TM 9-6625-1753-14
deflect toward the plate with the larger positive potential. After passing through the plates, the deflected beam strikes the
triggering switches to the difference amplifier. At the output of the amplifier, we observe the second waveform, a series of
shield instead of going through the aperture. This produces the desired result of blanking the CRT screen preventing signal
symmetrical pulses. The squaring of the original sine-wave input is the result of the tunnel-diode action.
display) during the retrace and lockout time. In order to obtain the blanked condition there must be at least a 75-volt potential
difference between the beam gating plates. Figure 3-6 a, b, and c illustrates the unblanked and blanked condition just
(3) When the pulse output of the difference amplifier triggers the normal-sweep gate multivibrator, positive and
described.
negative gates are produced, as shown by waveforms 3 and 4. The negative gate reverse-biases the disconnect diodes,
activating the Miller integrator circuit which produces the linear sawtooth output shown by waveform 5. The sawtooth output
(2) The positive o tput (gate), taken from Q8206 of the normal-sweep gate multivibrator, is applied to unblanking
u
goes to the horizontal deflection amplifiers.
cascode amplifiers Q8405, Q8403 (zone D14). This gate coincides with the ramp (sweep) output of the sawtooth generator.
After passing through the unblanking amplifiers, the gate goes to emitter follower Q8402. Voltage divider R8412 and R8113:
(4) To prevent premature triggering of the normal-sweep gate multivibrator, the output of the turn-off emitter follower
establishes a 15-volt operating potential for amplifier Q8403. Emitter follower Q8401 is the B+ operating voltage source for
keeps the trigger diode reverse-biased. Waveform 6 shows that although negative triggers are applied to the trigger diode,
Q8402. It produces a +125-volt output, limiting the collector potential of emitter follower Q8402 to this value. Therefore, the
the positive sawtooth and lockout circuit outputs are large enough to keep the diode reverse-biased for the required time
negative-gate output of the unblanking amplifiers (approximately 75 volts peak to peak), after passing through Q8402,
interval. Notice that during the hold-off time, the linear-rising ramp doesn't return to its initial operating level immediately. This
operates between voltage levels of +50 and +125 volts, and it is applied to one of the beam gating plates (pin 6). With the
is the retrace interval where the lockout circuit functions, preventing premature triggering. Waveform 7 shows output of the
oscilloscope in the normal sweep mode (DISPLAY LOGIC switch S840 at NORM), the other beam deflection plate (pin 13)
turn-off emitter follower, that forward-biases the turn-off diode, to trigger the normal-sweep gate multivibrator back to its
receives a constant +50-volt potential via resistor R2028.
original operating state.
(3) Remember that a 75-volt difference between the beam-gating plates is required to remove the display from the
(5) The positive-gate output of the normal-sweep gate multivibrator goes to the unblanking amplifiers in the beam
CRT screen. When the output of emitter follower Q8402 is maximum, the voltage difference between the plates is 75 volts (+
gate circuit. After it is inverted, the negative gate, as shown by waveform 8, passes through an emitter follower to a beam
125 to +50 volts) and the CRT screen is blank. (This occurs during retrace and lockout time.) During the time interval the
gate deflection plate. During the period the gate is negative (forward-sweep time) the CRT is unblanked. When the gate
output of Q8402 is minimum (negative gate), there is no voltage difference between the plates: a sweep trace or signal, as the
becomes positive (retrace time), the CRT is blanked.
case may be, appears on the CRT screen (during forward sweep) time).
(6) Besides going to the beam gate deflection plate in the CRT, the emitter follower output is differentiated (waveform
(4) The negative-gate output of emitter follower Q8402 also goes to differentiator C8441, R8442, R8443.
The
9) and sent to the dual trace plug-in. This differentiated signal provides the proper trigger for the blanking multivibrator in the
differentiated output provides the proper trigger to the blanking multivibrator in the dual trace plug-in (par. 3-9).
dual trace plug-in.
g. Summary.
3-14.
Delayed Sweep (Fig. 4-5)
(1) Before going into a description of the delayed sweep circuitry (par. 3-14), let's briefly summarize the generation of
The delayed sweep circuitry, operating in conjunction with the normal sweep circuitry, provides us with four more operating
the trigger pulse and sawtooth signals. The waveforms in figure 3-7 are used to cover the main points of preceding detailed
modes. These modes are: triggered-strobe, triggered-delayed, armed-strobe, and armed-delayed.
description.
a. Triggered-strobe.
(2) The first waveform (assuming a sine-wave signal) shows the input signal that passes through the various
(1) In this mode, a segment (strobe) of the sweep displayed on the CRT appears brighter. To accomplish this, the
delayed sweep circuitry is activated during the normal sweep interval. This, in a sense, superimposes the delayed sweep on
the normal sweep. The start of this brightened segment represents the start of the delayed sweep; the length of the segment
represents the length of the delayed sweep.
(2) Setting DISPLAY LOGIC switch S840 to TRIG-STROBE activates the delay trigger pick-off circuit (consisting of
comparator Q8601, Q8602, emitter follower Q8603, differentiator amplifier Q8604, transformer T8601, and DELAY VERNIER
control R8617), the delayed sweep circuit, and the strobe circuit (Q8001, Q8002). In this mode, the normal sweep (delaying
sweep) operates as described in paragraph 3-13.
(3) The normal sweep signal from sawtooth generator V8203, V8204 is applied to comparator Q8601, Q8602 via
diode CR8602 (zone B10). The other input to the comparator is a dc voltage that is determined by the setting of the SWEEP
CAL, DELAY VERNIER, and DELAY ZERO controls, R8241F, R8617, and R8618, which form a voltage divider. This fixed
voltage is applied to the comparator via emitter follower Q8603 and diode CR8603. The comparator circuit functions in the
Figure 3-7. Triggering and normal sweep waveforms.
3-10
