Tektronix 502A Instruction Manual page 26

Circuit Description — Type 502A
voltage, and the voltage at the plates of the Disconnect
Diodes, moves negative. This cuts off the diodes.
The current available through the Timing Resistor (R160)
is diverted into the Timing Capacitor (C160).
to force the grid of the Miller Runup Tube negative.
the grid of the Miller Runup Tube starts negative the plate
starts positive. This raises the voltage at the grid and
cathode of the Runup C.F.
of the Runup C.F. rises it causes the voltage at the upper
end of C 1 60 to increase, which in turn prevents the grid of
the Miller Runup Tube from going negative.
The Miller Runup Tube has a gain of about 200, so that
a grid voltage change of only .75 volt produces a plate
voltage change of 150 volts.
plained, the small negative change in grid voltage will
provide a nearly linear runup of voltage at the cathode of
the Runup C.F.
Maintaining the voltage across the Timing Resistor (R160)
nearly constant provides a nearly constant current into the
Timing Capacitor (Cl 60), which in turn causes a linear saw ­
tooth output voltage to be generated.
The linear rise in voltage at the cathode of the Runup
C.F. V 1 61 B is used as the sweep time base. Timing Capacitor
C 1 60 and Timing Resistor R160 are selected by means of the
TIME/CM switch SW160. The Timing Resistor determines the
current that charges the Timing Capacitor. By means of
the Timing Switch, both the size of the capacitor being
charged and the current charging the capacitor can be
selected to cover a wide range of sweep rates. Thus, the
timing circuit determines the rate at which the spot moves
across the crt.
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.
linearly at the cathode of VI 61 B, there will be a linear
rise in the voltage at the arm of the SWP LENGTH Control.
This will increase the voltage at the grid and cathode of
VI83 and at the grid and cathode of V145B.
voltage at the cathode of V145B rises the voltage at the
grid of V135 will rise. When the voltage at this point rises
to a point where V135
Gating Multivibrator will
state with V135 conducting and V145
at the plate of V145 will then rise, carrying with it the
voltage at the plates of the Disconnect Diodes VI 52. The
diodes then conduct and the lower-half (V152B) provides a
discharge path for the Timing Capacitor through R147 and
R148, and through the resistance in the cathode circuit of
V 1 61 B. The plate voltage of the Miller Tube now falls line ­
arly, under feedback conditions essentially the same as when
it generated the sweep portion of the waveform, except
for a reversal of direction.
The resistance through which C 1 60 discharges is much
less than that through which it charges (the Timing Resistor).
The capacitor current for this period will therefore be much
larger than during the sweep portion, and the plate of the
Miller Runup Tube will return rapidly to its quiescent voltage.
This produces the retrace portion of the sweep sawtooth,
during which time the crt beam returns rapidly to its start ­
ing point.
The Hold-Off Circuit prevents the Time-Base Generator
from being triggered during the retrace interval. In addition,
3-8
This tends
s the voltage at the cathode of
Due to the feedback, as ex ­
s the sweep voltage rises
comes out of cutoff, the Sweep-
rapidly revert to its original
cutoff. The voltage
the Hold-Off allows a finite time for the Time-Base circuits
to regain a state of equilibrium after the completion of
a sweep.
During the trace portion of the sweep sawtooth
s
Hold-Off Capacitor C 1 80 charges through
result of the rise in voltage at the cathode of V183 .
the same time the grid of V135
the cathode-follower V145B, until V135
and starts conducting.
action that initiates the retrace.
interval C 1 80
Resistor. The time-constant of this circuit is long enough,
however, so that during the retrace interval, and for a short
period of time after the completion of the retrace, C 18 0
holds the grid of V135
triggered. However, when C 1 80 discharges to the point
that the cathode-follower V145B is cutoff, it loses control
over the grid of V135
established by the ST BILITY
hold-off time required is determined by the sweep rate,
i.e., by the size of the Timing Capacitor. For this reason
the TIME/CM
Hold-Off Circuit simultaneously with that of the Timing
Circuit.
The ST BILITY
grid of V135 . This control should be adjusted so that the
voltage at the grid of V135
the circuit from free-running.
sweep can only be produced when a negative trigger pulse,
from the Time-Base Trigger circuit, can drive the grid of
V135 below cutoff. However, should a free-running sweep
be desired, the TRIGGERING LEVEL control can be turned
full right; this closes the RECURRENT switch and connects
the grid circuit of V135
R116. This permits the grid of V135
mediately upon removal of the hold-off voltage, at which
point the next sweep is initiated.
When the MODE switch is in the NORM L position, as
s the
above, the emitter of Q124 is open, making it inoperative.
lso the anode of D 1 26 is grounded, preventing any effect
upon the sweep generator circuit.
When the MODE switch is thrown to the SINGLE SWEEP
position, the emitter of Q 1 24 is connected to ground and
the anode of D126 is connected to a small positive voltage.
To consider the action
MODE switch is in the RESET position and then allowed to
return to SINGLE SWEEP.
In the RESET position, SW126 grounds the plate of V135 .
This places the same potential on both the cathode of D126
and the emitter of Q124, which reverse biases Q124.
Q 1 24 is reverse biased, the voltage at the collector goes
negative and the RE DY light B124 will light when the
collector voltage reaches about — 60 volts. D124 is also
reversed biased as its anode voltage goes negative. The
reverse biasing of D124 prevents the collector voltage of
Q124 from affecting the grid voltage of V135
the sweep.
The grid voltage of V135
a level which will be determined by R111 , R185 and R186.
This voltage will be at a value which will allow an incoming
trigger pulse to switch the Multivibrator and produce a
sweep.
is being pulled up, through
comes out of cutoff
s mentioned previously, this is the
t the start of the retrace
starts discharging through
high enough so that it cannot be
and the grid returns to the level
DJUST R 111, The amount of
switch changes the time-constant
DJUST R111 regulates the d c level at the
is just high enough to prevent
djusted in this manner, a
to the — 150 volt supply through
to fall to cutoff im ­
of the circuit, assume that the
will rest during this time at
the
V183 as a
t
the Hold-Off
of the
s
until after