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Theory of Operation— 2236 Service
The base voltages of Q770 and Q780 are at nearly the same
dc level due to forward-biased diodes CR765 and CR768
between the two emitters.
Transistors 0770, Q775, and Q779 form a cascode-feed-
back amplifier for driving the right crt horizontal deflection
plate with R775 setting amplifier gain and C775 providing
high-frequency compensation. For low-speed signals, Q779
serves as a current source for Q775, and at high sweep
rates, the ramp is coupled through C779 to the emitter of
Q779. This provides additional pull-up output current to
drive the crt at high sweep rates. The amplifier consisting of
Q780, Q785, and Q789 drives the left crt horizontal deflec
tion plate in the same manner as described above with ze-
ner diode VR782 level shifting the collector signal of Q780.
The BEAM FIND function is implemented when S390 is
pushed in to disconnect the cathode of CR764 from the
-8 .6 -V supply. The voltage on the cathode of VR764 goes
positive, causing CR780 and CR770 to be forward biased.
Current from R764 causes the output common-mode volt
age of the two shunt-feedback amplifiers to be shifted nega
tive to reduce the available voltage swing at the crt plates.
This prevents the trace from being deflected off-screen
horizontally.
Z-A XIS AMPLIFIER
The Z-Axis Amplifier, shown on Diagram 7, controls the
crt intensity level via several input-signal sources. The effect
of these input signals is either to increase or decrease trace
intensity or to completely blank portions of the display. The
Z-Drive signal current as determined by the A and B Z-Axis
Switching Logic and the input current from the EXT Z AXIS
INPUT connector (if in use) are summed at the emitter of
common-base amplifier Q825 and thereby determine the
collector current of the stage. This transistor provides a low-
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
INTENSITY 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 Q583 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.
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 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 fixed level so that the
INTENSITY controls and the Z-Drive signal have no control
over the crt intensity. A bright trace or dot will then be
displayed.
3-16
Содержание 2236
Страница 10: ...2236 Service viii The 2236 Oscilloscope ...
Страница 28: ...Specification 2236 Service 1 18 ADD JAN 1985 Figure 1 2 Physical dimensions of the 2236 Oscilloscope ...
Страница 74: ...Theory of Operation 2236 Service 3 30 ...
Страница 75: ...Theory of Operation 2236 Service Figure 3 13 Typical waveforms for the Delta Time function with Valt HI 3 31 ...
Страница 102: ...Performance Check Procedure 2236 Service 4 21 Figure 4 1 Test setup for DMM common mode check ...
Страница 240: ...2236 Service CTM INTERFACE SIGNALS F ig u re 9 4 O s c illo s c o p e b a sic b lo c k diag ram 4294 46 ...
Страница 241: ... VERT DEFL 2236 Service 4 2 0 6 3 5 4 2 0 4 4 7 F ig u re 9 5 O scillo sco p e d e ta ile d b lo c k diag ram ...
Страница 242: ...2236 Service 4204 34 Figure 9 6 CTM basic block diagram ...
Страница 243: ...DMM POWER S U P P LIE S 2236 Service F ig u re 9 7 C T M d e ta ile d b lo c k diag ram ...
Страница 253: ...A10 COUNTER TIMER MULTIMETER BOARD F ig u re 9 8 C irc u it b o a rd lo c a tio n Illu s tra tio n ...
Страница 254: ...S I 0 O I 5 1 S O i 4204 21 C IR C U IT BOARD INTERCONNECTIONS ...
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Страница 319: ...2236 Service TROUBLESHOOTING GUIDE 4206 99 4204 69A ...
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