Tektronix 502A

Circuit Description — Type 502A
at the plate of V45A to rise, which in turn will complete
the negative step-voltage output from the Trigger Multivi 
brator circuit.
The Trigger Input Amplifier V24 amplifies the triggering
signal that in turn is used to drive the Trigger Multivibrator.
The amplified signal is always taken from the plate of V24B,
but the grid of either tube (V24A or V24B) can be connected
to the input circuit. When the Slope switch SW 20 is in the
— position the grid of V24A is connected to the input circuit
and the grid of V24B is connected to a bias source adjust 
able by means of the TRIGGERING LEVEL Control R17. With
this configuration V24 is a cathode-coupled amplifier, and
the signal at the output plate is in phase with the signal at
the input grid. The circuit operation is then as follows:
With the Slope switch in the — position, triggering of the
Time-Base Generator will occur on the falling (negative 
going) portion of the triggering signal. Recalling that a neg 
ative-going signal is required at the grid of V45B to drive
the Trigger Multivibrator into the other state of its bistable
operation, this signal must be of the same polarity as the
original signal at the input circuit.
However, when it is desired to trigger the Time-Base Gen 
erator on the rising or positive-going portion of the trigger 
ing signal the signal at the grid of V45B must be opposite
in polarity to that at the input circuit. This is accomplished
by placing the Slope switch in the + position. With this
arrangement the grid of V24B is connected to the input circuit
and the grid of V24A is connected to the bias source. This
eliminates V24A from the amplifier circuit and V24B becomes
a plate-loaded amplifier. The output waveform will therefore
be opposite in polarity to the grid waveform.
The TRIG LEVEL CENT Control R24 determines the division
of current through both tubes, and is adjusted so that the
quiescent voltage at the plate of V24B lies in the center
of the hysteresis of the Trigger Multivibrator. The TRIG 
GERING LEVEL Control R17 is adjusted to vary the bias on
the tube to which it is connected. This in turn varies the
quiescent voltage at the plate of V24B about the level es 
tablished by the TRIG LEVEL CENT Control. The operator
can select the point on the waveform at which he wishes to
trigger the Time-Base Generator.
When the Time-Base Trigger circuit is switched into the
automatic mode of triggering, (TRIGGERING LEVEL) control
turned counterclockwise the AUTOMATIC switch SW17 con 
verts the Trigger Multivibrator from a bistable configuration
to a recurrent (free-running) configuraton. This is accom 
plish by coupling the grid circuit of V45A to the grid circuit of
V45B via R32. In addition, the dc-coupling between the
Trigger Input Amplifier and the Triggering Multivibrator is
removed when the switch is in this position. The automatic
free-running mode is not to be confused with action of the
Recurrent switch, shown on the Time-Base Generator dia 
gram, which causes the Sweep-Gating Multivibrator to free-
run.
The addition of R32 to the circuit causes the Triggering
Multivibrator to free-run in the absence of a triggering sig 
nal. For example, assume the grid of V45B is just being dri 
ven into cutoff. The voltage at the plate of V45B starts to
rise, carrying with it the voltage at the grid of V45A. Since
the two grids are coupled through R32, this causes the
voltage at the grid of V45B to start rising. The time-constant
of the R32-C31 network is such that it takes about .01 second
for the voltage at the grid of V45B to rise exponentially from
its starting point, below cutoff, to a point where plate current
can start.
As V45B starts to conduct its plate voltage drops, which
in turn lowers the voltage at the grid of V45A. The voltage
at the grid of V45B then starts dropping exponentially. When
this grid drops below cutoff again, the circuit has completed
one cycle of its approximately 50-cycle triangular waveform.
With the circuit configuration just described, the hori 
zontal sweep can be triggered with repetitive signals, over
a wide range of frequencies, without readjustment. When
not receiving triggers, the sweep continues at approximately
a 50-cycle rate. Thus, in the absence of any triggering signal
the sweep generates a base line which indicates that the in 
strument is adjusted to display any signal that might be con 
nected to the vertical deflection system.
TIME-BASE GENERATOR
The Time-Base Trigger produces a negative-going rectang 
ular waveform which is coupled to the Time-Base Generator
circuit. This waveform is differentiated in the grid circuit
of V135A to produce sharp negative-going triggering pulses
to trigger the Time-Base Generator in the proper time
sequence. Positive-going pulses are also produced in the
differentiation process, but these are not used in the opera 
tion of the circuit.
The Time-Base Generator consists of four main circuits:
A Lockout circuit, a bistable Sweep-Gating Multivibrator, a
Miller Runup circuit, and a Hold-Off circuit. The main com 
ponents in the Lockout circuit are the transistor QI 24 and
MODE switch SW126. The Multivibrator circuit consists of
V135A, V145A and the cathode-follower V135B. The essential
components in the Miller Runup circuit are the Miller Runup
Tube V161A, the Runup C.F. V 1 61 B, the Disconnect Diodes
V152, the Timing Capacitor C160 and the Timing Resistor
R160. The Hold-Off circuit consists of the Hold-Off C.F.'s
V183A and V145B, the Hold-Off capacitors C 1 80 and C 1 81
and the Hold-Off Resistors R181 and R180, A or B (shown on
the Timing Switch diagram).
With the MODE switch in NORMAL, the quiescent state
of V135A is conducting and its plate voltage is down. This
cuts off V145A through the cathode-follower V135B, the
voltage divider R141-143 and the cathode resistor R144.
The quiescent state of the Miller Runup Tube is deter 
mined by a d c network between plate and grid. This network
consists of the neon lamp B 1 67, the grid-cathode impedance
of the Runup C.F., and the Disconnect Diodes. The purpose
of this d c network is to establish a voltage at the plate of
the Miller Runup Tube of such a value that the tube will
operate above the knee, and thus over the linear region,
of its characteristic curve.
In the quiescent state the grid of the Miller Runup Tube
rests at about — 2 volts. There is about 25-volts bias on the
Runup C.F., and about a 60-volt drop across the neon lamp.
This establishes a quiescent voltage of about 4-32 volts at the
plate of the Miller Runup Tube.
A negative trigger pulse, arriving at the grid of V135A,
will then cause the Sweep-Gating Multivibrator to switch
rapidly to its other state. That is, V135A will be cutoff and
V145A will start to conduct. As V145A conducts its plate
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