Theory ot Operation—465B Service
Q1175
and
Q1375 are common-base amplifiers that
convert the output
current signals from
U
1
290
i nto voltage
signals
to be
amplified in the Second Amplifier circuit.
Gain
adjust
R1482 allows setting of the overall gain of the
Channel 1 Vertical Preamplifier
by
adjusting the signal
voltage
to
the
ba3ea
of Q1155 and Q1355.
Second Amplifier
Q1155 and Q1355,
in conjunction with 01344 and
Q1346
in the Vertical Switching circuit, form a push-pull
cascode
amplifier. CR1263. CR1264. and RT1267 provide
temperature compensation for the high-frequency
gain
amplifier
to
ensure constant gain in the
presence of
varying ambient temperature. As temperature
increases,
the
resistance
value of
RT1267 decreases, and the reverse
bias on both CHI 263 and CR1 264 decreases CR1 263
and
CR1264 are voltage-variable capacitors whose capac-
itance
increases
as
reverse bias decreases. The increase
in capacity at higher temperatures
provides additional
high-frequency peaking to
counteract
the
effects of
increased
temperature on the amplifier's gain
The push-pull signals at the emitters of Q1155 and
Q1355
are converted
to a
single-ended signal
by
Q1150.
Q1250, Q1045,
and Q1145. The current signal fromQ1145
is converted
to
a voltage signal by common-base
amplifier
stage
01038 and applied to the bases of Q1 125 and Q1028
Q1028
provides the output signal to the CH 1
VERT
SIGNAL OUT
output
connector located ontheinstrument
rear panel
CR1014, CR1024, CR1023.
and CRl013protect
the emitter
circuit of Q1028 in the event large
signal levels
are
accidentally connected to the
CH 1 VERT SIGNAL
OUT
connector. The output
signal at the emitter of Q1125
is used as
the trigger signal
sourceintheCH
1
positions
of
the
Trigger SOURCE switches
and
as
the signal source
for
emitter follower
Q1004 R1126 adjusts the dc
level of the
CH
1 trigger source signal
When intheX-Y mode,
Q1004
provides the X-axis signal
from the Channel 1
Preamplifier
to
the Horizontal
Amplifier.
R1055
is the
Channel 1
Vertical
POSITION
control.
When set to its mid-position, the constant current supplied
by Q1062 flows equally through each side of R1055 into
the collectors
ot Q1155 and Q1355. As the POSITION
control is rotated off its mid-point, one side of the amplifier
receives more current while the other side of the amplifier
receives
less
current This proportionally changes the
amount of current flowing into
the
Delay
Line
Drivers,
therefore causing the trace to
be
positioned vertically on
the ert The mid-range operating point of the POSITION
control is set by
adjusting R1135
CHANNEL 2
PREAMP
Introduction
The Channel 2
Preamplifier
circuit
is virtually the same
as the Channel 1
Preamplifier circuit The
only
differences
between the two circuits are
described
in
this part. Input
signals for vertical deflection on the ert
are connected to
the CH 2
OR Y input connector. When the
TIME/DIV
switch is set to the
X-Y mode, the Channel 2 input signal
provides the vertical (Y-axis)
deflection
First
Amplifier
Basically, the First
Amplifier stage in Channel 2
operates the
same
as
the First Amplifier stage in
Channel
1 However, the
Channel 2 circuit also contains the
INVERT switching
function This allows the Channel 2 ert
display
to be inverted.
When pushed in, the INVERT switch
changes the biasing on the output
transistors of
U1790
so
that the normally inactive transistors (B
and
C)
now carry
the signal Since
their
outputs are
cross-coupled from side
to
side, the output
signal
is of
opposite polarity from the
signal available when the
INVERT switch
is in
the normal
(button out) position,
Channel
2
Invert Balance poten-
tiometer
R1975 allows the dc balance of
the
stage to be
adjusted to eliminate baseline shift in
the display when
switching from
a
normal to an inverted display.
VERTICAL
SWITCHING LOGIC
Q
Introduction
The Vertical Switching Logic (Figure 3-3)
determines
he input signal or combination of input signals to be
connected
to
the Vertical Output Amplifier. Possibleinput
signal combinations that may be
displayed are
selected by
a
read-only memory (ROM) integrated circuit that is
controlled
by
the
VERT MODE switches and the X-Y
position of the
TIME/DIV
switch
Diode Gates
The Channel
1,
Channel
2,
and Trigger View Diode
Gates, consisting of
four diodes each, act as switches that
are
controlled
by
the Vertical Switching Logic circuit.
Outputs
Qu. Qi,
and
Q
2
of U1705(pins
2, 7,
and
10)
control
the switching transistors that switch the Diode Gates on
or
off These output signals
alsoarefedintothe
Ao. Ai,
and A
2
inputs of ROM
U 1605
(pins 5,
6,
and
7),
to indicate the state
ot the
switches (Refer to the Maintenance section of
this
manual for a Logic
Table of the ROM functions.) A LO
indicates that a particular switch is on,
and
a
HI indicates it
is off The ROM is programmed to use
the state indicators
from
U1705
and the selected
VERTMODEinputstoU1605
Aj
through A? (pins
4, 3,
2,
1,
and
15)
to turn
on the correct
Diode Gates for obtaining the selected signal,
or
combina-
tion of signals, to be displayed
3-6
HEV A JUN 1979
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