Standard Communications mt930b, Service Manual

Page: 70 / 155

Maintenance
Page 6-38 Service Manual MT930B
There are two methods of measuring the pedestal level in step 4, above. One
uses an oscilloscope, while the other uses video analyzer VM-700A. Set the os-
cilloscope to DC input before using it. When using the VM-700A, make settings
in the order of waveform, menu, clamp-couple, and DC coupling.
Note that the value will not be saved after setting the video level to 75 in step 3,
above unless a save operation is performed, and that the video level will not be-
come 75, even when composite is set to gang.
Video De-emphasis Adjustment
Adjustment Procedure:
1. Set the video signal to NTSC, pre-emphasis to 525, and frequency dis-
placement to 27.6 MHz p-p.
2. Set the UUT de-emphasis to 525, and connect a video analyzer to COM-
POSITE OUT. Adjust the unit video level so the bar level is 100
±
1 IRE.
3. Set the video analyzer to Bar Line Time, and observe the bar waveform.
Adjust RV03 so the left corner is more or less a right angle. Verify that the
Line Time Distortion is 1% or less.
4. Set the video analyzer to C/L Gain Delay. Adjust RV06 so the C/L gain is
within 110 ± 1%.
5. Set the video analyzer to Bar Line Time, and verify that the Line Time Dis-
tortion is 1% or less. If 1% is exceeded, repeat steps 3 and 4.
6. Set the video signal to PAL, pre-emphasis to 625, and frequency deviation
to 24.0 MHz p-p.
7. Set the UUT de-emphasis to 625, and connect a video analyzer to COM-
POSITE OUT. Adjust the unit video level so the bar level is 100 ± 1 IRE.
8. Set the video analyzer to Bar Line Time, and observe the bar waveform.
Adjust RV09 so the left corner is more or less a right angle. Verify that the
Line Time Distortion is 1% or less.
9. Set the video analyzer to C/L Gain Delay. Adjust RV12 so the C/L gain is
within 110 ± 1%.
10. Set the video analyzer to Bar Line Time, and verify that the Line Time Dis-
tortion is 1% or less. If 1% is exceeded, repeat steps 8 and 9.
DG/DP Characteristic Check
Adjustment Conditions:
• Connect the IF filter board and the loop-thru cable in the specified manner.
•
Set the measurement device as follows:
•
RF level: –30 dBm
•
C/N ratio: 30 dB or more
• Condition 1: Video signal: NTSC
•
Pre-emphasis: 525
•
Frequency Deviation: 18.4 MHz p-p
• Condition 2: Video signal: NTSC