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graticule center.
When
the TRIGGER switch is set to CH 1
ONLY,
the lead from the divider is grounded
and the chan-
nel 1 only trigger is applied to V533A.
Channel
1 Only Internal Trigger.
The channel 1 trigger
is derived
from
the cathode
of V423,
channel
1 Input CF
stage.
The cathode is a convenient low-impedance point for
extracting the signal without affecting the frequency response
of the channel
1 input amplifier.
In addition, this point is
isolated from the gating pulses that control the diode switches
during multi-trace operation.
The
trigger
is applied
to the
Interna!
Trigger
Input
EF
(Emitter Follower)
stage Q503
whose
output drives the In-
ternal Trigger Amplifier Ist Stage Q504/Q514.
A relatively
high
gain-bandwidth
product
is obtained
from
@Q504
and
its associated
circuitry.
This high performance
can only be
gotten by returning the emitter of Q504 to a low impedance
source with a voltage level quite close to the base of Q504.
Transistor Q514 provides this low impedance voltage source.
When
channel
1 is properly de-balanced, the cathode volt-
ages of V423
and
V433
are at the same
level.
Thus, by
using
the
cathode
of V443
as a reference
and
inserting
diode
D512 to give the same amount of voltage offset as
Q503, the bases of Q504 and Q514 are at the same voltage.
Hence
by emitter follower action the emitter of Q514
pro-
vides the proper voltage at a low impedance for the emitter
of Q504
to look into.
R510 degenerates the stage slightly
to stabilize the gain.
Diode
D502
protects Q503
in the event that a positive
overload voltage is applied to the channel 1 Input connector.
When the overload occurs, D502 conducts and limits the re-
verse bias of Q503.
The amplified trigger signal is then applied to the base of
@524,
the
Internal
Trigger
Amplifier
2nd
Stage.
Here
the
trigger is further amplified
so that its amplitude
approxi-
mates
the
amplitude
of
the
composite
trigger.
Inductor
L525
in the
collector
circuit
of Q524
aids
the
high
fre-
quency
response
of the stage.
With
the TRIGGER
switch
SW530
set to CH
1 ONLY,
divider resistors R529 and
R528
offset the
output
of Q524
to provide
proper
dc voltage
level at the grid of V533A.
C529
compensates
the small
amount
of attenuation
in this divider.
The
channel
1 trigger signal, applied
via the TRIGGER
switch to the grid of V533A, appears at the cathode.
From
the cathode of V533A the trigger is applied through pin 1]
of the interconnecting
plug to the time base circuitry.
The
CHAN.
1 INT. TRIG. DC LEVEL adjustment R521
is used to
set the de level at the cathode of V533A to zero volts.
The
adjustment is made when no signal is applied to channel 1
and the channel
1 trace is vertically positioned to coincide
with the graticule center.
CHANNEL
SWITCHING
CIRCUIT
The Channel Switching Circuit is composed of these stages:
Ring
Counter
(Q545/Q555/Q565/Q575),
Blocking
Oscillator
Q590, Blanking
Pulse Regenerator V533B, and the Sync Pulse
IN EF {Emitter Follower} Q583.
In the following
discussion
the operation of the Channel Switching Circuit is described
in each of these modes of operation: (1) single-trace opera-
tion,
(2}
chopped-mode
multi-trace
operation,
(3) alternate-
mode multi-trace operation, and (4) X-Y multt-trace operation.
®
Circuit Description — Type
3A74
The
individual
stages in the Channel
Switching
Circuit are
variously used in specific modes of operation, and they are
subsequently
described whenever
convenient.
Ring
Counter
The
Ring
Counter
consists
of Q545,
Q555,
Q565,
Q575,
and associated
circuitry.
Each transistor controls the appli-
cation of one Switched
Amplifier to the Output Amplifier.
The
Ring
Counter
is tetra-stable;
that
is, each
of its four
states is stable (meaning that one transistor is turned on and
the other three transistors are turned off).
Each stable state
of the Ring Counter produces a positive-going current gate
which closes the series diode switches between one channel
and
the Output
Amplifier.
During
multi-trace operation, a
trigger pulse is required to make the Ring Counter "count"
and thereby sequentially apply each Input Amplifier channel
to the Output Amplifier.
Single-trace
operation.
Assume
for this discussion that
channel
1 is turned on (MODE
switch set to NORM.
or INV.}
and
the other channels
are turned
off by means
of their
MODE
switches.
Under these conditions, the Ring Counter is
locked into one of its stable states.
In this case, then channel
1 transistor Q545
is conducting
and
the other transistors in
the Ring Counter are reverse biased; that is, locked out.
The dc level at the base of each
transistor in the Ring
Counter is determined
by a matrix consisting of three 10-k
resistors and
a 3.37-k
resistor to ground.
For each
channel
which is off there is an additional 10-k matrix resistor which
connects
from
the junction of D580
and
R580 through the
MODE
switch
contacts
to the base
of the reverse biased
transistor.
At the base
of @545
the resistor matrix
is R542, R543,
R544, and
R545.
Currents
in the collector circuits of the re-
verse biased transistor flow through the Q545 resistor matrix
and hold the base of Q545 at about —5.9 volts.
Base matrix currents flowing through R550, R560, and R570
serve
two
functions.
First, they shift the base voltages
in a
less negative direction and thus add to the reverse biasing
of Q555,
Q565
and
Q575.
Secondly,
these currents have a
controlling influence over the operation of the Blocking Oscil-
lator,
"Constant current" resistor R580 can supply about 0.85 ma
of current to the base matrices without its D580/R580 junc-
tion going negative.
Since the MODE
switches for channels
2, 3, and 4 are set to OFF, as previously assumed,
resistors
R550,
R560 and
R570
draw
about 0.4ma
each or a total
current of about 1.2 ma.
Since this is more current than R580
can supply, its D580/R580 junction voltage level decreases to
the
point
where
D580
reverse
biases
and
D581]
forward
biases.
D581
now supplies the current to the base matrices
that R580 was unable to supply.
With D581
conducting, the voltage level at the anode of
D590
is not sufficiently positive to forward
bias D590 and
turn on Q590
in the Blocking
Oscillator stage.
Under these
conditions, there will be no blocking oscillator action during
single-trace operation.
With
no trigger pulses from Q590,
the Ring Counter cannot switch from the turned-on channel
to a turned-off channel.
In addition, with the Blocking Oscil-
lator inoperative, no Blocking Oscillator initiated transients
will occur to ''feed-thru'' and interfere with the single-chan-
nel display.
4-7
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