Tektronix 3A74 Instruction Manual page 26

Circuit Description — Type 3A74
resistors. This current comes from two sources which add to-
gether at the Q444/Q454 emitters. Under no-signal con-
ditions 3.8 ma comes through each connecting bus from the
series diodes of the channel then connected; 3.3 ma comple-
mentary current comes through each transistor (Q444 and
Q454) to make up 7.1-ma total demanded by the 12.4-k
effective emitter resistors (see Fig. 4-3).
Under signal conditions, the 3.3ma complementary cur-
rents in the Output Amplifier 1st Stage Q444/Q454 follow
the applied signal and/or positioning current variations in
the turned-on channel. These variations in current develop
voltage signals across collector-load resistors R444 and
R454. Voltage gain up to this point, as measured between
the single-ended input of V423 and the push-pull output of
the Output Amplifier Ist Stage, is nominally 50. Gain is
mostly determined by the emitter degeneration resistors in
the Input Amplifier stage and collector resistors R444 and
R454 in the Output Amplifier Ist Stage.
Peaking coils L444 and 1454 improve the high-frequency
response of the stage by providing an almost critically
damped response rather than a simple rc response. Networks
L445/R445 and L455/R455 compensate the stage for high-
frequency attenuation due to the characteristics of the transis-
tors and associated circuitry. Networks C460/R460 and
C470/R470 are time constants that improve the transient
response of the Type 3A74.
The POSITION RANGE adjustment R449, in conjunction
with R448 and R458, differentially adjusts over a small range
the quiescent currents through Q444 and Q454. Thus the de
levels throughout the Output Amplifier may be shifted so
that the electrical center of the Output Amplifier coincides
with the center of the crt screen. When R449 is properly ad-
justed, the positioning range of the individual channels is
centered; that is, the positioning range up is equal to the
positioning range down. As measured at the collector of
each transistor in the Output Amplifier Ist stage, the range
of R449 is about 2.8 volts. This is equivalent to a total of
about 5.6 divisions on the crt.
Driver EF Q463/Q473. These transistors are connected
as emitter followers to provide a low-impedance driving
source for the Output Amplifier 2nd Stage. Thus, the input
capacitance of the Output Amplifier 2nd Stage V464/V474 is
isolated from the collector loads of Q444 and @Q454, thereby
increasing the frequency response of the Type 3A74.
The Driver EF stage de couples the signal and the dc-
positioning voltages from the collectors of Q444 and Q454
to the grids of V464 and V474. Voltage dividers R461 /R462
and R471/R472 provide the emitters of Q463 and Q473
with a positive voltage source and sufficient source impe-
dance for the stage to function as an efficient emitter fol-
lower, Resistors R462 and R472 are in the dividers to keep
the grids of V464 and V474 from going to +125 volts in
the event that Q463 and/or Q473 are removed from their
sockets.
Output Amplifier 2nd Stage V464/V474. Output tubes
V464 and V474 and the associated circuitry are a push-pull
amplifier stage which drives the vertical deflection plates of
the oscilloscope. The gain of the stage is nominally 20.
Thus, the total gain of the Type 3A74, as measured from
the single-ended input at the grid of V423 to the push-pull
output at the plates of V464 and V474, is about 1000 (50
times 20).
4-6
In the cathode circuit, CALIB. control R477 is adjusted to
compensate for variations in crt deflection plate sensitivity
by altering the gain of the Type 3A74. This control is a
master gain control for all the channels. When adjusted,
R477 varies the cathode degeneration and thus the gain of
the stage. Maximum gain, and therefore maximum deflec-
tion of an input signal, occurs when R477 is set for zero
resistance.
Diodes D468 and D469 in conjunction with R468 improves
the linearity of the Output Amplifier 2nd Stage for large
signals by reducing cathode degeneration when the display
is more than several major divisions from electrical center.
In the plate circuits variable inductors L464 and L474
provide a means of adjusting the stage for optimum transient
response. R465 sets the output potential at an optimum value
for the crt deflection plates.
Internal Trigger Circuits
The internal trigger circuits provide a choice of two in-
ternal trigger sources: composite and CH 1 ONLY. Either
of these sources can be selected by means of the TRIGGER
switch SW530.
Composite Internal Triggering. This means of triggering
refers to the trigger signal derived from the plate of V474
in the Output Amplifier 2nd Stage. If more than one channel
is on (multi-trace operation}, the trigger is a composite of
successive signals, sampled in time, arriving from the "on"
channels.
During single-trace operation, the trigger is not time-shared
since only one channel is operating. Though not time-shared
as in multi-trace operation, the single-channel trigger is still
called a composite trigger to distinguish it from the CH 1
ONLY trigger. Naturally, during single-trace operation, the
composite trigger will resemble the displayed signal of the
operating channel.
In multi-trace operation, the composite trigger contains
both the ac and de components of the time-sampled signals
from the "on' channels. The dc component is made up from
the de level of the channel's POSITION control and, if the
applied signal is de coupled, also includes the de level of
the applied signal.
The composite signal, derived from the plate of V474, is
applied across a divider consisting of V535, R535 and R536
in units S/N 101 to 589; in S/N 590 and up units the divider
consists of C535, R535, R536 and R530. A portion of the
signal is picked off from the divider for use as a composite
internal trigger source. When the TRIGGER switch SW530
is pushed in (COMPOSITE on the schematic diagram), the
composite trigger is applied to the grid of V533A in the In-
ternal Trigger Output CF (Cathode Follower) stage V533A.
Ferrite bead L532 in the grid circuit of V533A reduces the
V533B blanking pulse radiation that couples into V533A grid
circuit due to the close proximity of the circuitry.
At the cathode of V533A, the trigger is applied through
pin 11 of the interconnecting plug to the time-base circuitry.
The COMPOSITE INT. TRIG. DC LEVEL adjustment, R530, is
used to set the cathode de level of V533A to zero volts
under no-signal conditions with only channel 1 turned on
and with the trace vertically positioned to coincide with the
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