SERVICE

MODEL 8559A

output to the mainframe, 4 4 must receive a positive voltage. The Blanking Driver is driven by the Vertical/ Baseline Comparator and the Sweep Ramp High/Low Limit Comparator. Either of these circuits can produce the positive input needed by the Blanking Driver to produce a blanking output.

Penlift Driver (G)

The display is blanked during retrace and during the dead time of the sweep ramp. Retrace blanking from the Sweep Generator/Bandwidth Control Assembly A9 is applied to the emitter of the buffer amplifier Q1. When the sweep ramp is turned off (dead time), the retrace blanking signal rises to + 1 0 1 This voltage appears at the base of Q4, blanking the display. Simultaneously, the + 10Vsignal is applied to base of Q5, causing the collector of Q10 to rise to +15Y Transistor Q10 provides the signal used to lift the pen of the X-Yrecorder during the analyzer's sweep retrace and dead time. Zener diodes VR2 and VR3 limit the output to 35V t o protect Q10 from high voltage and inductive transits generated by the X-Y recorder.

Sweep Indicator Driver (H)

The front panel SWEEP indicator lights when the retrace blanking signal is low (OV). Transistor 4 2 2 is turned on by the low retrace signal and switches on the SWEEP light-emitting diode.

VERTICAL DRlVERlBLANKlNG ASSEMBLY A15, TROUBLESHOOTING

Display Held in Blanked Mode: When this occurs, it may be necessary to increase the display intensity (on HP 180 series mainframes) to make the trace visible. A bright dot appears at the beginning of the trace and the BL CLIP control does not work. Most common failures are 4 8 and 4 1 6 (always change both).

The S + T line from the Frequency Control Assembly A7 can cause the comparators (block C) to latch-up.

The Sweep Generator/Bandwidth Assembly A9 retrace line input line can lock-up retrace.

Display Offset in Linear: Most common failure is 417.