Tektronix 2235A Instruction Manual page 68

Theory of Operation
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2235A Instruction
altered as necessary to maintain 43 V across C940. T o
shut off Q9070 during each oscillator period, Q908 is
used to discharge the gate-drain capacitance. Pin 10 of
U930 goes LO, reverse biasing CR908 and turning on
Q908 to shut off the F f l .
Once the supply is running, power to U930 will be sup-
pliedfrom the winding connected to pins 6 and 7 of T906.
Diode CR920 half-wave rectifies the voltage across pins
6 and 7 to keep filter capacitor C925 charged and to
maintain supply voltage to U930 through Q930.
Instrument protection from excessive output voltage is
supplied by silicon-controlled rectifier Q935. Should the
Preregulator output voltage exceed 51 V , Zener diode
VR935 will conduct, causing Q935 to also conduct. The
Preregulator output current will then be shunted through
Q935, and the output voltage will very quickly go to zero.
With the supply voltage of U930 no longer being pro-
vided by the winding connected to pins 6 and 7 of T906,
the Preregulator will shut down and Q935 will be reset.
The supply will then attempt to power up, but may again
shut down if the overvoltage condition is again reached.
This sequence continues until the overvoltage condition
is corrected.
lnverter
The lnverter circuit changes the dc voltage from the
Preregulator to ac for use by the supplies that are
connected to the secondaries of T948.
The output of the Preregulator circuit is applied to the
center tap of T948 (shown on Diagram 10). Power-
switching transistors Q946 and Q947 alternate con-
ducting current from the Preregulator output through the
primary windings of T948. The transistor switching action
is controlled by T944, a saturating base-drive
transformer.
When the instrument is first turned on, one of the
switching transistors will start to conduct and its col-
lector voltage will drop toward the common voltage
level. This will induce a positive voltage from the lead of
T944 which is connected to the base of the conductirlg
transistor and reinforce conduction. Eventually T944 will
saturate, and as the voltage across T944 (and T948)
begins to reverse, the conducting transistor will cut off
because of the drop in base drive. The other transistor
will not start conduction until the voltage on the leads of
T944 reverse enough to bias it on. This process will
continue, and the saturation time of T944 plus the
transistor-switching time will determine the frequency of
lnverter operation (typically 20 kHz). After the initial
lnverter start up, the switching transistors do not
saturate; they remain in the active region during
switching.
Diodes CR946 and CR947 serve as a negative-peak
detector to generate a voltage for controlling the output
of the error amplifier. Capacitor C943 charges to a
voltage equal to the negative peak voltage at the col-
lectors of Q946 and Q947, referenced to the Preregulator
input voltage. This voltage level is applied to the divider
composed of R937, R938, and R939. The error amplifier,
composed of Q938 and Q939, is a differential amplifier
that compares the reference voltage of VR943 with the
voltage on the wiper of potentiometer R938. The current
through Q939 will set the base drive of Q944. This
voltage will bias Q946 and Q947 to a level that will main-
tain the peak-to-peak input voltage of T948. The ampli-
tude of the voltage across the transformer primary
winding, and thus that of the secondaryvoltages ofT948,
is set by adjusting -8.6 V Adjust potentiometer R938.
At turn on, Q938 is biased off and Q939 is biased on. All
the current of the error amplifier will then go through (2939
to bias on Q944. The current through Q944 controls the
base drive for Q946 and Q947. Base current provided by
base-drive transformer T944 will charge C944 negative
with respect to the lnverter circuit floating ground
(common) level.
Crt Supply
High-voltage multiplier U975 (Diagram 10) utilizes the
2-kV winding of T948 to generate 12 kV to drive the crt
anode. It also uses an internal half wave rectifier diode to
produce -2 kV for the crt cathode. The -2 kV supply is
filtered by a low-pass filter composed of C975, C976,
R976, R978, and C979. Neon lamp DS870 protects
against excessive voltage between the crt heater and crt
cathode by conducting if the voltage exceeds approxi-
mately 75 V .
Focus Circuit
Focus voltage is also developed from the -2 kV supply
via a voltage divider composed of Q885, R890, and
FOCUS potentiometer R893. The focus voltage tracks
the A-intensity level through the action of Q885. The
emitter voltage of Q804, set by the A INTENSITY control,
is applied to the emitter of Q885 through R885. When the
emitter voltage of Q804 changes, the current through
Q885 changes proportionally and alters the voltage at
one end of FOCUS control R893.
Low-Voltage Supplies
The low-voltage supplies utilize center-tapped secon-
dary windings of T948. The
+
102 V supply uses CR954
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