Tektronix 3B2 TIME-BASE Instruction Manual page 13

saturation and continues to conduct until the multivibrator
is reset. When Q64 goes info saturation, its collector moves
negative to produce a negative signal that passes to SWEEP
DELAY switch SW170 and to the Miller circuit.
As long as the Miller circuit has a negative input, it
generates a delay-ramp signal. Such a negative input turns
off disconnect diode V72, and Miller capacitor C80 starts
charging through timing resistor R80, As C80 charges, the
grid voltage of Miller tube V91A moves in a negative direc-
tion. However, as this happens, the plate of V91A goes
positive and the positive change is coupled through cathode
follower V91B to the upper plate of C80, This increasing
voltage on C80 serves to keep the charging current of C80
constant, thus providing a linear rather than exponential
voltage change across C80 and at the cathode of V91B.
Therefore, the output of the Miller circuit (V91B cathode)
is a linear ramp that passes on to the delay-comparator
circuit and back to the gated multivibrator.
Delay Ramp Stops
The Miller ramp is coupled to the gated multivibrator
through D99 to the base of Q123. When the ramp exceeds
+15 volts, Q123 is back biased and turns off (emitter cannot
exceed +15 volts because of D116). When Q123 turns off,
it ends the ramp by switching D55 to its low-voltage state
which turns Qé64 off. With Q64 off, its collector moves
positive. This positive change brings disconnect diode V72
into conduction and Miller capacitor C80 discharges, thus
stopping the delay ramp rise.
At times, the sweep produced on the crt may end before
the delay ramp reaches +15 volts to stop the delay ramp
rise. When this occurs, the delay generator reset signal turns,
the delay ramp off. This reset signal is coupled through R115
and C115 to the emitter of Q@123, When the reset signal goes
negative at the end of the crt sweep, the negative pulse
formed turns Q123 off and thus stops the delay-ramp rise.
The next delay ramp cannot be started by a pulse from the
trigger generator until Q123 is allowed to conduct so that
D55 can be switched back to its high-voltage state. As long
as the ramp signal to Q123 through D99 is +15 volts or
greater, Q123 will not conduct. However, when the ramp
stops, the signal reduces and will allow Q123 to conduct.
Two other signals, however, can prevent Q123 from con-
ducting. One signal, the negative trigger lockout, is applied
to the emitter of Q123 through D114 to prevent conduction
during the time the Sweep Generator produces sweep volt-
ages for the crt. The second signal, from the holdoff circuit,
appears at D98 to keep Q123 from conducting for a short
time after the sweep is generated to allow the Sweep Gen-
The holdoff-circuit signal applies back bias to D98 to
hold Q123 cut off. D98 is back biased through D96 and D97
with positive charges on holdoff capacitors C100 and C270.
Circuit Description — Type 3B2
volts through R103 as the delay ramp starts. When the
delay ramp stops with SWEEP DELAY switch SWI170 set to
IN, Q103 conducts bringing one end of C100 to ground and
C100 provides a positive charge of about 10 volts at D96
(+12.2 volts minus the negative starting voltage of the
ramp). Thus, with the SWEEP DELAY switch set to IN, the
holdoff signal from C100 is a minimum of +10 volts and
may be increased depending on the value of delay-ramp
voltage generated. With the SWEEP DELAY switch set to
OUT, C100 remains connected to —12.2 volts through R103
and in some cases may never be charged to a positive value
by the delay ramp. Any positive charges developed on C100
are discharged at the end of the delay ramp.
Delayed-Trigger Pulse Generator
The linear ramp output of the delay Miller circuit at pin 8
of V91B is applied to the grid of voltage comparator V134A.
A variable de voltage, set by DELAY TIME control R149, is
on the other grid of the comparator (V134B). During quies-
cence, V134A is cut off and V134B is conducting. As the de-
lay Miller circuit starts running up, the grid of V134A gets
progressively more positive. When the ramp voltage on the
grid of V134A reaches and exceeds the voltage on the grid
of V134B, the comparator circuit switches states (V134A con-
ducts and V134B turns off}. As the current falls, V134B
switches D135 to its low-voltage state. The switching signal
of D135 is coupled directly to the base of Q164 where it is
amplified. The amplified signal is the delayed-trigger pulse
coupled to the DEL''D TRIG OUT connector through Q174
and to the SWEEP DELAY switch.
The amount of delay produced by the delay circuit depends
on how long it takes for the delay Miller circuit fo run up
to the voltage set by the DELAY TIME control. Hence, this
time can be changed either by varying the rate of rise of the
delay Miller circuit with DELAY TIME switch SW80, or by
varying the comparator switching voltage with DELAY TIME
V138 in the delay comparator circuit supplies a relatively
constant current to V134 to permit V134 to operate linearly
over a wide range. V149 regulates the voltage supplied
to the DELAY TIME control to increase the stability of the
The sweep generator operation starts with a trigger pulse
input from the Delay Generator. The incoming trigger
turns Q214 on. When Q214 turns on, tunnel diode D215
switches to its high-voltage state. When D215 switches, Q224
turns on. This, in turn, places a more-negative voltage on
the plates of disconnect diode V252. With the disconnect
diodes turned off, Miller capacitor C260 starts charging
through timing resistor R260. When C260 starts charging, the
control grid of Miller tube V261A attempts to move negative.
With its grid moving negative, the plate of V261A moves
positive. This positive change is coupled through a cathode
follower and back to the upper plate of Miller capacitor
C260. This increasing voltage across the Miller capacitor
serves to keep the charging current of C260 constant—thus
providing a linear rather than exponential voltage change
across C260 and at the cathode of V261B.