TM 9-6625-1753-14
C2
d. Lockout Circuit.
3-19.
Triggered-Strobe Mode (Figs. 3-16, 3-17, and 6-4)
(1) The lockout circuit prevents the sweep gate multivibrator from being triggered until the sweep retrace interval has
a. When the DISPLAY LOGIC switch is set to TRIG STROBE, the delay pick-off, the delayed sweep, and strobe circuits
reverted the saw to its quiescent base level. The sweep does not retrace in zero time since the clamping of the Miller
are activated. In the "triggered" mode, the delaying sweep receives the external trigger signal while the delayed sweep
integrator timing capacitor is accomplished by a diode switch circuit having a small impedance. The timing circuit discharge
receives its trigger signal later from the delaying sweep. The triggering signal for the delayed sweep in this mode is derived
(saw retrace) takes longer than the flipping of the sweep gate multivibrator. Consequently, the trigger excitation pulses must
from the delaying sweep at the exact instant this sweep reaches a level determined by the adjustment of the delay pick-off
be held off to prevent the formation of a new saw during the retrace interval. The sawtooth waveform must be permitted to
circuit. This mode from now on will be referred to as the delayed-triggered mode. The normal and delaying sweep circuit
retrace completely to its quiescent base, which re-establishes the beam at the left-hand side of the CRT screen. This is
functions as previously described. Major differences between the triggered strobe and the normal mode circuits may be seen
accomplished by holding the cathode of series triggering diode CR8203 at a suitable positive voltage for a given period after
by comparison of the block diagram.
the completion of the sweep.
b. Referring to figure 3-16, the saw from the delaying sweep circuit is applied to the horizontal deflection amplifier and to
(2) During the forward sweep interval, a portion of the positive sweep voltage is fed back to the base of turn-off
the input of delay pick-off comparator circuit Q8601 and Q8602. This circuit is a differential amplifier which compares the
emitter follower Q8208, which charges the lockout capacitor through coupling diode CR8206. When the saw reaches its
voltage level of the input delaying sweep against a fixed voltage obtained from the DELAY VERNIER control. When the
predetermined full screen length, the sweep gate multivibrator flips and clamps the Miller integrator, which then causes the
voltage level of the delaying sawtooth waveform reaches a certain level, as determined by the setting of the DELAY VERNIER
sweep to retrace. During the return-trace interval, diode CR8206 is cut off and the lockout capacitor is discharged through
control, the output of the comparator circuit is applied to delayed trigger amplifier Q8604, which drives coupling transformer
R8261. The sweep gate multivibrator will not retrigger until the preselected lockout timing capacitor is chosen to give lockout
T8601. The negative pulse from the secondary of this transformer is through series-coupling diode CR8105, to trigger
beyond the time that the sweep has returned to rest at its initial starting point.
delayed sweep gate multivibrator Q8106 and Q8107.
(3) The duration of the lockout interval is determined by the sweep-charging time constant and the Miller integrator
c. The positive gate developed at the collector of Q8106 is coincident with the delayed TIME/DIV control setting and is
clamp. Consequently, the TIME/DIV switch must also change the lockout circuit hold-off timing capacity. This technique
prevents additional triggers from initiating the sweep gate multivibrator until the sweep retrace interval is completed and the
saw reverts to its quiescent state at the left-hand side of the screen.
(4) Trigger coupling diode CR8203 prevents triggers from disturbing the sweep gate multivibrator during all of the
sweep-forward interval. The lockout circuitry prevents retriggering during the retrace interval.
e. Unblanking.
(1) The cathode-ray tube employed in this oscilloscope has two additional beam-deflecting plates, which cut off the
electron beam independently of the control grid. Beam control pulses (trace brightening and retrace blanking pulses) can
easily be decoupled since these beam deflection plates are electrically close to ground. In conventional circuits these pulses
must be coupled to the control grid, which is operated at high potential. The extra beam deflection plates are controlled by
sweep gate multivibrator Q8206 and Q8207.
(2) The positive pulse developed at the collector of Q8206 of the delaying sweep gate multivibrator is applied to the
base of unblanking cascode amplifier Q8405 and Q8403. This gate signal is coincident with the sweep forward time and is
also available at a front-panel connector labeled + GATE. The resulting negative pulse developed at the collector of Q8403 is
applied to the beam deflection plate of the cathode-ray tube through emitter follower Q8402. Therefore the display is visible
only during the forward sweep interval and is blanked during the retrace and lockout interval. Transistor Q8401 serves as an
emitter follower voltage source for the collector of Q8402, and it also prevents the collector voltage of Q8403 from exceeding
125 volts.
(3) A portion of the negative pulse from unblanking emitter follower Q8402 is applied to differentiating network C8441
and R8443. The output from this network provides the proper trigger to the blanking multivibrator of the dual trace plug-in.
(4) These delaying sweep time base plug-in units may be used in the X or Y cavity of the main frame. Only the X
cavity permits retrace beam blanking signals to be coupled to the cathode-ray tube. Therefore, sweep operation in the vertical
direction will show the sweep retrace.
Figure 3-16. Triggered strobe mode, functional block diagram.
3-17
