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MODEL
8559A
SERVICE
SWEEP GENERATOWBANDWIDTH CONTROL ASSEMBLY A9, TROUBLESHOOTING
When making measurements at or near test points, be careful not to short
adjacent points or circuit components together.
Auto Scan Time (AST) Accuracy:
Observe front panel switch positions to help isolate the problem area.
Auto scan time can also be affected by circuits on the Marker Assembly A8 and the VIDEO FILTER control
position. If the AST
groblem is band related, the Marker Assembly A8 is the most probable cause. If the AST
problem is bandwidth or scanwidth related, the most probable cause is the Sweep
Generator/Bandwidth Con
-
trol Assembly
A9.
The greater the load placed on the AST line, the greater the current demand. The greater the
current demand, the faster the sweep rate.
Failure to Sweep:
Check the
+
10V (nominal) supply. If it is greater than
+
11.5V, the sweep will be inhib-
ited.
If the
+
10V (nominal) supply is low, check the Bandwidth Filter No. 1 and No. 2 Assemblies A1 1 and A13 for
a shorted crystal filter pole. Test from
All/A13TP2 to ground and A1 1/A13TP5 to ground with an ohmmeter
to locate the possible short.
Begin troubleshooting the sweep generator by determining if the Current Source (block F) is operating and if the
Comparator (block
H)
will toggle.
The inability to trigger retrace, during the beginning of a sweep, is commonly caused by the failure of
U1 or
CR7.