Tektronix 2215A, Инструкция по эксплуатации

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impedance termination for the input signals and isolates the
signal sources from following stages of the Z-Axis Amplifier.
Common-base transistor Q829 establishes a constant
current through R832. This current is divided between Q825
and Q829 with the portion through Q829 driving the shunt-
feedback output amplifier consisting of Q835, Q840, and
Q845. The bias level of Q825 therefore determines the
amount of emitter current available to Q829. Feedback-re­
sistor R841 establishes the transresistance gain which con­
verts the input current to output voltage. Emitter-follower
Q835 is dc coupled to Q840, and for low-speed signals
Q845 acts as a current source. Fast transitions couple
through C845, providing additional current gain through
Q845 for fast voltage swings at the output of the Amplifier.
External Z-Axis input voltages establish proportional in­
put currents through R822 and R823, and Amplifier sensitiv­
ity is determined by the transresistance gain of the
shunt-feedback amplifier. Diode CR823 protects the Z-Axis
Amplifier if excessive signal levels are applied to the EXT Z
AXIS INPUT connector.
The intensity of the crt display in the A, B, and Alt Hori­
zontal modes is determined by the INTENSITY controls and
associated circuitry. The A INTENSITY potentiometer con­
trols the base voltage of Q804 to determine the amount of
emitter current that will flow through the transistor and
therefore the level of the Z-Axis signal. Likewise the B IN­
TENSITY potentiometer will control the base voltage of
Q814 and the intensity of the B and Alt Sweep displays.
When only the A Sweep is displayed, Q586 and CR583
are biased off. The current through R818, as set by the
A INTENSITY potentiometer, will flow through CR818 and
Q825 to fix the voltage level at the Z-Axis Amplifier output.
For a B-only display, Q586 is biased on to reverse bias
CR818 and prevent A-intensity current from reaching Q825.
Current determined by the base voltage of Q814 will flow
through CR817 to Q825 and determine the B Sweep inten­
sity. For an alternating A and B display, Q586 will be biased
off when the A Sweep is displayed. During the portion of the
A Sweep in which the B Sweep runs, current from R816 is
allowed to flow through CR816 by the B Z-Axis Logic circuit
to provide an intensified zone.
When CHOP VERTICAL MODE is selected, the Chop
Blank signal is applied to the collector of Q825 through
CR824 during the display-switching time. Signal current is
shunted away from CR825, and the forward bias of Q829
increases to the blanking level. When blanked, the output of
the Z-Axis Amplifier drops to a level that reduces the crt
beam current below viewing intensity during the chop­
switching transition.
Theory of Operation— 2215A Service
For an X-Y display, CR818, CR817, and CR816 are re­
verse biased. The XY signal is LO to reverse bias CR551
and allow current in R820 to flow through CR820. The crt
intensity is then controlled by the A INTENSITY potentiom­
eter which sets the current in R820 through Q804.
BEAM FIND switch S390 controls the base bias voltages
of Q825 and Q829. When the BEAM FIND button is out,
- 8 . 6 V is supplied to a base-biasing network. When the
button is pushed in, the —8.6 V supply is removed and the
voltage at the anode of VR828 rises to about - 5 .6 V. This
turns off Q829 so that the amplifier output voltage is deter­
mined by R835, R846 and the voltage at the BEAM FIND
switch, as set by other parts of the Beam Find circuitry. The
output voltage of Q835 will then be at a level such that the
beam is visible regardless of Z-Drive signals, or the setting
of the INTENSITY control.
Dc Restorer
The Dc Restorer circuit produces the crt control-grid bias
and couples both dc and ac components of the Z-Axis Am­
plifier 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 Fig­
ure 3-5 during the following discussion.
Ac drive to the Dc Restorer circuit is obtained from pin 16
of T948. The drive voltage has a peak amplitude of about
±100 V, a frequency of about 20 kHz, and is coupled into
the Dc Restorer circuit through C853 and R853. The cath­
ode of CR851 is biased by the voltage applied from the
wiper of Grid Bias potentiometer R851, and the ac-drive
voltage will be clamped whenever the positive peaks reach a
level that forward biases CR851.
The Z-Axis Amplifier output voltage, which varies be­
tween + 10 V and + 75 V, is applied to the Dc Restorer at
the anode of CR853. The ac-drive voltage will hold CR853
reverse biased until the voltage falls below the Z-Axis Ampli­
fier output voltage level. At that point, CR853 becomes for­
ward biased and clamps the junction of CR851, CR853, and
R854 to the Z-Axis output level. Thus, the ac-drive voltage
is clamped at two levels to produce a square-wave signal
with a positive dc-offset level.
The Dc Restorer is referenced to the - 2 kV crt cathode
voltage through R858 and CR854. Initially, both C855 and
C854 will charge up to a level determined by the difference
between the Z-Axis output voltage and the crt cathode volt­
age. Capacitor C855 charges from the Z-Axis output
through R858, CR854, and CR855, to the crt cathode. Ca­
pacitor C854 charges through R858, CR854, R854, and
CR853 to the crt cathode.
3-15