Tektronix 503, Instruction Manual

Page: 19 / 147

Circuit Description— Type 503
to determine the amplifier gain. The amount of positive
feedback is controlled by varying R460. The second stage
provides its own driving current and none is diverted from
the plate circuit of the first stage. The effect of R460 is most
pronounced in the 1 m V/CM position of the SENSITIVITY
■'switch since a higher open-loop gain is required in this
postion.
Output Amplifier
The Output Am plifier stage (V474-V484) drives the deflec­
tion plates of the CRT. The gain o f this stage is adjusted
by means of the .2 V G A IN ADJ. R478 and the VARIABLE
control R488.
Vertical positioning of the CRT beam is accomplished
by a cross-connected dual POSITION control R470. Adjust­
ment of this control increases the current through one tube
while decreasing the current through the other tube, there­
by changing the DC level at one output-tube plate in the
opposite direction from the other. A t the same time, a cur­
rent (proportional to the POSITION control rotation) is
applied to the feedback resistors R457 and R467 of the
hybrid feedback pairs. This current causes the grid potential
of V474 and V484 to change so that the cathode potentials
remain constant. This assures that all of the current change
in R476 also occurs in V474, and all of the current change
in R486 occurs in V484. Thus no positioning current flows
in R488, R478, and R479. Therefore, rotation of the VARI­
ABLE SENSITIVITY control w ill not cause trace shift when
the trace is anywhere on the CRT screen.
Sweep triggering is accomplished by sampling a portion
of V474 output through a divider network consisting of
R490, R491, and R492. When the SOURCE switch is in the
INT. position, this signal is applied to the Trigger Input
Amplifier.
HORIZONTAL AMPLIFIER
X-Y Operation
W ith the Type 503 set for X-Y operation (HORIZONTAL
DISPLAY) switch in the HORIZ. AMPLIFIER (SWEEP DIS­
ABLED) position, the Horizontal Amplifier is virtually identi­
cal to the Vertical Amplifier. A given signal or signals
applied to the Horizontal Amplifier input connectors will
produce the same deflection as the same signal or signals
applied to the Vertical Amplifier, but in the horizontal
plane, rather than the Vertical plane. The HORIZONTAL
DISPLAY switch disables the sweep by opening the cathode
circuits of the Sweep Gating M ultivibrator tubes, V135A
and V145A. This mode of operation is used for plotting
two time-related functions against one another.
Sweep Operation
In this mode of operation, the Horizontal Input A m pli­
fier is connected for single-ended input, with one input
grounded and the sawtooth waveform from the Sweep Gen­
erator applied to the other input.
Neither of the input
attenuators is connected in the circuit.
The feedback circuits which control the gain of the Hori­
zontal Amplifier operate in the same manner for sweep
operation, except that a different set of resistors is used
in the Input Amplifier cathode circuit. Also, the VARIABLE
control is shorted out by the HORIZONTAL DISPLAY switch,
and one section of the dual POSITION control is removed
from the circuit. Beam positioning is accomplished by using
half of the POSITION control to change the current through
the divider resistors R326 and R327, changing the DC level
at the grid of V334.
SWEEP TRIGGER
The Sweep Trigger circuit consists of the Trigger Input
Am plifier V24 and the Trigger M ultivibrator V45. The Trig­
ger Input Am plifier has the dual function of amplifying the
triggering signal, or amplifying and inverting the triggering
signal. The Trigger M ultivibrator produces rectangular pulses
whose leading edges are differentiated to produce nega­
tive spikes to trigger the Sweep Generator.
Trigger Source
Three trigger sources may be selected by means of the
SOURCE switch. In the INT. position, the triggering signal
is taken from the output of the Trigger Pickoff network of
the Vertical Amplifier. In the EXT. position, the Trigger
Input Am plifier is connected to the EXT. TRIG. IN connector
on the front panel. In the LINE position, the Trigger Input
Amplifier is connected to the 6.3 VAC heater winding ter­
minal 10 of power supply transformer T601.
Trigger Slope
The negative-going pulse required at the output of the
Trigger M ultivibrator is produced only when a negative­
going signal appears at the grid of the multivibrator input
section (V45A). W ith the SLOPE switch in the — position,
a negative-going signal at the grid of V24A appears at
the plate of V24B as a negative-going signal. W ith the
SLOPE switch in the + position, a positive-going signal
applied to the grid of V24B is inverted, and appears as a
negative-going signal at the input of the Trigger M ultivi­
brator.
Trigger Level
The Trigger Input Amplifier operates as a comparator cir­
cuit as follows: with the SLOPE switch in the — position,
the LEVEL control sets the DC level at the grid of V24B.
When the grid of V24A is driven to the same level by the
incoming signal, the plate of V24B passes through the
hysteresis range of the multivibrator, and causes the multi­
vibrator to switch states. W ith the SLOPE switch in the +
position, the LEVEL control is connected to V24A grid and
the signal appears on V24B grid. When the signal drives
V24B grid to the same level as V24A grid (set by the LEVEL
control), V248 plate w ill pass through the hystresis point
of the Trigger Multivibrator, causing it to switch states.
Trigger Multivibrator
A cathode-coupled multivibrator (Schmitt circuit) is used
to generate a fixed amplitude and risetime (period depen­
dent on input signal period) square wave which is differen­
tiated and used to start the sweep. When the LEVEL control
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