TM 9-6625-1753-14
C3
However, the output of the armed multivibrator prevents all input triggers from firing the delayed-sweep gate multivibrator.
When the comparator trigger is applied to the armed multivibrator, it produces an output that allows the next input trigger to
b. Normal Sweep. Normal sweep is provided into the horizontal deflection circuit when the DISPLAY LOGIC switch, in
fire the delayed-sweep gate multivibrator. This sweep gate is then fed back to reset the armed multivibrator, locking out
the normal and delaying triggering sweep channel (NDT), called the normal sweep channel is set to NORM.
subsequent input triggers. The sweep gate then goes through the beam gate circuit, to produce strobe effects similar to those
(1) Sweep gate. The first trigger-resulting from a voltage level comparison in the trigger circuit-fires the sweep gate
described in (1) above.
multivibrator, producing a negative gate. This gate is fed to a linear sweep generator to start the ramp sweep.
(4) Armed-delayed. In this switch position only the delayed sweep is displayed, as in (2) above However the delayed
(2) Linear sweep. The slope of the sweep signal is set by the particular RC time constant switched in by the
sweep is not activated until the comparator output triggers the armed multivibrator, and, as in (3) above, it produces an output
TIME/DIV selector switch. The sweep begins to rise at a linear rate and when the ramp voltage reaches a predetermined
that allows the next input trigger to fire the delayed-sweep gate multivibrator.
level, a turn-off trigger resets the sweep gate multivibrator. This causes the ramp to collapse, producing a sweep retrace.
d. External Sweep. For external sweep, an external signal is injected at the X AMP input instead of the normal or
(3) Lockout output. The lockout circuit does two things. It uses the sweep feedback (coupled through the turn-off
delayed sweep inputs to the horizontal deflection circuit. This permits the use of non-linear signals for the sweep with which
trigger circuit) to keep additional triggers out of the sweep gate multivibrator, and to sustain the collapse time of the sweep,
we can generate waveforms such as lissajous patterns.
thus allowing full recovery and inhibiting premature triggering.
e. Horizontal Deflection Amplifier. The horizontal deflection amplifier functions the same in all sweep modes, including X
(4) Beam gate circuit. The CRT contains a baffle with an aperture through which the scanning electron beam passes
amplifier only (external sweep). The output from the sweep generators or from the external source is sent through a series of
on its way to the phosphor screen. At the top and bottom of this baffle are two plates, and as long as these two plates are
emitter follower and amplifier stages before being applied to the horizontal deflection plates. These stages convert the single-
kept at the same potential the beam is unaffected. For example, if the inverted sweep gate is clamped to +50 vdc, and +50
ended input signals (output of the sweep generators) into pushpull signals of sufficient power to drive the CRT deflection
vdc is applied to the other plate, the beam gate plates allow the electron beam to go through the aperture during trace time.
plates.
During retrace, one of the beam gate plates is raised to +125 volts by the beam gate circuit. This 75-volt difference of
potential draws the electron beam away from the aperture, thus blanking the screen during retrace.
Section II. FUNCTIONAL THEORY-POWER SUPPLY
c. Delayed Sweep. The delayed (armed) sweep channel (DA), called the delayed sweep channel, closely resembles the
normal sweep channel. For the normal sweep channel description, the NORM position of the DIS-PLAY LOGIC switch was
3-4. Low-Voltage Power Supply, Older 765MH Models (Fig. 4-3)
described. Now let's consider the other five positions.
(1) Triggered-strobe.
a. Primary Power. Primary ac power from a convenience outlet is brought into the unit via connector W1001 (sone A1).
(a) In this switch position, a bright segment (strobe) appears to be superimposed over the normal presentation.
The unit is turned on by turning the SCALE ILLUM control clockwise, thus closing the contacts of switch S102, and applying
This illusion is created by using the beam gate plates in the CRT. To do this the normal sweep is routed to the deflection
the ac voltage to the primary windings of transformers T1001 and T2. SCALE ILLUM control R1002 is also used to control the
plates through the TRIG-STROBE position of the DISPLAY LOGIC switch.
graticule scale brightness level. If the source is 115 volts, switch S101 is set to the 115V position, connecting the two primary
(b) The normal sweep is also fed to a comparator trigger circuit. One input to this circuit is a dc reference level,
windings of T1001 in parallel. When the source is 230 volts, switch S101 is set to the 230V position, connecting the primary
set by the DE-LAY VERNIER control. When the sweep run-up reaches this reference level, the comparator trigger circuit
windings in series.
fires, sending a trigger to the delayed-sweep circuit.
b. Temperature Control. Temperature control is provided by a fan, a heater, and thermal interlock system. If the
(c) The trigger fires the delayed-sweep gate multivibrator. From the multivibrator, one gate output goes to the
temperature drops below 41F, normally open thermal relay S1201 closes, and the heater warms the equipment. If the
delayed sweep generator. However, the sweep isn't applied to the CRT deflection plates, but is used to generate a turn-off
temperature rises above 68F, the fan cools the equipment. If the temperature exceeds 190F, interlock thermal cutout relay
trigger at the end of the sweep. Meanwhile, the other sweep gate output goes to the strobe circuit.
S103 opens the ac source. Ac voltage for induction motor in the fan is supplied through power switch S102. With 115/230-
(d) The CRT beam gate circuit was covered in b.(4) above. In the triggered strobe switch position, this circuit
volt switch S101 set to 115V, the fan voltage is supplied through temperature switch S1 to one fan connection; and through
works in the same way, except that the potentials are no longer equal. The non-gate level may be about 30 volts, creating a
thermal cutout S103, interlocks J8001-31 and J9001-31 and the 115-volt connection of S101 to phase-shifting capacitors C1
potential between beam gate plates of 20 volts. This 20 volts isn't large enough to completely deflect the beam, but it does
and C2, directly to the fan. With switch S101 in the 230-volt position, the fan is connected in series with one section of
deflect it partially. As a result there is a less intense display. When the delayed beam gate (50 volts) is applied, the potential
transformer T1001 (black and black-red wires), and in parallel with the second section.
of both plates is the same, permitting the full electron beam to go through the aperture. Since the signal is more intense, the
c. Low-Voltage Rectification. Four of the (tapped) secondaries of transformer T1001 apply stepped-down ac voltages to
display brightens during this strobe period.
four different full-wave rectifiers. Capacitor/resistor network filters the rectified outputs to reduce the ac ripple. These dc
(2) Triggered-delayed. In this switch position, only the delayed sweep appears. (The delayed sweep is routed
voltages are then processed by individual voltage regulators, or where applicable, are routed internally as unregulated
through the DISPLAY LOGIC selector switch, and the normal sweep is terminated.) The circuit performs as described above,
voltages.
except that one output of the sweep gate is differentiated, thus providing a trigger to turn off the normal sweep at the end of
the delayed sweep.
NOTE
(3) Armed-strobe. In this switch position the trigger from the comparator trigger circuit goes only to the armed
The low-voltage regulators described below are in some MH models.
Similar
multivibrator. At this time the input trigger circuit is also enabled.
regulators, with different reference designators, are used in MH/F models.
3-2
