Circuit
Description-Type
453/R453
C456I
r
-----1
SLOPE
1--
--
-1
l SW455
I
I
I
I
I
C466I
R459
-12
v
+
12
v
R467
R469
+75
v
C467
'--i
R473
Q473
Current
path
;1rc;11:111rtRl!lt•I
*
Reverse-biased
diode
[:>"Ito----
Fig.
3-10.
Trigger
path
for
positive-slope
triggering
(simplified
A
Trigger
Generator
diagram).
near
zero
volts. The
base
of
Q485
is
held
at
about
-0.65
volts by the
forward
voltage
drop
of
0484.
Since the
base
of
Q495
is
the most positive·, it conducts
and
raises the emitter
level of
Q485
positive
enough
to hold it off. C485
charges
to
about
+ 13 volts
where
it
is
clamped
by
0486
and
0493.
The
base
of
Q494
is
clamped
at
about
+ 12.6 volts by
0493
which reverse biases it. Since there
is
no current flow
through Q494, its
colledor
level
goes
negative.
When
a
trigger
signal
is
present, the positive-going pulses
from the Auto Pulse Amplifier
stage
turn
Q485
on through
0484.
The collector
of
Q485
goes
negative
and
C485 dis-
charges
rapidly
through Q485,
R490
and
R485.
As
C485 dis-
charges,
the current flow through
R490
biases
Q495
off.
When
C485
is
fully
discharged,
the current flow through
R490
ceases
and
Q495
comes
back
on
to reset the multi-
vibrator.
Now
C485 begins to
charge
towards
+75
volts
through
R486.
Current also flows through
R494
and
the
base
of
Q494
goes
negative
to
bias
it on. The collector level
of
Q494
rises positive to
produce
the
auto
gate
output
for the
A
Sweep
Generator
circuit.
3-14
For low-frequency signals (below
about
30
hertz), C485
recharges
to
about
+ 13 volts
in
about
85
milliseconds. Then
Q494
is
biased
off to
end
the
auto
gate
(display free runs
or
is
unstable). However,
if
a repetitive
trigger
signal turns
Q485
on
again
before
C485 has
charged
to + 13 volts, C485
is
discharged
completely
again
and
once
more starts to
charge
towards
+75
volts. Since the
base
of
Q494
remains
negative
enough
with a repetitive
trigger
signal to hold it
in
conduction, the
auto
output
level
is
continuous for a
stable
display (with correct A
LEVEL
control setting).
A
SWEEP
GENERATOR
General
The A
Sweep
Generator
circuit
produces
a
sawtooth
volt-
age
which
is
amplified by the Horizontal Amplifier circuit
to
provide horizontal
sweep
deflection on the
CRT.
This
output
signal
is
generated
on
command
(trigger pulse) from
the
A
Sweep
Generator
circuit. The A
Sweep
Generator
cir-
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