4.Step the Video BW down to .1 Hz. At each step, the amplitude of the LO feedthrough should decrease.
5.Refer to Table
6.If latch A3U102’s output is not correct, trigger an oscilloscope on LLOGSTB (UlO2 pin 9) and monitor U102 pin 1 and other latch inputs while changing the video bandwidth.
7.If the inputs are incorrect, troubleshoot the analog bus. Correct inputs with bad outputs indicate a faulty U102.
8.Check that the outputs of A3UlllA, ASUlllB, and A3U107A/B/C/D are correct for their inputs. The outputs should be high with noninverting inputs higher than the +1.4 V threshold voltage. If a voltage drop is noticed across these components, suspect A3CR109 or A3Q317B. Since no dc current flows through any of the series resistances or FETS (drain to source), no voltage drops should occur.
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| Table | Latch Outputs |
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Video BW | Pin2 | Pin 5 | Pin 7 |
| Pin 10 | Pin 12 | Pin 15 |
300 Hz | H | L | L |
| L | L | L |
1kHz | L | L | L |
| L | L | H |
3 kHz | L | H | L |
| L | L | L |
10kHz | L | L | L |
| L | H | L |
30kHz | H | L | H |
| L | L | L |
100 kHz | L | L | H |
| L | L | H |
300 kHz | L | H | H |
| L | L | L |
1MHz | L | L | H |
| L | H | L |
3 MHz | L | L | L |
| H | L | L |
Video Filter Buffer Amplifier
See function block W of A3 Interface Assembly Schematic Diagram (sheet 5 of 6).
The video filter buffer amplifier provides outputs for video trigger, positive and negative peak detectors, and the analog
Current source U307C provides twice the current of Q316. Resistor R145 and current source U307D shift the dc level. Resistor R260 terminates the peak detector inputs in 500 ohms. The unterminated gain is 1.1. Diode CR114 prevents latchup during positive overdrive conditions while CR113 protects Q318 during overdrive. Diode CR117 is a 12.7 V zener that limits the peak detector’s output to t1.5 V. Typically, limiting occurs at +l.l V.