Trigger Amplifiers And Switching - Tektronix 2205 Service Manual

Theory of Operation— -2205 Service
Coupling capacitor C547 and resistors R547 and
R548 form a differentiating circuit that produces
positive-going and negative-going short duration
pulses. These pulses are inverted by U537B to gen­
erate the Chop Blank signal to the Z-Axis Amplifier.
The pulses blank the crt during CHOP switching
times.
The Ait Sync signal applied to one input of U537A
is HI except during Holdoff. This allows the output of
U537C to be inverted by U537A which drives the
clock input of U540A. Since the 15 output of U540A
is connected back to the D input, and both the SET
and RESET inputs are HI. the outputs of U540A tog­
gle (change states) with each clock input. The Verti­
cal Output Amplifier is then driven alternately from
the Channel 1 and Channel 2 Preamplifiers at the
CHOP rate.
ALTERNATE DISPLAY.
CHOP ENABLE line is held HI, disabling CHOP
multivibrator U537D. The output of U537C will be HI
and the Chop Blank signal from U537B will be LO.
Input signals to U537A are the HI from U537C and
the ALT SYNC from the Holdoff circuitry in the
Sweep Generator. The output of U537A will then be
the inverted ALT SYNC signal that clocks Channel
Select flip-flop U540A. This causes the outputs of
U540A to toggle at the end of each sweep so that
the Channel 1 and Channel 2 Preamplifiers alter­
nately drive the Vertical Output Amplifier.
Beam Find keeps the vertical trace within the
graticule area for locating off-screen and over­
scanned traces. BEAM FIND switch S390 adjusts the
Vertical Output Amplifier biasing to limit the voltage
swing at the crt plates. When S390 is in normal out
position, the voltage level on pin 3 of U225 is about
zero volts. When the BEAM FIND switch is in, pin 3 of
U225 goes to -8.6 V, the output of U225 goes low
and bias Q202 and Q203 such that the amplifier
dynamic range is limited.
Vertical Output Am plifier
The Vertical Output Amplifier provides final
amplification of the input signals for application to
the vertical deflection plates of the crt. Signals from
the preamplifier are applied to a differential amplifier
composed of Q230 and Q231 with frequency com-
pensatlon provided by R241, R280, and C241, and
overall gain set by R233. Transistors Q232, 0236.
and Q237 form a cascade-feed-back amplifier for
driving the positive deflection plate with R243 setting
amplifier gain and C243 providing high frequency
compensation. Emitter follower of Q232 buffers the
input and provides tow impedance drive to the two
output transistors Q236 and Q237. For slow speed
signals, Q236 serves as a current source for Q237,
3-6
and at high frequencies, the signal is coupled
through C232 to the base of Q236. This provides
additional pull-up output current to drive the crt at
high frequencies. The amplifier consisting of Q234,
Q238,and Q239 drives the negative deflection plate
the same way the positive deflection plate is driven.
vide signals to the Trigger Generator circuit from
either the Vertical Preamplifiers, the EXT INPUT OR Z
connector, or the power line. The SOURCE switch
selects Channel 1, Channel 2, external, or line as
the trigger source.
Internal Trigger
In ALT. the
internal Trigger Amplifier with channel selection
determined by the VERTICAL switch.
plished by Q450 and Q451 for Channel 1. and Q452
and Q453 for Channel 2. The circuitry associated
with Channel 2 is the same as that for Channel 1
except that it does not have a trigger offset
adjustment,
applied to Q450 and Q451. These emitter-follower
transistors each drive one input transistor in U335,
and the collectors of the U335 input transistors in
turn supply emitter current to two current-steering
transistors. The biasing network of the input tran­
sistors of U335 is adjustable while the biasing net­
work of the input transistors of U310 is fixed. Potenti­
ometer R338 adjusts the emitter bias levels of the
two input transistors of U335 so that dc offsets
between channels can be matched.
output differential amplifier pair is fixed by the divider
network composed of R443 and R444. The other
base voltage is controlled by the CH 1 TRIG signal
from the Trigger Channel Switch circuitry. When the
CH 1 TRIG signal is LO, the transistors in each output
pair, with the collectors connected together, are
biased on and the other transistors are off. The
collector signal currents are equal in magnitude but
opposite in polarity and signal cancellation occurs. If
the CH 1 TRIG signal is HI, the other transistors in
each pair will be biased on and an output signal will
be developed across R339 and R340 to drive the
Internal Trigger Amplifier.
SOURCE switch S555. The CH 1, VERT MODE, and
CH 2 positions of S555 forward biases CR348 and

TRIGGER AMPLIFIERS AND SWITCHING

The Trigger Amplifiers shown on Diagram 3, pro­
Signals from the Vertical Preamplifiers drive the
Trigger pickoff from the Preamplifiers is accom­
Signals from the Channel 1 Preamplifier are
The base bias voltage of one transistor in each
Internal trigger channels are chosen by the