Tektronix 3A74 Instruction Manual page 30

Circuit Description — Type 3A74
slightly positive due to the positive bias applied by +125
volts through R585. In alternate the positive emitter bias is
applied in the form of a positive pulse from the Sync Pulse
In Emitter Follower Q583.
In either mode of operation when @590 is forward biased,
the initial current flowing through the transistor and the pri-
mary of 1592 causes a build-up of magnetic flux. Due to
this varying magnetic flux the secondary of 1592 impresses
on the base of Q590 a negative turn-on voltage. This for-
ward biases the transistor further, causing more current, which
causes more flux and a further increase in the negative-going
base voltage. This regenerative action results in the B. O.
being suddenly turned from full off to full on. At full on Q590
is saturated, that is to say that there is very little voltage be-
tween emitter and collector, both being at approximately
—6 volts.
The current increasing through the collector load (T592}
raises the collector voltage positive while transformer coup-
ling and inversion through the secondary applies a negative-
going voltage to the base. This base voltage, in turn by
emitter follower action pulls the emitter voltage negative
until the transistor saturates. During this regenerative turn-on
of the B. O., the rapidly falling emitter voltage causes a
negative-going trigger pulse to be coupled through C588 to
the common emitter bus of the Ring Counter. This trigger
pulse causes the Ring Counter to switch to the next channel.
The rapidly rising primary current also causes the tertiary
winding of T1592 to apply a positive pulse to the Blanking
Pulse Regenerator V533B.
Once the transistor is saturated, there is no increase of
collector current. Therefore, there is no changing magnetic
flux to sustain the voltage on the base of Q590. The base
then seeks its reference level (ground), and this tends to turn
the transistor off. The reverse regenerative action occurs
which causes the base voltage to rise very fast to about +5
volts. Thus, Q590 is suddenly turned off after having been
momentarily on. Diode D590 is in the emitter circuit of Q590
to prevent breakdown of the emitter base junction during the
regenerative back biasing of the base by 7592.
Once the emitter-to-base junction of Q590 ceases to be
forward biased, no more emitter current flows. The emitter
therefore returns to its quiescent level as determined by the
time constants and voltages of the emitter circuitry. In
chopped mode R585 causes the emitter to seek a quiescent
level that will again forward bias Q590, and thus initiate
another cycle of B. O. action. The quiescent voltage level
and the time constant determined by R585, R587, C585,
and C587 is such that approximately 2 yseconds elapse
between B. ©. actions. Thus, in chopped mode the Block-
ing Oscillator free runs at approximately a 500-ke rate.
In alternate mode C585 and R585 are disconnected by
SW580 so that the quiescent level sought by the emitter is
not sufficiently positive to automatically turn @Q590 on.
However, the emitter level is close enough to the base
voltage to allow the B.O. to be triggered into operation by
a positive pulse received through the ALT. position of SW580
from C583 in the Sync Pulse In Emitter Follower circuitry.
Thus, during alternate-mode operation, the B. O. operates in
a triggered mode.
4-10
Sync Pulse In Emitter Follower
When the CHOP.-ALT. switch is set to ALT., the Sync Pulse
in EF stage provides a low-impedance drive for the Blocking
Oscillator stage. Incoming sync pulses from the Time-Base
unit are applied to the Sync Pulse In EF stage and the pulses
at the output of the stage are used to trigger the Blocking
Oscillator stage, which in turn, triggers the Ring Counter to
switch the channels.
During quiescence, Q583 is zero biased and is effectively
cut off. (Quiescence is the period of time when no sync pulses
are applied to Q583.) If there is a time base in the X-axis
opening, a positive-going square-wave sync pulse is generat-
ed by this time base during the sweep retrace interval. The
sync pulse is applied to pin 3 of the Type 3A74 interconnect-
ing plug. The time constant of C582 and R582 in the base
circuit of Q583 differentiates this square-wave sync pulse
and the positive-going leading edge drives Q583 into con-
duction.
NOTE
A time-base source is not mandatory for the proper
functioning of the Sync Pulse In EF stage. Any
5-volt positive-going gate or pulse applied to pin
3 of the interconnecting plug will affect Q583 in
a similar manner. For example, during X-Y opera-
tion, the sync pulse arriving through pin 3 of the
interconnecting plug, comes from another multi-
trace unit.
When Q583 goes into conduction, a positive-going pulse
is produced at its emitter: With the CHOP.-ALT. switch set
to the ALT. position, the pulse couples through C583 to the
emitter circuit of the Blocking Oscillator. Then, as described
earlier, the Blocking Oscillator is triggered into action. The
Blocking Oscillator produces a negative-going trigger pulse
which is coupled through C588 to drive the Ring Counter.
The Ring Counter, in turn, switches the channels during the
retrace interval.
Blanking Pulse Regenerator
V533B and its associated circuitry fulfills two purposes:
(1) to provide a blanking pulse which will shut off the crt
beam during the switching time between channels, and (2)
to supply a sync pulse output which will trigger the switching
circuits in another multi-trace unit. This second purpose is
used in X-Y display applications.
Each time there is a blocking oscillator action, V533B
functions. However, since chopped-mode multi-trace opera-
tion is the predominant mode during which the pulses are
used, it is this mode which will be discussed herein. Assume
for this discussion that the Type 3A74 is in the Y-axis opening
of the oscilloscope and a time-base unit drives the X-axis.
V533B operates in a modified bootstrap circuit where a
regenerated blanking pulse is taken from its cathode and a
regenerated sync pulse is produced at its plate. In its qui-
escent state (when not driven by the Blocking Oscillator),
V533B is essentially cut off. During chopped-mode multi-
trace operation, each time the free-running B.O. turns on,
the collector current in the primary of T592 induces a positive-
going pulse into its 6-turn tertiary winding.