Tektronix 1A1, Инструкция по эксплуатации

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Circuit Description— Type 1A1
When the MODE switch is set to the ALT position, the
MODE switch connects +225 volts via R355 and pin 16 of
the interconnecting plug to the plate of the Sync Amplifier
tube in the oscilloscope circuitry. The Sync Amplifier stage
operates as a cathode follower when the Type 1A1 is used
with the oscilloscope, because the base winding of T330 is
connected in the cathode circuit of the tube. The connection
is made via a single-pin connector on the Output Amplifier
card connector and pin
8
of the interconnecting plug.
In its quiescent state, Q330 is not conducting. At the end
of each sweep cycle a positive-going pulse is generated
in the base winding of T330, causing Q330 to conduct. The
collector winding of T330 supplies regenerative feedback
to the base winding to drive Q330 into saturation, and col­
lector current ceases to increase.
Since the collector current becomes constant, no feedback
voltage is induced, Q330 is reverse biased, and the collaps­
ing field produces a slight backswing voltage. During the
time that the Alternate Trigger Blocking Oscillator is going
through its cycle, the approximate voltage induced in the
output tertiary winding is about a
2
-volt positive-going
pulse which is applied to the Switching Multivibrator stage.
This is the trigger which flips the Switching Multivibrator.
The Switching Multivibrator in turn, switches the channels
via the diode switches.
Diode D330 is in Output Amplifier cards Model 4-up to
clip the slight backswing voltage caused by the collapsing
field of T330. This prevents double triggering of the Switch­
ing Multivibrator.
The dots above the individual T330 windings, as shown
on the schematic diagram, are phasing dots. They show
that there is no phase reversal if pulse polarities are com­
pared between the dot end of the windings. However, there
is a phase reversal if the signal at the dot end of one wind­
ing is compared with the signal at the no-dot end of the
other windings. The signals can be compared between the
ends of the windings that are not at AC or DC ground.
When the MODE switch is set to any position other than
ALT, the +225 volts is disconnected from the switch end of
R355. The result is that the Sync Amplifier stage in the
oscilloscope no longer functions as a cathode follower to
drive the Alternate Trigger Blocking Oscillator stage. Since
the Alternate Trigger Blocking Oscillator stage is not being
driven, the stage is inoperative during these modes of
operation.
Blanking Multivibrator (Q 3 4 3 /Q 3 5 3 )
The Blanking Multivibrator stage Q343/Q353 is a mono­
stable, common-emitter, collector-to-base clamp multivibrator.
When triggered during chopped mode of operation, this
stage produces a blanking pulse of sufficient amplitude and
duration to blank the beam during the switching interval.
Timing of the switching and blanking multivibrators in the
Switching Circuit of the Type 1A1 allows for the delay in the
vertical amplifier of the oscilloscope. That is, the blanking
pulse arrives at the CRT cathode at the same time that the
switching-transient portion of the composite signal arrives
at the vertical deflection plates. Correct timing and wave­
shape assures that the beam is blanked out during the
switching time between channels. However, due to the
nature of the circuitry, some intensification of the unblanked
trace does occur during the sweep.
Setting the MODE switch to the CHOP position causes
the Switching Multivibrator to free run, as stated earlier.
The ramp pulses at the junction of C301 and C311 are
applied through R341 to the base of Q343 in the Blank­
ing Multivibrator stage.
In its quiescent state Q343 is cut off and Q353 is con­
ducting. C343 is charged to about +12.5 volts at the base
of Q353; clamp diode D345 is forward biased by about
0.2 volt. The instant that the Switching Multivibrator switches
states, the ramp pulse terminates as it drops rapidly from
its peak amplitude. The terminating ramp causes the volt­
age at the base of Q343 to drop from +12.7 volts to about
+ 11 volts. This sudden drop in voltage drives Q343 into
conduction. The rise in voltage at the collector of Q343 is
coupled to the base of Q353. D345 unclamps and Q353
goes toward cutoff. With D345 unclamped, C343 discharges
at a constant rate through R345 toward the —150-volt
supply. Finally, a point is reached where Q353 is cuf off.
At about this time D345 conducts and clamps the base of
Q353 at +12.5 volts, thus completing the first half of the
cycle. For the last half of the cycle the ramp at the base
of Q343 causes Q343 to go from saturation to cutoff as
Q353 goes into conduction. Near the end of the cycle, the
ramp pulse drives Q343 into cutoff. Then, the ramp pulse
terminates and begins its slow rise to repeat the cycle.
Negative-going blanking pulses, about
6
volts or more in
amplitude, are produced at the collector of Q353. The pulse
reaches its peak about 0.25 /.is after the Switching Multi­
vibrator has triggered the Blanking Multivibrator. The 0.25-
I-
ls
delay equals the delay of the applied signal as it goes
through the vertical amplifier and delay line of the oscillo­
scope. Thus, the switching portion of the signal and the
blanking pulse arrive at the same time at the CRT.
To get the pulse to the CRT, the pulse at the collector of
Q353 is coupled through C353 and then through a single­
pin connector on the Output Amplifier card to pin 16 of the
interconnecting plug. From pin 16 the blanking pulse goes to
the plate circuit of the Sync Amplifier tube in the oscillo­
scope. This tube is inoperative as a Sync Amplifier (or a
cathode follower) during this mode of operation. The com­
ponents in the plate circuit of the tube combined with
R355 in the Type 1A1 Blanking Multivibrator stage form an
RC coupling network to couple the pulse to the grid of the
Blanking Amplifier, the pulse is applied through the CRT
Cathode Selector switch to the cathode of the CRT to blank
the beam during the time that the channels switch.
Diode D353 is in Output Amplifier cards Model 7-up to
protect Q353 from being damaged by the large transients
which occur at some positions of the MODE switch.
If the MODE switch is set to the ALT position, no pulses
from the Switching Multivibrator stage are coupled through
C302 and C311 to the Blanking Multivibrator. Conduction
of diodes D301 and D311 effectively shorts out the coupling
capacitors, thus, preventing the pulses from triggering the
Blanking Multivibrator. Shorting out the coupling capacitors
is an indirect result obtained when the Switching Multivibra­
tor functions as a bistable circuit in the alternate mode. On
the other hand, no blanking pulses need be generated in this
mode because switching from channel to channel occurs
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