Tektronix 323, Instruction Manual

Page: 49 / 170

Circuit Description— Type 323
o large resistor to the summing node. The POSITION con­
trol can move the fine potentiometer wiper 30° independent
of the coarse potentiometer, moving the trace 1 division
in X I mode, and 10 division in X 10 mode. Continued
movement of the POSITION control causes both of the wip­
ers to move. Combined movement of the coarse and fine
wipers permits an approximate 14 division total horizontal
movement range in X I mode.
The R404-R405 circuit compensates for changes in the
sweep start position which would otherwise occur as a re­
sult of changing sweep rates. Refer back to the Sweep Gen­
erator schematic. It was previously stated that the sweep
start reference was established by connecting the collector
of Q326 to the gate of Q311 by way of D309 and the
emitter-base junction of saturated Q343. The D309 current
flows through the timing resistors. Smaller resistors are in­
serted when faster sweep rates are selected. This causes
the D309 current (and therefore the voltage across it) to
increase slightly with increases in sweep rates. Q311, Q317
and Q326 respond to these slight changes and the quiescent
output voltage changes accordingly.
The slight increase in quiescent voltage from the sweep
generator during fast sweep rate selection causes an in­
crease in current through R401 and R402. This current change
is compensated for by decreasing the resistance of the R404-
R405 circuit as the sweep rate is increased. The sweep
starting position, therefore, remains relatively independent
of sweep rate.
When X 10 HORIZ MAG operation is selected, a cur­
rent divider network is connected into the feedback circuit
in such a manner that only one-tenth of the R430-C432 feed­
back current passes through R431-C433, with the remaining
current flowing in the path provided by R433, R437, R438
and R439. This reduction of degenerative feedback current
requires that the output voltage be ten times as large as in
the X I situation to provide sufficient feedback current to
balance the input signal at the base of Q415. Due to out­
put voltage limitation, this can only occur as the input sweep
voltage passes through slightly more than 10% of its sweep
sawtooth.
An instant exists during a magnified sweep when all the
R430 feedback current flows to the summing node at the
base of Q415, and no current flows through R433. This
occurs when the voltage at the R430-R431 junction is equal
to the voltage at the R437-R438 junction. The sweep posi­
tion is then totally under the control of the sawtooth sweep
voltage, just as it is during X I sweep presentation. This
is the magnified sweep registration point, or the point on
the CRT at which the same instantaneous presentation will
appear during either X I or magnified sweep. R439 (SWP
MAG REGIS) permits adjustment of the R437-R438 junction
voltage, so that this registration point will occcur at the
graticule center vertical line. When R439 is properly set,
the 1 division of horizontal presentation which straddles the
center graticule vertical line during X I operation will be
magnified and appear as a 10 division display when the
X 1 0 HORIZ MAG knob is pulled out. Any 10% portion of
the sweep sawtooth can be displayed as a 10 division sweep
during X 10 HORIZ MAG operation by adjusting the Hori­
zontal POSITION control.
R433 (X 10 GAIN), which is located in the feedback cur­
rent shunt path, permits adjusting the X 1 0 feedback current
to provide calibrated X 1 0 HORIZ MAG sweep presenta­
tions. R433 must be adjusted after R439 (SWP MAG REGIS)
because R439 also affects the impedance of the shunt feed­
back path.
Current Control Circuit
Q440, Q450, and their associated resistors, diodes and
capacitors make up the current control circuit. The follow­
ing conditions exist during quiescence, when the electron
beam is deflected to the left edge of the CRT graticule:
+ 160 V exists on the left deflection plate while + 10
V is applied to the right plate;
D442 sets the base of Q440 at +168V , and the Q440
emitter at about +168.6 V;
The 6.4 V across R441 causes about 425/rA of current
to flow through R441 and the Q440 collector, where if
splits to provide operating current for Q427, and the cur­
rent which the + 1 6 0 V demands through R430 and R454;
The base of Q450 is at approximately the same po­
tential as the base of Q440, because of the small volt­
age across the R444-R446 voltage divider;
The emitter of Q450 is at about the same potential
as the emitter of Q440, with essentially no current flow­
ing through R451;
Approximately 150 /tA of current is demanded through
R450, passing through Q450 and providing operating cur­
rent for Q459, plus the small amount of current required
by the 10 V across R457.
When the trace is positioned to the right edge of the
graticule, conditions are changed as follows:
The voltage on the left deflection plate is at about
+ 10 V, while that on the right plate is about +160 V;
The current demanded through R430 and R454 decreases
in proportion to the voltage decrease, thus demanding
less current from Q440;
The voltage across R446-R444 increases to about 160 V,
lowering the base voltage and increasing the drive to
Q450;
The Q450 emitter voltage follows its base voltage,
creating an unbalance across R451;
The unbalance across R451 causes sufficient current to
flow through R451 and Q450 to supply the current re-
by the + 1 6 0 V across R457.
Note that only a very small change of voltage (about
0.5 V) occurs across R450, and its current remains relatively
constant. Also note that R441 current remains constant under
the two conditions, with the increased current demand of
one circuit being compensated for by the decreased demand
of the other circuit. Thus, with current shifting from the
Q440 side to the Q450 side of the amplifier, demands on
the + 1 7 5 V power supply remain constant.
During fast sweep operation, the lowered impedance of
C446 permits a greater portion of the left deflection plate
voltage change to be applied to the base of Q450. The
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