Tektronix 2213, Инструкция по эксплуатации

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Theory of Operation—2213 Service
voltage on C957. This voltage will bias Q940 and Q942 to a
level that will maintain the peak-to-peak input voltage of
T940. The amplitude of the voltage across the transformer
primary winding and thus, that of the secondary voltages of
T940, is set by adjusting —8.6 V Adj potentiometer R946.
At turn on, Q948 is biased off and Q954 is biased on. All
the current of the error amplifier will therefore go through
Q954 to bias on Q956. Diode CR956 allows the base of
Q956 to go positive enough to initially turn on Q940 or
Q942. The current through Q956 controls the base drive for
Q940 and Q942. Base current provided by base-drive trans­
former T942 will charge C957 negative with respect to the
Inverter circuit floating ground (common) level.
NOTE
The following paragraph applies only to instruments
having the Current Lim it board.
Voltage from CR940 and CR942 also provides a
measurement of the minimum collector voltage of Q940 and
Q942 with respect to the Inverter circuit floating ground.
This voltage is fed back to the Preregulator through optical
isolator U931 to control the output voltage from the
Preregulator circuit. As the negative peak voltage at the
collectors of the switching transistors is regulated by the
error amplifier with respect to the output of the Preregulator,
control of the dc level from the preregulator will control the
minimum voltage with respect to the floating ground.
Potentiometer R952 (Head Room Voltage Adjust) is used to
set this minimum voltage level to a point that prevents
saturation and excessive power dissipation of the Inverter
switching transistors.
C R T Supply
High-voltage multiplier U990 utilizes the 2-kV winding of
T940 to generate 8 kV at one output 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
three-stage low-pass filter composed of C990, R992, R990,
C992, R994, and R995. Neon lamp DS870 protects against
excessive voltage between the crt heater and crt cathode by
conducting if the voltage exceeds approximately 75 V.
A uto Focus C ircuit
Focus voltage is also developed from the — 2 kV supply
via a voltage divider composed of R884, R882, AUTO
FOCUS potentiometer R883, R881, R880, R879, R878,
R872, Auto Focus Adjust potentiometer R875, and Q877.
The focus voltage tracks the intensity level through the
action of Q877. The Intens Level signal from the Auto
Intensity circuit (Diagram 6) is applied to the emitter of Q877
through R877. When the Intens Level signal changes due to
a changing display intensity, the current through the divider
resistors changes proportionally. Auto Focus Adjust
potentiometer R875 is adjusted to produce the best focus
tracking.
L o w -V o ltag e Supplies
The low-voltage supplies utilize the secondary windings
of T940 and are all full-wave, center-tapped bridges. The
+100 V supply uses CR961 and CR963 for rectification and
uses C961 for filtering. Diodes CR965 and CR967 rectify ac
from taps on the 100-V winding, and C965 filters the output
to produce + 30 V dc. The diode bridge consisting of CR971
through CR974 produces the + 8 .6 V and -8 .6 V supplies.
Filtering of the + 8 .6 V is accomplished by C971, C975, and
L971; while filtering of the —8.6 V is done by C972, C976,
and L972. Voltage regulator U985 uses the rectified +8.6-V
supply to produce the + 5 V output. Diode CR985 protects
the regulator by not allowing the output voltage to go more
positive than the + 8 .6 V input voltage.
DC R estorer
The DC Restorer circuit produces the crt control-grid
bias and couples both dc and low-frequency components of
the Z-Axis Amplifier output to the crt control grid. Direct
coupling of the Z-Axis Amplifier output to the crt control grid
is not employed due to the high potential differences
involved. Refer to Figure 3-8 during the following discussion.
The ac drive to the DC Restorer circuit is obtained from
pin 16 of T940. The drive voltage has a peak amplitude of
about 150 V and a frequency of about 20 kHz. The
sinusoidal drive voltage is coupled through C863 and R863
into the DC Restorer circuit at the junction of CR860,
CR863, and R864. The cathode end of CR860 is held at
about + 8 5 V by the voltage applied from the wiper of Grid
Bias potentiometer R860. When the positive peaks of the
ac-drive voltage reach a level that forward biases CR860,
the voltage is clamped at that level.
The Z-Axis Amplifier output-signal voltage is applied to
the DC Restorer at the anode end of CR863. The Z-Axis
signal voltage level varies between + 1 0 V and + 7 5 V,
depending on the setting of the AUTO INTENSITY control.
The ac-drive voltage will hold CR863 reverse biased until the
voltage falls below the Z-Axis Amplifier output voltage level.
At that point, CR863 becomes forward biased and clamps
the junction of CR860, CR863, and R864 to the Z-Axis
output level. Thus, the ac-drive voltage is clamped at two
levels on the positive swing of the cycle to produce an
approximate square-wave signal with a positive dc-offset
level.
The DC Restorer is referenced to the — 2 kV crt cathode
voltage through R867 and CR867. Initially, both C865 and
C864 will charge up to a level determined by the difference
between the Z-Axis output voltage and cathode voltage.
Capacitor C865 charges from the crt cathode through
R867, CR867, CR868, and R865 to the Z-Axis output.
Capacitor C864 charges through R867, CR867, R864, and
CR863 to the Z-Axis output.
REV NOV 1982
3 -2 5