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of the
Output
Amplifier
2nd
stage
because
the
grids
of
V464 and V474 will be virtually grounded.
Vertical
Positioning.
The
POSITION
control
R438,
con-
nected
between
the differential
inputs to the diode switches,
provides differential currents that act as positioning signals
superimposed
on
the output
signal
currents
of Q424
and
Q434,
When
the POSITION
control is set to its electrical
center, approximately 1.3 ma flows in each leg.
When the POSITION
control is moved to either end from
center
a change
of about 3 volts per side occurs at the
collectors of Q444 and Q454 (First Output Amplifier stage).
This
voltage
range
corresponds
to at least
+6
divisions
positioning range at the crt.
Diode
Switches.
The
combined
differential
current sig-
nals from
the Input Amplifier stage and
positioning
circuit
are applied to diode switches D440, D441, D442 and D443.
These
diodes
act
like a double-pole
double-throw
switch.
Each pair, D440 and D443, or D441
and D442, is 'on' while
the other pair is "off.
Switching of the diodes to connect
or disconnect a channel is controlled by the MODE switch via
the Ring Counter in the Channel Switching Circuit.
Assume
that channel
1 is turned
off (MODE
switch
is set
to OFF) and the POSITION
control is centered.
With channel
1 turned off, Q545 in the Ring Counter stage is not conduct-
ing (see Fig. 4-2).
The —13.1-volt level at the cathodes of
shunt diodes
D441
and
D442
is more
negative
than
their
anodes
(—12.8
volts).
As a result, D441
and
D442
are for-
ward biased and return channel 1 input Amplifier and posi-
tioning currents through R548 to —100 volts.
With the shunt
diodes
forward
biased,
series
diodes
D440
and
D443
are
reverse
biased.
The
reverse- biased
series
diodes
turn
the channel off; that is they effectively disconnect channel 1
from the Output Amplifier.
If channel
1 only
is turned
on (MODE
switch
is set to
NORM.
or INV.}, Q545
in the
Ring
Counter
conducts.
The
voltage
level
at the
junction
of the shunt diodes
reverse
biases the shunt diodes
permitting the series diodes to be
forward biased.
As a result the Input Amplifier and position-
ing currents are applied through the series diodes and con-
necting busses to join with the current from the first stage of
the Output
Amplifier.
The
de-current
paths
are shown
in
Fig. 4-3.
in multi-trace operation (more than one channel is turned
on with the MODE
switch), positive-going gating pulses from
the Ring Counter turn the channels on in sequence.
Assume
that all four channels are operating and channel 1 is "gated"
on first (see Fig. 4-4).
With
channel
1 gating
pulse at its
—11.8-volt
'"'on"
level, D441
and
D442
are
reverse
biased.
Diodes D440 and D443 are forward biased.
With D440 and
D443
conducting, the signals are applied through
D440 and
D443, and then through the connecting busses to the Output
Amplifier.
At
the
termination
of the
channel
1 gating
pulse,
the
—13.3-volt
"off" level disconnects channel
1, reverse biasing
series
diodes
D440
and
D443,
and
forward
biasing
shunt
diodes
D441
and
D442.
With
channel
1 turned off by the
—13.1-volt
level, the channel
1 signals cannot be applied
to the Output Amplifier.
While the —13.1-volt level holds
channel
1
off,
each
of the
other
channels
is switched
on
(gated on) in sequence by its own gating pulse.
®
Circuit Description — Type
3A74
TIME
—
{
i }
Ch 1
Ch 1
—11.8v (On) + 1On
On
1.3
—13.1
v (Off)
(a) Gating
pulses
applied
to Ch.
1
D441/D442
junction.
Ch
2
On
{b) Gating
pulses
applied
to Ch.
2
D441/D442
junction.
I
i j
t
Ch3
!
Ch 3
On
—11.8v
orf
a
1
i
21.3 v!
{
L
|
—13.1v (Offi
:
\
;
(c) Gating pulses
\
\
'
applied
to Ch.
3
roy
D441/D442 junction.
|
1
Ch 4
Ch 4
|
1On;
On
—11.8¥
oh
13
_
( ote
13.1 v
(Off)
(d) Gating pulses
applied
to Ch.
4
D441/D442
junction.
Fig.
4-4.
Waveforms
showing
the
time
relationship
of
the
gating
pulses
as they
switch
on
their respective
channels
during
four-trace
operation.
Note
the
1-2-3-4
sequence.
As soon as channel 4 turns off, the channel 1 gating pulse
turns channel
1 on again to repeat the cycle {see Fig 4-4].
Note that only one channel is on at any one time.
Output
Amplifier
The Output Amplifier, which is the common
amplifier for
all channels,
consists of three stages: Output Amplifier 1st
Stage Q444/Q545,
Driver EF (Emitter Follower) Q463/@Q473,
and Output Amplifier 2nd Stage V464/V474.
Output Amplifier 1st Stage Q444/Q454.
The bases of
@444 and Q454 are connected directly to —12.2 volts. These
transistors
operate
in a
common-base
configuration.
The
voltage at their emitters is approximately —12.4 volts.
Emit-
ter
resistors
R447,
R448,
R457,
R458
and
the
POSITION
RANGE adjustment R449 combined together give each transis-
tor an effective emitter resistor of 12.4k referred to —100
volts (see Fig. 4-2 and 4-3). With the emitters at —12.4 volts
about 7.1 ma
flows in each
of the 12.4-k effective emitter
4-5
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