Tektronix T921 User Manual

Circuit Description-T921/T922/T922R
REV . B JUL 1977
CRT & VERT AMPL CIRCUIT DESCRIPTION
PROBEADJUST
The Probe Adjust circuit provides an output of approximately
0 .5V peak-to-peak negative from ground at approximately 1 kHz .
When the output (pin 6) of U24 is positive, the voltage divider,
R22-R23, sets pin 3 at a positive voltage . Feedback through R24
charges C24 until the pin 2 level reaches the same positive voltage
as pin 3 . When pin 3 and pin 2 are at the same voltage, U24 output
(pin 6) switches from
positive to negative . The output of U24 is
about 7 .2 V either positive
or negative . Then C24 starts charging
negative . When pin 2 and pin 3 are at the same voltage again, U24
output (pin 6) switches positive, and the cycle repeats .
During
the positive half cycle, CR26 is forward biased and
CR27 is reverse biased keeping the output at
ground
level .
During
the negative half cycle CR26 is reverse biased and
CR27 is
forward biased causing current
to
flow from ground through
R27,
CR27 and R26 to -8 V . This sets the output level to approximately
-0 .5 V .
VERTICAL OUTPUT AMPLIFIER
The vertical output amplifier circuit provides final amplifica-
tion for the signal to drive the vertical deflection plates of the crt .
0112, 0122, 0134, 0136, Q144, and Q146 form a common-
emitter shunt-feedback amplifier . Shunt-feedback transistors
0134, Q136, Q144, and
Q146 are stacked . Resistors R118K,
R118L, R118H, and R118J provide the
feedback
. The
output
voltage at the collectors of Q136 and
Q146 is proportional to the
collector current of Q112 and Q122 through feedback resistors
R1 18K-Rl18L and R1 18H-R1 18J .
When
BEAM
FINDER button S100A is pressed, R117 is placed
in series with
R1 18B and R1 18C, limiting Q112 and 0122 emitter
current . This limits the maximum vertical deflection
to within the
crt screen area . Another section
of
the BEAM
FINDER switch
limits the horizontal deflection .
ZAXIS CIRCUIT
The Z Axis Amplifier controls the crt intensity level from
several inputs : the INTENSITY control,
unblanking signal from
the
sweep circuit, chop blanking signal from the vertical amplifier,
and
external signals from the EXT Z AXIS INPUT connector (also
intensity limit signal during X-Y operation) . The INTENSITY
control, R412, varies the trace intensity from
off
to maximum
brightness, overriding all other inputs to the
Z Axis Amplifier . The
unblanking signal from the sweep circuit blanks the signal during
retrace and holdoff . The chop blanking signal (with the vertical in
the chop mode) blanks the crt during the channel switching
interval to eliminate vertical chopping noise form appearing on
the display . The EXTZ AXIS INPUT connector,
J419, allows
control of
the trace intensity from an external source .
The current signals from the various inputs are connected to
the emitter of Q416 . The algebraic sum of the signals determines
the collector conduction level . In case of
overdrive from any
input, Q416 cuts off and CR416 conducts the excess- current to
ground and thereby prevents the output stage from saturating .
Transistors Q424, Q426, 0434, and associated circuitry form
an inverting operational amplifier . Components R423 and C423
are the feedbackelements . Any current into the input summing
point, the base of Q424, results in an output voltage at the
collectors
of Q426
and 0434 . This output voltage controls the
display intensity level by changing the do voltage level at the
junction of R462, C463 and C464 .
HIGH VOLTAGE OSCILLATOR
Transistor 0458 and associated circuitry make up the high-
voltage oscillator that produces the drive for high-voltage
transformer T460 . When the instrument is turned on, current
through 0454 provides forward bias for Q458 . Transistor Q458
conducts and the collector current increases, which develops a
voltage across the primary (Q458 collector) winding of T460 . This
produces a corresponding voltage increase in the feedback
winding of
T460, which
is connected
via R457
to
the
base of
Q458,
and Q458 conductseven
harder
. Eventually the rate of collector
current increase in Q458 becomes less than that required to
maintain the voltage across the collector winding, and the output
voltage drops . This turns off Q458 by way of the feedback voltage
to the base .
The
voltage waveform at the collector of Q458 is a
sine wave at the resonant frequency of T460 . During the negative
half cycle, 0458 remains off and the field collapses in the primary
of T460 . When the field is collapsed sufficiently, the base of Q458
becomes forward biased into conduction again and the cycle
begins anew . The amplitude of sustained oscillation depends
upon the average current delivered to the base of Q458 by the
regulator circuitry . The frequency of oscillation is approximately
50 kHz . Components C458 and R458 decouple the unregulated
+100
V
supply line .
HIGH-VOLTAGE REGULATOR
Transistors Q446-Q454 and associated circuitry control the
output voltage of the High Voltage supply . Components R443 and
C443 provide a slow start up for the high-voltage oscillator . When
the instrument is turned on, the +100 V supply charges C443
through R443 . The voltage increases until it is sufficient to
forward bias CR443, holding the voltage at slightly above
+8 volts
.
This
forms the reference for the high-voltage regulator .
The resulting current in R444A
(100,uA) turns on Q446 and
Q454 providing base current for Q458 . This starts the high voltage
oscillator, causing a negative voltage to develop at the crt
cathode .
Resistors R444B, C, D, and R468 sample the cathode voltage .
The high voltage increases until the
cathode voltage is -2000
V .
At this point the current in R444B
is approximately the same as
the current in R444A with Q446 barely conducting .
Anyc
signal at
applied
T460 . RE
If
the
(less nec_
of
Q446-
conduct
0458 thr
closer tc
soonert
T460 . Tt-
at the cr
By samp
manner,
relatively
Comp
regulato
unblanks
Resis -
high-vol -
The I -
One win
filament
ting cath
high ac
anode a
rectified
Compon

Tektronix T921 Specifications

General

Brand	Tektronix
Model	T921
Category	Test Equipment
Language	English