MODEL 8559A

SERVICE

SWEEP GENERATOWBANDWIDTHCONTROL 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 Control 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 A11 and A13 for a shorted crystal filter pole. Test from All/A13TP2 to ground and A11/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.