Tektronix 455, Instruction Manual, Page: 54 / 246

Circuit Description—4 5 5 /A2/B 2
A trigger pickoff stage in each Preamplifier circuit provides
a sample of the Channel 1 and Channel 2 input signals to the
Horizontal Module fo r sweep triggering purposes. A sample
of the Channel 2 signal is also supplied to the Main Module
for the output to the rear panel CH 2 OUT connector.
In the X-Y mode, the Channel 1 signal is connected to the
input of the Horizontal Preamplifier in the Horizontal
Module to provide the X-Axis deflection.
The Channel 2 Preamplifier circuit contains an invert
feature to invert the channel 2 signal to the crt, CH 2
trigger, and to the CH 2 OUT connector when the INVERT
button is pressed.
The outputs of both CH 1 and CH 2 Preamplifier circuits
are connected w ithin the HIC to the vertical switching circuit.
The vertical switching circuit selects the channel (s) to be
displayed, determined by the settings o f the VERT MODE
switch. In the A L T (alternate) mode, sync pulses from the
Horizontal Module trigger the Channel Switching M ulti­
vibrator to alternately connect the outputs of the pre­
amplifiers to the Delay-line Driver and Vertical O utput
Am plifier circuits.
When the VERT MODE switch is in the CHOP (chopped)
mode, the Channel Switching M ultivibrator free-runs at
about 250 kHz, providing a dual-trace presentation of the
signals on both channels. Chopped blanking pulses are sent
to the CRT Z-Axis circuit to blank the crt during switching
transitions.
The o utput o f the vertical switching circuit is connected to
the Vertical O utput A m plifier through the Delay-Line
Driver c ircu it and the Delay Line. The Vertical O utput
Am plifier circuit provides the final signal amplification
needed fo r driving the vertical deflection plates. The BEAM
FINDER switch, in the Main Module, connects to this
circuit and, when pressed, limits vertical deflection to the
viewing area as an aid in locating off-screen displays.
B2 H O R IZ O N T A L M ODULE
The B2 Horizontal Module contains the Sweep Generator,
Trigger Generator, Sweep Control and Horizontal Preampli­
fier circuits. The A and B Triggering circuits produce a gate
pulse that starts the A or B Sweep Generator circuits. The
input signal to the A and B trigger circuits can be individual
selected from the Channel 1 signal, the Channel 2 signal, th
external trigger input connectors, or a sample o f the line
voltage powering the instrument. Each trigger circuit con­
tains LEVEL, SLOPE, COUPLING and SOURCE controls.
The A Sweep Generator circuit, when initiated by the A
Trigger circuitry, produces a linear sawtooth o utput signal,
the slope of which is controlled by the A T IM E /D IV switch.
The TRIG MODE switch controls the operating mode of the
A Trigger circuitry. In the AUTO position, the absence of
an adequate trigger signal for about 100 ms after the end of
sweep holdoff causes an A sweep start gate to be generated.
In the NORM position, a sweep occurs only when correctly
triggered by an adequate trigger signal. Pushing the SGL
SWP pushbutton allows only one sweep to be initiated.
The B Sweep Generator is basically the same as the A Sweep
Generator circuit. However, it only produces a sawtooth
output signal after a delay time determined by the A TIM E/
DIV switch and the DELAY TIM E POS dial. If the B Trig­
gering SOURCE switch is set to the STARTS AFTER DE­
LAY position, the B Sweep Generator begins to produce
the sweep immediately following the selected delay time.
If the switch is in one of the remaining positions, the B
Sweep Generator circuit does not produce a sweep until
it receives a trigger pulse from the B Trigger circuit which
occurs after the selected delay time.
Both A and B Sweep Generators produce an unblanking
gate signal to unblank the crt so that the display can be
presented. These unblanking pulses are coincident w ith
their respective sweep outputs.
The + gate signals, which are also coincident w ith the
sweep outputs, are supplied to a rear panel connector. The
A gate or B gate can be selected by moving plug P2800.
The Sweep Control circuit produces an alternate-trace sync
pulse that is connected to the Channel Switching circuit in
the Vertical Module. This pulse switches the display be­
tween channels at the end o f each sweep when the alter­
nate-trace mode is used.
The o utput of either the A or B Sweep Generator is
amplified by the Horizontal Preamplifier and Horizontal O ut­
put A m plifier circuits (Main Module) to produce horizontal
deflection fo r the crt, except in the fu lly counterclockwise
position (X-Y) of the T IM E /D IV switch. This circu it con­
tains a X10 magnifier to increase the sweep rate 10 times
in any A or B T IM E /D IV switch position. Other horizontal
deflection signals can be connected to the Horizontal Pre­
amplifier by using the X-Y mode o f operation. When the
T IM E /D IV switch is set to X-Y, the X signal is connected
to the Horizontal Preamplifier circuit through the Channel
1 Vertical Preamplifier circuit in the Vertical Module.
4-2
REV. A, OCT 1975