A3 Assembly's Video Circuits - HP 8560A Service Manual

A3 Assembly's Video Circuits

Voltages from A3JlOl to A3's Variable Gain Amplifier correspond (approximately) to
on-screen signal levels. (One volt corresponds to the top of the screen and zero volts
corresponds to the bottom of the screen.) This is true for both log and linear settings except
when the analyzer is in 1 dB/div or 2 dB/div. In these cases the log expand amplifier is
selected, and 1 V corresponds to top-screen and 0.8 or 0.9 V corresponds to bottom-screen.
The analyzer can be set to zero span at the peak of a signal to generate a constant dc voltage
in the video circuits during sweeps.
1. Disconnect W26 from A3JlOl and W20 from A2J4.
2. Connect W26 to A2J4.
3. Set the HP 8560A to the following settings:
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OHz
SWEEP TIME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 ms
RES BW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..lMHz
LOG/div . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..lOdB/div
4. If a trace is displayed, troubleshoot the A3 assembly. If a trace is absent, connect an
oscilloscope to the rear-panel BLANKING OUTPUT.
5. The presence of a TTL signal (TTL low during 20 ms sweep) indicates a good A3
Interface Assembly. Troubleshoot the IF Section.
6. If the BLANKING OUTPUT is always at a TTL high or low, troubleshoot the A3's
trigger circuits.
7. Reconnect W26 to A3JlOl and W20 to A2J4.
8. Remove the A3 assembly's shield.
9. If the video filters appear to be faulty, refer to "Video Filter" in this chapter.
10. If there appears to be a peak detector problem, refer to "Positive/Negative Peak
Detectors" in this chapter.
11. Connect the HP 8560A's CAL OUTPUT to the
follows:
C E N T E R F R E Q
SPAN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFLVL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..'llbdBm
12. If the analyzer works correctly in 5 dB/div and 10 dB/div but not in 1 dB/div or
2 dB/div, refer to "Log Offset/Log Expand" in this chapter. Continue with step 13 if the
problem involves on-screen amplitude errors which appear to originate in the video chain.
13. Monitor A3TP9 with an oscilloscope. If the voltage is not approximately i-1 Vdc,
troubleshoot the A4 Log Amplifier. (Refer to the IF troubleshooting procedure.)
14. To confirm proper video input to the video circuit, 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.
7-12 ADC/lnterface Section
. . . . . . . . . . . . . . . . . . . . . . . .
INPUT 50R and set the controls as
300 MHz
0 Hz