positions 1 - 2 and jumper B2-GD TB6 in positions 2 - 3 on group delay daughter board (internal to B2 module). If frequency response problem is corrected by deactivating receiver group delay corrector, contact Comark for further details.

NOTE: Diplexer group delay correction is essential for those transmitters with separate amplification (externally diplexed), but often unnecessary for those transmitters with common amplification (no external diplexer to correct). The diplexer group delay corrector is often left deactivated in IOX transmitters with common amplification.

To establish proper video levels in B3 luminance regulator The video signal makes a detour to the B3 module luminance regulator. This circuit fixes the video white level to 100IRE, either automatically if the VITS test line is present, or manually if the test line is absent. The output of the luminance regulator must be set to 100IRE at white before the depth of modulation may be adjusted. 9. Apply luminance stairstep test signal to transmitter. 10. Place exciter switch B3 - 8 in the MAN position to disable video AGC. 11. Using SMB to BNC adapter, connect video output of exciter B3 module appearing at B3 - J7 (side of B3 module) to waveform monitor input.

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12. Adjust exciter potentiometer B3 - 5 to obtain correct video levels on waveform monitor. 13. Place exciter switch B3 - 8 in the AUTO position to activate video AGC. 14. Adjust exciter potentiometer B3 - 7 to obtain correct video levels on waveform monitor if the NTSC test line is not present.

NOTE: The NTSC test line is defined as a 100 IRE white bar on Field 1, Line 17 of the vertical interval. The line selection may be changed to other values via the selection jumpers E1, E2 located inside of the exciter B3 module. Consult B3 module section exciter manual for further details.

15. Adjust exciter potentiometer B3 - 9 to obtain correct video levels on waveform monitor if the NTSC test line is present. 16. Place exciter switch B3 - 8 in the MAN position to disable video AGC, if not desired. 17. Reestablish proper connection to exciter connector B3 - J7.

To set depth of modulation and sync level The depth of modulation adjustment sets the DC offset applied to the video before IF modulation. It is imperative that the correct video levels be verified in the previous section before attempting to modify this adjustment. The sync level adjustment sets the amplitude of the re-inserted sync pulse. 18. Connect RF sample from EXC SAMPLE port at rear of transmitter system cabinet to RF input of television demodulator. 19. Connect waveform monitor input to video output of television demodulator. 20. Activate zero carrier pulse (chopper) on television demodulator. 21. Adjust exciter potentiometer B2 - 30 (depth of modulation) to establish proper 100IRE white level on waveform monitor. Adjust vertical scale of waveform monitor, as necessary, to maintain zero carrier chopper aligned to 120IRE level.

NOTE: As an alternative, the DC offset may be checked directly using an oscilloscope connected to video sample from connector B2 - J7. Note that a BNC tee and 75 ohm video termination must be used to terminate the sample cable at the oscilloscope input. When viewed on an oscilloscope, the potentiometer B2 - 30 is adjusted offset the 100IRE level to 0.114 volts above ground. See illustration immediately below.

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22. Adjust exciter potentiometer B2 - 6 to obtain 40IRE of sync on waveform monitor. 23. Readjust exciter potentiometers B2 - 30 and B2 - 6 in iterative fashion until correct sync amplitude and white amplitude are achieved simultaneously.

To set white clipper threshold The white clipper limits video excursions above a user-established threshold. This prevents the transmitted RF waveform from approaching zero carrier and causing poor reception (audio buzz) during scenes with heavy white video content. 24. Place exciter switch B3 - 8 in the MAN position to disable video AGC. 25. Adjust exciter potentiometer B3 - 5 to obtain 120 IRE at maximum white on waveform monitor. (See left photo) 26. Adjust exciter potentiometer B2 - 4 until white clipper limits maximum white level to 110 IRE. (See right photo)

27. Readjust exciter potentiometer B3 - 5 to establish a 100 IRE maximum white level.

Other Checks Once proper set-up of the video portions of the exciter has been verified, perform these additional checks, as necessary.

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28. Verify correct horizontal interval timing with waveform monitor. The video timing is fixed and does not generally require adjustment. 29. Using multiburst video test signal, verify correct frequency response of entire exciter via waveform monitor and television demodulator connected to RF sample port J4 (rear panel of exciter). If frequency response is not acceptable, consult exciter manual schematics for further information on suitable test points for troubleshooting. Temporarily disable B1 module corrections via exciter switches B1 - 8, 13, and 23 to observe effect on exciter frequency response and possibly locate origin of problem. 30. Procedure complete. Here at Comark, we are constantly striving to improve the satisfaction of both our new and existing customers. Please do not hesitate to contact Comark Customer Service with any questions you may have concerning the contents of this service bulletin.

Comark Communications LLC 104 Feeding Hills Road Southwick, MA 01077 U.S.A. (800) 345-9295 www.comarktv.com

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