HP 85662A, Руководство по устранению неисправностей и ремонту

Страница: 145 / 594

Video Trigger
Video Trigger comparator U13 generates a negative-going transition when the VIDEO
signal rises above the voltage at U13 pin 2, which depends on the position of the front-panel
TRIGGER LEVEL control. R49, R50, and R51 divide the TRIG LEVEL voltage (0 V to
V) to approximately -0.1 V to V at U13 pin 2. The LOW FAST SWEEP signal,
connected to the strobe inputs (pins 5 and 6) of U13, disables the output of U13 in Fast
Sweep mode when the fast sweep ramp is being generated. R52 provides hysteresis for U13,
and CR1 protects U13 from excessive input voltage on the VIDEO line. The V and -10
V supply voltages for U13 are derived through VR4 and VR5.
External and Line Triggers
The EXT TRIG input from the IF-Display Section rear panel is applied to
Q14 through divider R61 and R62. When EXT TRIG rises above approximately
V, Q14
saturates and pin 9 goes low, causing U16 pin 3 to go low, producing a trigger.
A voltage, resistively divided from a winding of the IF-Display Section line transformer, is
buffered by Schmitt trigger inverter
the output of which is applied to U16 pin 2, the
line trigger input of the trigger multiplexer. Negative-going transitions at U16 pin 2 are valid
line triggers.
Free Run Mode (Sweep lime 10 ms)
In Fast Sweep mode, during the fast sweep dead time, pin 8 is high. This causes
to oscillate at a rate (determined by R59 and Cll) of approximately 500 Negative-going
transitions at pin 6 produce triggers in the Fast Sweep, Free Run mode. is gated
off during the fast sweep ramp. Note that the Free Run Oscillator is not used to trigger
sweeps in sweep times
ms. (See High Sweep Control circuit description.)
Fast Sweep Auto Trigger
When the or key is pressed and the sweep time is 10 ms or less (Fast Sweep
mode), a fast sweep ramp will be started automatically about 25 ms after the start of the last
fast sweep if no video or external trigger occurs first.
or keys are
pressed, fast sweep triggering will be in Normal mode. In Normal mode, no fast sweep will be
started until a trigger occurs. The CRT graticule and characters will not be refreshed until a
fast sweep has occurred.
Fast Sweep Auto Trigger Circuitry
In Auto Trigger mode, the Instrument Bus sets U5 pin 12 High. pin 1 goes low when
the fast sweep ramp begins, triggering a 25 ms (low) pulse at pin 4. If another fast
sweep ramp is started within 25 ms of the beginning of the previous fast sweep ramp,
will be retriggered, and
pin 4 will not go high for at least another 25 ms. If triggers
cease to occur, pin 11 will go high 25 ms after the last fast sweep. This causes trigger
multiplexer U16 to select the output of the Free Run Oscillator in either Video or External
mode, triggering a sweep. Auto triggering will not occur in sweep times
ms, because in
this case the Auto Trigger circuitry is disabled by the FAST SWEEP ENABLE line to
pin 5.
If [SHIFT) or (SHIFT] keys are pressed, U5 pin 12 will be low, preventing automatic
triggering in the absence of video or external trigger.
3