Tektronix 502A, Instruction Manual

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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). This tends
to force the grid of the Miller Runup Tube negative. As
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. As the voltage at the cathode of
of the Runup C.F. rises it causes the voltage at the upper
end of C160 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. Due to the feedback, as ex­
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 (Cl60), 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. V161 B is used as the sweep time base. Timing Capacitor
C160 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. As the sweep voltage rises
linearly at the cathode of VI61B, 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
VI83A and at the grid and cathode of V145B. As the
voltage at the cathode of V145B rises the voltage at the
grid of V135A will rise. When the voltage at this point rises
to a point where V135A comes out of cutoff, the Sweep-
Gating Multivibrator will rapidly revert to its original
state with V135A conducting and V145A cutoff. The voltage
at the plate of V145A will then rise, carrying with it the
voltage at the plates of the Disconnect Diodes VI52. 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 161 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 C160 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,
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 the
Hold-Off Capacitor C180 charges through V183A as a
result of the rise in voltage at the cathode of V183A. At
the same time the grid of V135A is being pulled up, through
the cathode-follower V145B, until V135A comes out of cutoff
and starts conducting. As mentioned previously, this is the
action that initiates the retrace. At the start of the retrace
interval C180 starts discharging through the Hold-Off
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, C180
holds the grid of V135A high enough so that it cannot be
triggered. However, when C180 discharges to the point
that the cathode-follower V145B is cutoff, it loses control
over the grid of V135A and the grid returns to the level
established by the STABILITY ADJUST R111, The amount of
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 switch changes the time-constant of the
Hold-Off Circuit simultaneously with that of the Timing
Circuit.
The STABILITY ADJUST R111 regulates the dc level at the
grid of V135A. This control should be adjusted so that the
voltage at the grid of V135A is just high enough to prevent
the circuit from free-running. Adjusted in this manner, a
sweep can only be produced when a negative trigger pulse,
from the Time-Base Trigger circuit, can drive the grid of
V135A 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 V135A to the —150 volt supply through
R116. This permits the grid of V135A to fall to cutoff im­
mediately upon removal of the hold-off voltage, at which
point the next sweep is initiated.
When the MODE switch is in the NORMAL position, as
above, the emitter of Q124 is open, making it inoperative.
Also the anode of D126 is grounded, preventing any effect
upon the sweep generator circuit.
When the MODE switch is thrown to the SINGLE SWEEP
position, the emitter of Q124 is connected to ground and
the anode of D126 is connected to a small positive voltage.
To consider the action of the circuit, assume that the
MODE switch is in the RESET position and then allowed to
return to SINGLE SWEEP.
In the RESET position, SW126 grounds the plate of V135A.
This places the same potential on both the cathode of D126
and the emitter of Q124, which reverse biases Q124. As
Q 124 is reverse biased, the voltage at the collector goes
negative and the READY 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 V135A until after
the sweep.
The grid voltage of V135A will rest during this time at
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.
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