Sweep Generator And Trigger; Output Section; Vertical Output Amplifier; Horizontal Output Amplifier - Tektronix 213 DMM Instruction Manual

Theory of Operation-213 Service
Sweep Generator and Trigger
Integrated circuit U335 is a combination trigger and sweep
generator. The trigger input sources are either internal or
external as selected by S315. External trigger signals applied
through J315 are de coupled through a 10: 1 attenuator net-
work consisting of R314, R315, C314, and C315. Internal
trigger signals are derived from a sample of the Oscilloscope
Preamplifier output and are ac coupled through C316. The
triggering level, and the slope at which triggering occurs, is
set by R316. CR332 and CR333 provide input protection
to U335.
The sweep generator portion of U335 is a Miller integrator
that generates a linear sawtooth output waveform. Its slope
is controlled by the setting of S300, and is calibrated by
R312. The ratio of R330 and R332 sets the sweep length
to about 12 divisions. When the generator is triggered from
an internal source, it operates either in a normal or automa-
tic mode depending on the voltage at pin 15 of U335. In
the normal mode of operation a negative 5 volts is applied
to pin 15, and when the input signal is not sufficient to
cause triggering there is no trace. In the automatic mode
the negative 5 volts is removed, and when the input signal
is not sufficient to cause triggering the sweep free-runs at a
rate set by C334. Pin 4 of U335 provides an unblanking
gate signal coincident with the sawtooth waveform. This
gate controls the Z-Axis Amplifier and provides crt unblank-
ing during the horizontal sweep. C327 and C328 establish
the sweep holdoff time.
OUTPUT SECTION
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Vertical Output Amplifier
The Vertical Output Amplifier section consists of an output
amplifier driven by one of two differential amplifiers, one
for the digital multimeter function and the other for the os-
cilloscope function.
The Y signal from the Character Generator drives the digital
multimeter vertical differential amplifiers 0350 and 0355.
They are turned on when S355 applies voltage to the emit-
ters and bases, and their gain, which sets the vertical size
and position of the characters, is controlled by R352 and
R354.
The vertical signal from the Oscilloscope Preamplifier drives
the oscilloscope vertical differential amplifier 0346 and
0348. This amplifier is turned on by 0347 and 0374 which
operates when S355 applies voltage to them. 0347 is a
current source for the amplifier and its base voltage is set by
0374, a temperature compensated voltage supply.
The Vertical Output Amplifier directly provides the crt
vertical deflection voltages and consists of transistors 0340
and 0345 whose output is inductively peaked by L340 and
L344. Rectifiers CR350 and CR355 prevent saturation of
the output amplifier.
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Horizontal Output Amplifier
The Horizontal Output Amplifier section consists of a hori-
zontal output amplifier driven by a differential amplifier for
the digital multimeter function or a differential amplifier
for the oscilloscope function.
The X signal from the Character Generator drives the digital
multimeter horizontal differential amplifier 0380 and 0385.
Transistors 0380 and 0385 are turned on when S355 is in
the DMM mode, which applies voltage to their emitters and
bases. Amplifier gain, which controls the horizontal size of
the characters, is set by R382 and R384, whose ratio also
sets the horizontal position of the characters. A portion of
the vertical signal from the Character Generator Y output is
fed to 0385 through R358 to cause a slight tilt in the nu-
merical readout display giving it a more pleasing appearance.
The sweep generator output drives the oscilloscope horizon-
tal differential amplifier consisting of 0366, 0368, 0370,
and U365A through U365E. Transistors 0366 and 0368
are turned on by 0370 and 0374 which operate only when
S355 applies voltage to them. The horizontal position of
the signal is determined by a current ratio between 0366
and 0368 that is set by R378 which controls the base vol-
tage on 0368. Amplifier gain is set by the ratio of R366
and R368 to R360 and R364. Magnifier gain is controlled
by integrated circuit U365 operating as a gain cell whose
current gain is controlled by the voltage on the bases of
U365E and 0370. The magnifier gain is set by the ratio of
the current through R372 to the current through R366 and
R368 minus the current through R365 and R369. The cur-
rent ratio of U365A through U3650 follows the signal input;
therefore, the gain cell maintains a constant magnifier gain
setting.
The horizontal output amplifier provides the crt horizontal
deflection voltages and consists of high voltage transistors
0340 and 0345. Rectifier CR360 keeps 0360 from satura-
ting.
2-Axis Amplifier
The Z-Axis Amplifier (or unblanking amplifier) consists of
VR395, 0390, and 0394. When an unblanking gate from
the Sweep Generator or Character Generator is present at
the base of 0390, it turns off and 0394 turns on. When
0394 turns on, a positive voltage set by V R395 causes
the crt to unblank (or turn on). 0394 is a speedup device
which supplies additional current to unblank the crt more
rapidly.
CRT CIRCUIT
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The crt is a Tektronix designed standard battery operated
oscilloscope device which has its phosphor modified to en-
hance the DMM readout function.
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