392 Chapter8
ADC/Interface Section
A3 Assembly Video Circuits
13.Press CAL and IF ADJ ON OFF until OFF is underlined. Monitor
A3TP9 with an oscilloscope. If the voltage is not approximately
+1 Vdc, troubleshoot the Log Amplifier on A4. (Refer to the IF
troubleshooting procedure in Chapter 9 , “IF Section.”)
14.To confirm proper video input to the video circuit, set the spectrum
analyzer to Log 10 dB per division and change the reference level in
10 dB steps from −10 dBm to +30 dBm. At each 10 dB step, the input
voltage should change 100 mV. The input level should be +0.6 Vdc
for a +30 dBm reference level.
NOTE The on-screen amplitude level will probably not change as expected,
since the video circuitry is assumed to be faulty.
15.Monitor A3TP14 while stepping the reference level from −10 dBm to
+30 dBm. If the voltage does not step approximately 100 mV per
10 dB step, refer to “Video MUX” on page 394
16.If the Video MUX is working properly, monitor A3TP15 with the
oscilloscope and step the reference level from −10 dBm to +30 dBm.
If the voltage does not change 100 mV per 10 dB step, refer to “Video
Filter” on page 395
17.If the voltage at A3TP15 is correct, move the oscilloscope probe to
A3TP17 and step the reference level between −10 dBm and
+30 dBm. If the voltage does not change 100 mV per 10 dB step,
refer to “Video Filter Buffer Amplifier” on page 396
18.If the voltage at A3TP17 is correct, move the oscilloscope probe to
A3TP6. Set the following controls to keep the ADC MUX set to the
MOD_VIDEO input during the sweep:
Sweep time .................................................................... 50s
Detector mode .......................................................... Sample
19.Step the reference level from −10 dBm to +30 dBm while monitoring
the voltage change on the oscilloscope. If the voltage does not change
100 mV per 10 dB step, refer to “ADC MUX” on page 400
20.If the voltage at A3TP6 is correct, move the oscilloscope probe to
A3TP8 and step the reference level between −10 dBm and +30 dBm.
If the voltage at A3TP8 is not the same as that at A3TP6, replace
A3U110.
21.If the voltage at A3TP8 and A3TP6 are equal, move the oscilloscope
probe to A3TP7.
22.Change the reference level from −10 dBm to 0 dBm. The voltage
change on A3TP7 should be between 630 mV and 770 mV. If the
voltage change is outside of these limits, refer to “Variable Gain
Amplifier (VGA)” on page 402 The gain of the VGA should be
7 ±10%.
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