MODEL 8559A

SERVICE

to the Pulse Shaper. Resistor A16R2 and capacitor A16C8 on the motherboard attenuate the ac line signal to approximately 1V peak-to-peak (at the base of 435) and filter line spikes.

Single Sweep Triggering and Abort

When the TRIGGER switch is in the single sweep position, the sync line is grounded and the single line open. Transistor 433 is held off by the voltage developed across CRlO and R53. The voltage at the collector of 433 is at + 10V, putting the emitter of 438 at +9.4V This charges C15 to +2.4V through voltage divider R48 and R49.

A sweep is initiated when the trigger switch is set to the spring-loaded SINGLE position and + 15Vis applied to the single trigger switch (block N). When 437 turns on, a negative pulse is produced at the emitter of 433 due to voltage stored by C15. This pulse turns 433 on and starts the sweep cycle.

The sweep may be aborted (reset to - 5V) by pressing the single sweep switch while the sweep is in progress. During the sweep, the collector of 433 is at +0.5V This puts the emitter of 438 at OV and charges C15 to - 4V through voltage divider R48 and R49. Now when + 15V is applied to the single trigger switch (block N), 437 turns on and a positive pulse appears at the emitter of 433. Consequently, 433 turns off and the sweep is aborted.

Manual Sweep

Manual sweep control is obtained when the SWEEP TIME/DIV switch is set to MAN. In the manual position, ST7 is open (see Figure 8-37). Transistor 440 turns 433 on by supplying current to its base and 439 acts as a

 

 

@ RETRACE

 

- BUFFER

 

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OUT BUFFER

 

 

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15v M A N

CONTROL

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S T 7

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DURING

MANUALR3 6

SWEFP

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FIGURE 837. MANUALSWEEPMODE, SIMPLIFIEDSCHEMATIC