Tektronix 2445, Инструкция по эксплуатации

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Theory o f Operation—2445 Service
centering stable over a wide range of varying display
intensity. Readout jitte r adjustment pot R618 is used to
minimize thermal distortion in the output amplifier to
reduce jitte r in the display readout.
The vertical output signal at pins 28 and 33 of U600
(OUT A and OUT B) is applied to the vertical deflection
plates of the crt (diagram 8) via L628 and L633. The
deflection plates form a distributed-deflection structure
that is terminated by a hybrid resistor network. One
element of the terminating network is an adjustment
potentiometer used to match the network impedance to
that of the crt.
BANDWIDTH LIM ITING . Bandwidth-limiting coils
L644 and L619, along w ith capacitors built into U600,
form a three-pole filte r used to roll o ff high-frequency
response of the Vertical O utput amplifier above 20 MHz.
To lim it the vertical bandwidth, the BWL (bandwidth lim it)
input to U600 (pin 16) is pulled LO. It may be set LO
either by the BWL control data b it from A uxiliary Control
Register U140 (diagram 4) when the operator selects the
Bandwidth L im it feature or automatically by the output
of NAND-gate U975A in the Vertical Channel Switch
circuitry (via CR616) when the readout is being displayed.
TRACE SEPARATION. The voltage applied to the TS
(trace separation) input of U600 (pin 42) is used to offset
the output levels of the hybrid to vertically shift the
position of trace on the crt. During normal sweep displays,
the TS1 + TS2 signal applied to the base of Q600 by the
Display Sequencer (diagram 5) is HI, and the transistor is
turned on. The TRACE SEP level at the junction o f R642
and CR600 is shunted to ground, and no offsetting of
the output signal will occur. For those displays in which
trace separation should occur, the Display Sequencer
switches the base of Q600 to ground level to turn o ff the
transistor. The trace separation level set by front-panel
TRACE SEP control R3190 is now applied to the TS input
o f U600, and a corresponding offset of the displayed trace
w ill occur.
BEAM FIND. As an aid in locating off-screen or over­
scanned displays, the 2445 is provided w ith a beam-finding
feature. When the front-panel BEAM FIND button is
pushed, the beam-find input pin (BF, pin 15) o f U600 w iii
be pulled HI. While BF is HI, the dynamic range o f Vertical
Output A m plifier U600 is reduced, and all deflected traces
w ill be held to w ithin the vertical limits of the crt graticule.
OUTPUT PROTECTION CIRCUIT. A current-lim it
circuit composed o f transistors Q623 and Q624 protects
the Vertical Output Am plifier from a short-circuited output
or a bias-loss condition. Either of these fault conditions w ill
cause excessive current to flow into pins 30 and 31 of
U600. Current in FET Q624 is limited to the IDSS current,
so the voltage at pins 24, 30, and 31 w ill drop. This
decreases the forward bias on pass-transistor Q623 and
lowers the voltage at pin 23 of U600 enough and provides
some degree of protection for the device.
Horizontal Amplifier
The Horizontal A m plifier circuitry consists of Hori­
zontal Output Am plifier U800, a unity-gain buffer
amplifier made up of the five transistors in U735, and
associated components.
UNITY-G AIN BUFFER AM PLIFIER. The amplifier
circuit composed o f U735A, B, C, D, and E along w ith
their associated components, form a unity-gain amplifier
that buffers the ramp signal from A Sweep Generator U700
to the Horizontal O utput Am plifier. Transistors U735C and
D form a differential pair w ith the negative excursion o f
their emitters limited to —5 V (clamped by U735E).
Negative feedback from the collector o f U735C to its
base is via emitter-followers U735A and B (in parallel)
which drive to the A Sweep input (pin 18, A+) to Hori­
zontal Output A m plifier U800.
HORIZONTAL OUTPUT AM PLIFIER. Integrated
circuit U800 provides the final amplification o f the selected
horizontal-deflection signal required to drive the crt. One of
the single-ended input signals applied to the fo ur input pins
is converted to a differential-output signal at the output
pins of the amplifier. The four deflection signals to U800
are: the A Sweep (pin 18, A+), the B Sweep (pin 16,
B+), the Readout Horizontal signal (pin 17, RO), and the
Channel 1 signal (used for horizontal deflection of the X-Y
displays) at pin 20, the X+ input pin. Signal selection is
done by an internal channel switch and is controlled by the
HSA (horizontal select A) and HSB (horizontal select B)
signals from the Display Sequencer (see Table 3-4).
Table 3-4
Horizontal Display Selection
Control Level
HSA
HSB
Selected
Signal
H
H
Readout (X)
H L B Sweep Ramp
L
H A Sweep Ramp
L L X Input (from CH 1)
3-25