Chapter 8 401
ADC/Interface Section
A3 Assembly Video Circuits
2. Refer to Table 8-8 and check for correct logic levels at A3U108 pins 1,
15, and 16. Check for proper output signals at TP6. If the select lines
are not changing, suspect the ADC ASM or the VGA/ADC MUX
Control. If the select lines are changing, but the proper video inputs
are not being switched to the output, replace U108. In SAMPLE
mode, the input is MOD_VIDEO (pin 7); in POS PEAK mode, the
input is POS_PEAK (pin 5); and in NEG PEAK mode, the input is
NEG_PEAK (pin 6).
3. Check for the presence of the YTO ERR signal at A3J2 pin 42 with
an oscilloscope probe.
4. If ERR 300 YTO UNLK or 301 YTO UNLK occurs and the voltage is
near zero during a sweep and positive during retrace (YTO is being
locked), the fault is on the A3 assembly. If a constant dc voltage is
present, refer to the Synthesizer section troubleshooting procedure
in Chapter 11.
5. Set the spectrum analyzer to the following settings:
Span ............................................................................. 5MHz
Sweep time .................................................................... 50ms
6. Check for the presence of the SCAN RAMP signal by connecting an
oscilloscope probe to A3J2 pin 45 (component side of A3J2). Connect
the negative-probe lead to A3TP4.
7. A 0 to 10 V ramp should be present in both LINE and FREE RUN
trigger modes. If the waveform is present only in LINE trigger, ADC
control signal HBADC_CLK0 may be faulty. Refer to “ADC Control
Signals” on page 403.
8. If the scan ramp is present, but is not being switched to the output of
U108, replace U108. If the scan ramp is absent in either mode, do
the following:
a. Connect the oscilloscope probe to A3J400 pin 15 (HSCAN).
b. A TTL signal (high during 50 ms sweep time and low during
retrace) should be present, indicating A3 is working properly.
Refer to the Synthesizer section troubleshooting procedure in
Chapter 11. A faulty TTL signal indicates a bad A3 Interface
assembly.
Table 8-8 Logic Levels at A3U108
Detector
Mode
U108 pin 1 U108 pin 15 U108pin 16
SAMPLE H L H
POS PEAK H L L
NEG PEAK L L H
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