Standard Communications mt930b, Service Manual

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Maintenance
MT930B Service Manual Page 6-37
5. Turn off composite de-emphasis, and observe the composite output wave-
form on an oscilloscope. Be sure to connect a terminal resistor near the os-
cilloscope’s input terminal for this measurement.
6. Using the marker position as a reference, adjust C311 and R311 so the am-
plitude of 1 to 8 MHz is generally flat and the amplitude at 10 MHz is 110%
to 80% compared to that at 1 MHz.
7. Change the video signal to one that includes an NTC-7 combination. Set
both the pre-emphasis and de-emphasis to 525, adjust the video level, then
observe the video waveform for the composite output.
8. Set the C/N to about 10 dB so noise appears on the TV monitor. Adjust
R307 so the white noise and black noise are at about the same level.
9. Adjust R255 so chrominance-luminance (C/L) intermodulation is optimized.
Reference:
If the frequency response of the RF converter, etc., is faulty, problems such as
sparkle noise balance trouble will appear unless the minimum points for Ku and
C are made to match, using the equipment in step 8, above. It is also OK to ad-
just R307 for balancing when other noise-balance and C/L intermodulation
equipment is off. When drastically off, be sure to check the RF converter, etc.
There is a risk of trouble, such as dynamic threshold problems, when R255 is
turned too far CCW, and similarly, S/N problems (increased noise at 1 MHz and
under) during weak C/N when R255 is turned too far CW. Make sure to check
that the RF converter, etc., are not faulty and that there has not been a mistaken
operation.
During step 7, above, adjust R311 to the spot that raises the amplitude of the
high band before adjusting C311.
Note that in step 6, above, the correct waveform is not displayed because of the
effects of the cable if a terminal resistor is not connected or if the position is in-
appropriate.
Video Level/Sync Chip Clamping Adjustment
Adjustment Procedure:
1. Set the pre-emphasis to 525 and frequency deviation to 10 MHz p-p.
2. Connect a video analyzer to VIDEO #2, and observe the output level.
3. Set the video level to 75, and adjust RV18 so the bar level is 100
± 1 IRE.
4. Adjust RM02 so the pedestal level of the video output is within 0 ± 0.5 V.
5. Connect the video analyzer to COMPOSITE OUT, and observe the output
level.
6. Set the gang and emphasis for the composite level to ON, and adjust RG05
so the bar level is 100 ± 1 IRE.
7. Turn off pre-emphasis, set the UUT emphasis OFF, and adjust RG02 so the
bar level is 100 ± 1 IRE.
Reference: