Tektronix 323, Instruction Manual

Page: 53 / 170

Circuit Description— Type 323
field causes current to flow from the top of the winding,
through D543, causing C543 to charge up to approximately
+ 100 V.
+ 1 7 5 V Power Supply
A secondary winding of T538 has one side referenced
to the 100 V winding pickoff point and the other side con­
nected to D540. During the time the primary field is col­
lapsing, voltage is induced into this secondary, adding its
value to that at the +100 V pickoff point. Current flows
through D540 and R541, developing + 1 75 V across C540
and C541.
+ T 4 V and + 9 V Power Supplies
Two secondary windings are connected in series-aiding
to provide current through D545 and C545 to generate the
+ 14 V power supply. A 9 V Zener diode, D547, uses this
14 V supply to provide the + 9 V reference used in the
power supply. The remaining 5 V is dropped across R547.
High Voltage Power Supply
A higher turns-ratio secondary winding drives the High
Voltage Multiplier which consists of D575 and C573 through
C579. The multiplier has 3 negative high-voltage taps: one
at —1900 V which supplies the CRT cathode; one at —2250 V
supplying the INTENSITY control circuit; and one at —1200 V
for the FOCUS circuit. The —1900 V tap is connected into
the CRT directly-heated cathode circuit in a manner that
keeps the AC filament voltage from changing the cathode
potential with respect to the grid, thus eliminating CRT in­
tensity changes. The INTENSITY LIMIT control, R583, is an
internal adjustment which sets the minimum difference volt­
age which can exist between the control grid and cathode.
This avoids cathode damage caused by excessive cathode
current.
The least negative voltage taken from the High Voltage
Power Supply appears at CRT pin 13, the focus anode. The
setting of the FOCUS potentiometer, R581, in combination
with ASTIG potentiometer R597, determines the sharpness
of the trace presentation. Only the FOCUS control is used
during routine operation, and it is capable of providing
a sharp trace at any intensity setting once the ASTIG con­
trol has been properly set.
+ 5 V Power Supply
The + 5 V Power Supply is dependent upon the 9 V ref­
erence supply and upon the T538 secondary winding which
supplies power through diodes D549 and D550. C550 be­
comes charged up to nearly the peak value of the output
of the secondary winding, and then determines the voltage
at the emitter of series-regulator Q558. This dictates the
voltage at the base of Q558, which then determines the
voltage at the base of Q557. The current through R557 is
therefore a fixed value, determined by the difference be­
tween the 9 V reference supply and the charge on C550. It
may be noted that current through R558 is also fixed, being
dependent upon the voltage at the base of Q558.
Error sensor Q555 constantly compares a selected por­
tion of the 9 V reference supply against the + 5 V output.
This comparison determines the collector current of Q555.
Any changes of Q555 collector current must be accompanied
by equal and opposite changes of Q557 emitter-base cur­
rent. This controls the total Q557 emitter current. Any
changes in it must be accompanied by equal and opposite
changes in Q558 base-emitter current. This controls the
conduction of Q558. For example, if the + 5 V supply tends
to increase, Q555 decreases conduction, which decreases
Q558 current drive. Q558 decreases its conduction and
more voltage is dropped across it, keeping the + 5 V out­
put within limits. The + 5 V output is filtered by C559 and
L559 before being applied to external circuits.
— 5 V Power Supply
The output of the + 5 V Power Supply is used as the ref­
erence for the —5 V supply. The —5 V supply is derived
from the output of a T538 secondary winding which is
rectified by D560 and D561, and developed across C560.
This voltage determines the voltage at the base and emitter
of Q567 and
Q569. Current through R562 and R568 there­
fore remains relatively constant. A comparison between the
—5 V output at the collector of Q569 is made against the
+ 5 V supply, and a voltage near —0.6 V is applied to the
base of Q562. If the —5 V supply tends to go positive, cur­
rent through Q562 decreases. This decreases the
Q567
emitter-base current, which decreases its collector current.
The Q569 emitter-base current increases, causing an increase
in Q569 collector current, keeping the —5 V supply within
design limits.
CRT Circuit
+100 V appears at pin 5 whenever the Oscilloscope is
energized. Pin 9 has 0 V applied except during sweep
time or external horizontal operation, during which time
+ 100 V is applied. When the voltage at pins 5 and 9 are
unbalanced, the CRT beam is deflected into the pin 9 plate
and cannot strike the CRT phosphor. When +100 V is ap­
plied to both plates, the deflection effect is nulled, and posi­
tion control is exercised by the horizontal and vertical de­
flection plates.
The GEOMETRY control adjusts for a minimum amount of
bowing of vertical and horizontal lines, regardless of the
area to which they are positioned.
The TRACE ROTATION potentiometer (R592) controls the
current through a trace rotation coil, thus creating a mag­
netic field through which the CRT electron beam passes.
When TRACE ROTATION is properly adjusted, horizontal
sweep voltages will cause the trace to follow paths which
are parallel to the horizontal graticule lines.
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