TM 9-6625-1753-14
LENGTH resistor set to provide the proper dc operating level, the positive-going ramp signal will overcome the reverse-bias
dashed line in fig. 3-5). The lockout capacitor doesn't fully discharge until the retrace time interval is completed. Therefore,
voltage applied to CR8204 at the right time (halfway between the last graticule marking and the end of the CRT face).
trigger diode CR8203: is kept reverse-biased by the decreasing positive voltage from the lockout capacitor via diode CR8210
and transformer T8201 during the retrace time interval. This prevents the normal sweep gate multivibrator from being
(3) When the turn-off diode conducts, a positive voltage is applied to the base of normal-gate sweep multivibrator
triggered.
transistor Q8206. This positive voltage drives Q8206 into saturation, rapidly returning the multivibrator to its original operating
state. You will recall that in this state, the multivibrator produces a positive output, causing the disconnect diodes to conduct,
(4) In summary, the ramp output of the turn-off emitter follower reverse-biases trigger diode CR8203 during the
which, in turn, deactivates the Miller integrator circuit. Charging capacitor Cc (fig. 3-4A) is short-circuited and discharges; the
forward sweep interval, preventing any unwanted trigger pulses from changing the operating state of the normal-sweep gate
Miller integrator circuit is then ready for the next sweep cycle after completion of the lockout time interval (d below).
multivibrator. And, although the charge on the lockout capacitor is decreasing during the sweep retrace, it is still large enough
to keep the trigger diode reverse-biased, and continues to prevent any unwanted trigger pulses from changing the operating
d. Lockout Circuit.
state of the normal-sweep gate multivibrator.
(1) The lockout circuit preve nts the triggering of the normal-sweep gate multivibrator to occur during the sweep
e. HF Stability (Recurrent Sweep). When you wish to observe very high frequency signals (5 megacycles or higher), you
retrace interval. Although the disconnect diodes (when forward-biased) short-circuit the charging capacitor, they still offer
rotate the HF STAB control R8268 counterclockwise, setting RECUR-NORM switch S825 to the RECUR position. This places
some resistance to the capacitor current; preventing the capacitor from discharging at once. The time it takes the capacitor to
a ground at the anode of CR8208 and reverse-biases the diode. You will recall that the lockout capacitor discharges through
discharge to its quiescent level (-2 volts), is the sweep retrace time. Since you don't want the sawtooth generator activated
R8261 and the HF STAB control. Therefore, by rotating the HF STAB control you can vary the duration of the lockout time
before the retrace time of the sawtooth signal is completed, you must prevent the normal-sweep gate multivibrator from being
(the time interval that prevents the unwanted trigger pulses from triggering the normal-sweep gate multivibrator). Rotating the
triggered during this interval. Figure 3-5 is a simplified schematic of the lockout circuit.
HF STAB control counterclockwise decreases the discharge time of the lockout capacitor, which in turn shortens the lockout
time. Notice that by shortening the lockout time, you increase the natural frequency of the sawtooth generator. It can also be
(2) You will recall in paragraph a above that trigger diode CR8203 and the lockout circuit prevent the triggering of the
set so that exact submultiples of the high trigger frequency (dual trace plug-in or external input) can be chosen. Being able to
normal-sweep gate multivibrator during the forward sweep interval. Let's see how they perform this function. The sawtooth
choose the proper submultiples provides stable synchronization of very high frequencies (5 to 50 megacycles).
output of turn-off emitter follower, Q8208, besides going to turn-off diode CR8204, is also applied to diode CR8206 (zone A8)
f. Beam Gate Circuit.
(1) This oscilloscope uses a cathode-ray tube (CRT) that has two additional deflecting plates and a blanking shield
(baffle) with a center aperture. The blanking shield is located in front of the beam deflecting plates (the CRT face side). The
aperture is used to cut off the electron beam without regard to the potential on the CRT control grid. During the forward
sweep interval the electron beam passes through the aperture of the blanking shield because both deflection plates are kept
at the same potential. Upon completion of the sweep, during the retrace interval, one of the deflection plates receives a larger
potential than the other, creating a potential unbalance between the plates. This unbalance causes the electron beam to
Figure 3-5. Lockout circuit, simplified schematic.
causing it to conduct. With CR- 8206 conducting the lockout capacitor (selected by the setting of TIME/DIV switch S850)
charges to this linear increasing voltage (shown by the solid line in fig. 3-5). As the lockout capacitor charges, a positive-
going ramp voltage goes to trigger diode CR8203 via transformer T8201. This voltage reverse-biases the diode sufficiently to
prevent any unwanted trigger pulses from changing the operating state of the normal-sweep gate multivibrator. When the
sawtooth output of Q8208 is not present, voltage divider R8265, R8267 forward-biases diode CR8208, preventing the -50
volts applied to HF STAB resistor R8268 and resistor R8261 from forward-biasing trigger diode CR8203.
(3) You will recall that when the ramp voltage reaches its predetermined height (full sweep trace), turn-off diode
CR8204 conducts, causing the normal-sweep gate multivibrator to change state, thus applying a positive voltage to the
disconnect diodes. As previously stated, this will deactivate the Miller integrator and result in sweep retrace. During the
retrace interval, the lockout capacitor discharges through resistor R8261 and HF STAB control R8268 (as shown by the
Figure 3-6. Beam gate effect.
3-9
