Tektronix 3A74 Instruction Manual page 31

The induced impulse is coupled through a short coaxial
cable to the grid winding of 7595. Since the other end of
the grid winding connects to the grid of V533B, the posi-
tive-going pulse drives V533B into conduction.
The rising cathode current produces a changing magnetic
flux in T595. This induces a voltage in the 10-turn grid wind-
ing that causes the grid to bias the tube more into conduc-
tion. This regenerative bootstrapping continues until the posi-
tive pulse from the B. O. tertiary reaches its maximum, and
begins to fall off. This falling-off action causes a slight de-
crease in grid voltage which decreases the cathode current
and thus initiates a regenerative turn off of V533B. V533B
remains essentially cut off until the next Blocking Oscillator
pulse repeats the cycle.
The surge of current through 533B causes the cathode
voltage to rise sharply. When V533B regeneratively turns
off, the cathode volfage undergoes an rc decay determined
by R595 and the total capacitance on pin 24 of the inter-
connecting plug. The resulting amplitude of the blanking
pulse is about 50 volts peak. From pin 24 the pulse is
coupled to the cathode of the crt. In oscilloscopes which
have a CRT Cathode Selector (Z-axis) switch, the blanking
pulse is applied to the ert cathode when the switch is set
to the Chopped Blanking position. The duration of the re
decaying blanking pulse which is in excess of the crt beam
current cutoff voltage is such that the pulse blanks the crt
during the switch-over time between channels.
In order to stabilize the voltage at pin 24 between blank-
ing pulses, diode D595 in the circuit of V533B clamps the
downward swing of the blanking pulse at a voltage a little
below ground. By holding the cathode near ground it also
insures that V533B is essentially turned off in its quiescent
state. 1593 in the plate circuit of V533B provides a Sync
Pulse Out, which indicates when the Type 3A74 is changing
channels. This syne is a@ positive-going pulse that is applied
to pin 4 of the interconnecting plug.
Multi-Trace Operation. Two Type 3A74 units can be
used together (in those oscilloscopes which accept both X—
and Y-axis plug-ins} to provide up to four synchronously
chopped X-Y displays. This is accomplished by having
the Y-axis Type 3A74 in CHOPPED and the X-axis Type
3A74 in ALTERNATE. The Y-axis plug-in free runs and pro-
vides a Syne Pulse Out to the left-hand interconnecting
Cirevit Description — Type 3A74
plug pin 4, Due to the crossed inter-connections between
pins 4 and 3 of the left and right-handed plug-in con-
nectors, the Sync Pulse Out on the left-hand pin 4 becomes
the Sync Pulse In on the right-hand pin 3. The X axis Type
3A74 is in ALTERNATE and thus its channels are switched
at the same time as the Y-axis channels.
To insure that the Y-axis channel 1 is displayed with the
X-axis channel 1, and the Y-axis channel 2 with X-axis chan-
nel 2, etc., the Ring Counters are locked in step by a ring
counter syne pulse on pin 18 of both interconnecting plugs.
Since the two pin 18's are tied together, the channel 4 Ring
Counter transistors (Q575) of both Type 3A74''s are ac-
coupled to each other. The two Ring Counters are stable
only when channel 4 in both units are operating in phase
with each other; that is, Y-Channel 4 displayed with X-Chan-
nel 4. One or two like-numbered pairs of channels can be
turned off (for example, channels 1 and 1} by means of
their MODE switches and the remaining pairs of channels
will synchronize as long as both channel 4's remain on.
For proper blanking of the X-Y display the Y axis Type
3A74 must be in CHOPPED and the X-axis Type 3A74 must
be in ALTERNATE. In the reverse situation there is an
X-Y display but the blanking is not optimum because of
the time delay in the blanking pulse applied to the crt
cathode. Since the crt cathode can be connected to only
pin 24 of the Y-axis plug-in, it is imperative that a blank-
ing pulse appear here as soon as possible after switching
action is initiated. This is accomplished in a normal man-
ner only when the Y-axis Type 3A74 is CHOPPED.
If the Y-axis Type 3A74 should be in alternate there is ap-
proximately a 0.3 microsecond delay before a blanking pulse
is presented to the crt cathode. In this non-optimal (reverse
situation) case the initial switching action begins in the free
running B. O. of the X-axis Type 3A74. Subsequently a Sync
Pulse Out from the Blanking Pulse Regenerator passes out of
pin 4 of this right-hand unit. Traveling over to pin 3 of the
left-hand interconnecting plug, it passes into the Sync Pulse
In EF of the Y-axis Type 3A74. Since the Y-axis Type 3A74
is in ALTERNATE the Blocking Oscillator in this unit is
triggered, which subsequently causes the Blanking Pulse
Regenerator to present a blanking pulse to pin 24 of the
Y-axis interconnecting plug. This entire sequence of opera-
tion accounts for the 0.3 microsecond delay and, hence,
improperly-timed blanking.