Tektronix 465B User Manual

Theory of
Operation—
465B Service
changing
the charge
on C,.
When the
positive-going A
Sweep
Start Gate
is
applied
to
the
emitter of Q4497,
CR4592 becomes
forward
biased and
turns off
Q4497.
Now
the A Sweep
starts, and
the Miller
Integrator
circuit
begins to
change the
charge on
Ct.
Q4497 will
remain off
until
retrace is
initiated
and
the
A Start Sweep
Gate is
removed.
Then Q4497
will become
forward
biased again,
and C, will
rapidly charge
to its quiescent
valuef
orthe start
of the
next A Sweep.
Sawtooth
Generator
The
Miller
Integrator circuit is
comprised of
Q4498
and
Q4598.
It
works on the
principle
that if the
charging
current to
a
capacitor can be
held
constant,
then the
charging
curve
will
be
linearratherthan
exponential.
The
action
starts
when
Disconnect
Amplifier Q4497
is turned
off by
the A Sweep
Start Gate.
The
selected capacitor
for
the
chosen setting
of the
TIME/DIV switch (C
t
)
begins to
charge
through
the R,.
This causes
the junction
of
Ct
and
R, to
start positive
in the
direction of
+v,, thereby
causing
the
gate of Q4498
to start
positive. The
Q4498
sourcethen
starts
In a
positive direction
and
increases the
forward bias
on Q4598
causing
the collector
voltage to
move
in
a
negative
direction
(less
positive). This
couples back
through C,
and opposes
the positive
change at
the gate of
04498.
Capacitor C, is
attempting to
charge
toward +V,,
but the action
of Q4498
gate being
held
virtually constant,
and the
collector
of Q4598
going more
negative,
results in
the
reduction of
the charge
on C, (it
discharges).
The gate
of
Q4498 rises
positive
about 10
millivolts over
the entire
sweep
generation time.
Since
the voltage at
the gate
of
Q4498 remains
relatively
constant, both
the
voltage across
R,
and the
current
through R, (the
current
discharging
from C.)
remain
constant.
The linear
rate of
discharge of C,
results
in a
linear ramp
across it.
The
resultant output at
the
collector of
Q4598
appears as a
negative-going
ramp,
dropping from
approximately +13
volts to
approximately
+2
volts.
When
the ramp
reaches
+2 volts,
Q4581 sends a
pulse
to U7375
initiating
retrace.
Transistor
Q4497 turns on,
and
its
collector goes
more
negative. This
moves the
gate
of
Q4498 i n the
negative
direction, causing the
voltage on the
base
of Q4598 to go
more
negative,
thereby
causing its
collector
voltage to
go
more positive.
Now C, charges
rapidly through
Q4497 to its
quiescent
state in
preparation
for the
next A Sweep
start
gate.
Output Amplifier
The Output Amplifier
is a
common-base amplifier
with
the signal
current-driven
into the
emitter. It provides
the
output
sawtooth
current signal to
the
Horizontal Amplifier
and
provides a
measure of
isolation between
the Sawtooth
Generator
and the
Horizontal
Amplifier. The
Horizontal
Display
Switching
circuit connects to
this stage
and
controls the A
sawtooth
output in
the various
horizontal
modes of
operation. In
the
A
and A
INTEN modes of
operation, the
A sweep
signal
passes
through
Q4475 to the
Horizontal Amplifier.
In the ALT mode,
Q4475 is enabled
for the
A sweep and
turned off for the
B sweep by
the
Horizontal
Display Logic
circuit.
In the B
DLY’d mode,
Q4475 is held off, and
Q4465
in the B Sweep
Generator is
held on to pass the B
sweep sawtooth
to the
Horizontal
Amplifier.
A
Sweep
Start
Comparator
Just before
the sweep
starts to
run down, the
levels at
the bases
of
Q4588A and
Q4588B are
approximately
equal. When the
sweep
starts
to
run
down, the
base of
Q4588B goes negative,
following the
collector of Q4598.
This increases
the forward
bias
on CR4581
which in turn
decreases the
forward bias
on CR4582.
Very
shortly after
start
of the sweep,
CR4582
becomes
reverse
biased and
interrupts the
current
through
Q4588A. The
circuit
remains in this
condition
until after the
sweep
retrace is
complete.
When the circuit
returns to
quiescence,
Q4588A
again begins to
conduct
through
R4587. This
sets the
currents
through Q4497,
establishing
thestarting
pointfor
the sweep.
Sweep
Start adjustment
R4570 sets the base
level
of Q4588A.
This level is
also connected
tothe base
of
Q4565
in the B
Sweep
Generator to ensure
that the
B
sweep
starts at the
same
level
as
the A
sweep.
A Sweep
End of
Sweep
Comparator
The A Sweep
End of Sweep
Comparator is a
switching
circuit
comprised of Q4581
and Q4582.
At quiescence,
Q4582 is
conducting
and Q4581 is turned
off. When
the
sweep
starts to
run, the
negative-going
ramp at
the
collector
of Q4498 is
coupled
through the base
of Q4588B
and
through CR4581 to
the cathode
of CR4577.
When the
collector
of Q4498
reaches about +2
volts, the
cathode of
CR4577
reaches
about +4 volts.
CR4577
begins conduc-
ting and turns
on Q4581,
which then turns
off Q4582.
The
resulting
positive step
at the collector
of Q4581
is fed to
pin 16
of U7375 where
it is used
to initiate
retrace and
holdoff.
The
negative-going
pulse at
the collector
of
Q4582 is
applied
to
the A
Sweep Z-Axis
Logic Gate
to
blank the ert as
soon as a
sweep-end
command
is
generated.
B
Sweep
Generator
Differences
There are two
main
differences between
the A Sweep
Generator and
the B Sweep
Generator.
The first is
that
Q4545 is used as
a constant
current source
in the B Sweep
Start
Comparator circuit.
The second
is that one
of the
outputs of the
B Sweep
Generator is
controlled by
the
B
ENDS A
switch associated
with the A
Trigger
HOLDOFF
control.
In the B ENDS
A position,
the end of the
B sweep
also ends the
A display
on the crt.

Tektronix 465B Specifications

General

Brand	Tektronix
Model	465B
Category	Test Equipment
Language	English