Tektronix 503, Instruction Manual

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Circuit Description— Type 503
The positive voltage swing on the upper plate of the Timing
capacitor also tends to prevent the lower plate from swing­
ing negatively. Since the gain of V160A is about 200, the
potential on the upper plate moves about 100 volts with
respect to ground while the potential on the lower plate
moves about one-half volt. The result is an extremely linear
ramp at the cathode of V160B, which is applied through the
Horizontal Amplifier to the Horizontal deflection plates of
the CRT.
Sweep Length
The length of the sweep, that is, the distance the spot
moves across the CRT, is determined by the setting of the
SWP. LENGTH control, R176. As the sweep voltage rises
linearly at the cathode of V160B, there will be a linear
rise in the voltage at the wiper arm of the SWP. LENGTH
control. This will increase the voltage at the plate, and
therefore the cathode, of V152B and at the grid and cathode
of V145B. As the voltage at the cathode of V145B rises,
the voltage at the grid of V135A w ill rise. When the voltage
at this point rises to a point where V135A comes out of
cutoff, the Sweep-Gating M ultivibrator w ill rapidly revert
to its original state with V135A conducting and V145A
cut off. The voltage at the plate of V145A will then rise,
carrying with it the voltage at the plates of the Disconnect
Diodes.
D152 starts conducting, and brings the grid of V160A
quickly back up to its quiescent level. The rise in voltage at
the grid causes the tube to conduct more, so the plate
voltage drops, carrying with it the grid and cathode of
V160B. When the voltage at the cathode of V160B returns
to about —2.5 V, V152A conducts, clamping the voltage
at this point. The circuit has now returned to its quiescent
level and is ready for the next trigger.
H old-O ff
The Hold-O ff Circuit prevents the Sweep Generator from
being triggered during the sweep retrace interval. It does
this by holding the grid of V135A positive enough to keep
V135A in conduction until after the M iller Runup Circuit
has stabilized in the quiescent condition.
Under quiescent conditions, normal conduction through
V152B allows the H old-O ff Capacitor, C l60, to be charged
to about 70 volts. During the latter part of the sweep, the
rising voltage at the cathode of V152B discharges this
capacitor to a lower voltage, in the vicinity of about 55
volts. A t the end of the sweep, when the voltage at the
cathode of V160B drops, the voltage at the SWP. LENGTH
wiper arm also drops and V152B cuts off. The cathode tries
to follow the drop in voltage at the plate but is held up
by the charge on the Hold-O ff Capacitor. The H old-O ff
Capacitor charges again exponentially toward — 100 volts,
carrying the cathode of V152B and the grid of V145B nega­
tive. The cathode of V145B, and therefore the grid of V135A,
follows the grid o f V145B. A t some point in this exponential
charging curve, depending upon the settings of the STABILITY
ADJUST control and the FREE RUN switch, the grid of V135A
w ill
become negative enough that a negative trigger pulse
coming from the Sweep Trigger circuit can again take V135A
into cutoff.
The hold-off time, then, is determined by the value of
Cl 60 switched into the H old-O ff Circuit by the SWEEP
TIME/CM switch. The amount of hold-off time required is
determined by the sweep rate. For this reason the SWEEP
TIME/CM switch changes the amount of capacitance in the
Hold-O ff Circuit simultaneously with that in the Timing
Circuit.
Sweep Stability
The STABILITY ADJUST control. R il l, regulates the DC
level at the grid of V135A. This control is adjusted so that
the voltage at the grid of V135A is just high enough to
prevent the circuit from free running. When it is adjusted in
this manner, a sweep can be produced only when a nega­
tive trigger pulse from the Sweep Trigger can drive V135A
into cutoff. Turning the LEVEL switch fully clockwise closes
the FREE RUN switch and shorts out R il l. This places a more
negative voltage on the grid of V135A such that V135A cuts
off immediately upon decay of the hold-off voltage, at which
time the next sweep is initiated. The result is a free-running
sweep whose period is the total of the sweep time plus the
hold-off time at any given setting of the SWEEP TIME/CM
switch. (This is compared to a maximum repetition rate of
about 50 Hz when the LEVEL switch is turned fully counter­
clockwise to the AUTO, position.)
Unblanking
The positive rectangular pulse appearing at the cathode
of V135B during sweep time is applied as an unblanking
pulse to the CRT. Action of this pulse is discussed under
the description of the CRT circuit later in this section. It
should be noted that, when the HORIZONTAL DISPLAY
switch is in the HORIZ. AMPLIFIER position, the Sweep-
Gating M ultivibrator is disabled, and there is no current
flowing through V135A or V145A. Therefore, the cathode of
V135B is held at +2 1 0 volts and the CRT is continuously
unblanked.
CRT CIRCUIT
The CRT in the Type 503 oscilloscope makes use of an
extra set of deflection plates for unblanking during sweep
time. One of these plates has a fixed potential of about
+210 volts on it; the other is tied to the cathode of V135B
in the Time-Base Generator. Quiescently, this latter plate
is held at a relatively low potential, in the vicinity of + 8 0
volts. Therefore, the electron beam in the CRT is deflected
and absorbed by the + 2 1 0-volt plate; none of it reaches
the screen. During sweep time, however, the unblanking
pulse from V135B raises the potential of the second plate
from + 8 0 volts to about + 2 1 0 volts. When this happens,
both unblanking deflection plates attract the electron beam
in the same amount with the net result that the beam is not
deflected toward either plate, but
passes on through to the
CRT screen.
The INTENSITY control varies the conHol grid of the
CRT from about —20 volts to — 150 \ h respect to
the cathode. Connections are provided on tin. rear of the
oscilloscope cabinet to couple an AC signal to the control
grid to provide intensity modulation of the trace is desired.
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