Tektronix 310A, Instruction Manual

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Circuit Description—Type 310A
goes positive to turn V40B o ff. The output level of this
stage returns to the quiescent level near the supply voltage.
A uto Triggering. When the Trigger Coupling-Mode
switch is in the AUTO position, SW3C and SW3D change
the circuit configuration. SW3C opens the signal path to the
grid o f V40A through R30 and R31 so that the signal must
pass through C30. Resistor R42 is added to the plate circuit
o f V40A to increase the plate load o f V40A and effectively
lower the plate voltage o f V40A when it is on. The circuit
can operate in one o f tw o modes dependent upon the trig­
ger signal.
When there is no trigger signal present, or if the repeti­
tion rate o f the trigger signal is less than about 60 hertz, the
Trigger M ultivibrator stage operates as an astable m u ltivi­
brator w ith C30 determining the repetition rate. To under­
stand the operation o f the circuit under this condition,
assume that there is no trigger signal applied to the instru­
ment and V40A has just turned on. When V40A is on, its
plate drops negative. This places less voltage across divider
R46-R48 and the voltage at the grid o f V40B is negative
enough to hold V40B o ff. The voltage at the junction o f
R46-R48 is also connected to the grid o f V I0 A through
R55, SW3C, R31 and R40. The grid o f V40A starts to go
negative toward the level at the junction o f R46-R48 as
C30 charges. The cathode o f V40A follows the voltage
change at its grid and the cathode level o f V40B follows
also, since the cathodes o f V40A and V40B are connected
together. The cathode level o f V40B continues to go nega­
tive as C30 charges, until it drops negative enough to fo r­
ward bias V40B. Then V40B comes on and its plate drops
negative to produce a trigger pulse to the Sweep Generator
circuit. V40A is reverse biased since its grid continues to go
negative w ith the charging of C30 and its cathode level is
set by the conduction of V40B. When V40A turns o ff, its
plate rises positive and this voltage level is connected back
to the grid o f V40A through R46, R55, SW3C, R31 and
R40. Now the grid of V40A begins to rise positive as C30
discharges toward the level at the junction of R46-R48.
When C30 has discharged sufficiently to forward bias
V40A, it turns on and its plate goes negative. Now the cycle
begins again. As long as the instrument is in the AUTO
mode and there is no trigger signal, or the trigger signal
repetition rate is too low, the Trigger M ultivibrator stage
free runs at about a 50 hertz rate as just described to auto­
matically retrigger the sweep and provide a reference trace.
Whenever a trigger signal w ith a repetition rate higher
than about 60 hertz is applied to the grid of V40A, the
Trigger M ultivibrator stage operates in much the same
manner as described w ith the Trigger Coupling-Mode switch
set to AC or DC. The display is triggered at the average
voltage level o f the trigger signal (trigger signal always AC
coupled in AUTO trigger mode). The switching rate o f the
Trigger M ultivibrator stage when it is triggered at a repeti­
tion rate faster than 60 hertz is such that C30 does not
charge or discharge enough to affect the grid level o f V40A.
SWEEP G ENERATO R
General
The Sweep Generator circuit produces a sawtooth v o lt­
age which is amplified by the Horizontal A m p lifie r circuit
to provide horizontal sweep deflection on the CRT. This
output signal is generated on command (trigger pulse) from
the Sweep Trigger circuit. The Sweep Generator circuit also
produces an unblanking gate to unblank the CRT during
sweep time. Fig. 3-5 shows a detailed block diagram o f the
Sweep Generator circuit.
Sweep Gate Multivibrator
The trigger pulse from the Sweep Trigger circuit is
applied to the Sweep Generator circuit through C101. The
trigger pulse applied to C l01 is a fast-rise square wave w ith
the repetition rate and duty cycle determined by the Trig­
ger M ultivibrator stage. C l01 differentiates this square wave
to produce positive and negative going pulses which are
very fast (trigger pulses at grid o f V102A are d iffic u lt to see
at normal test oscilloscope intensity; it may be necessary to
increase the test oscilloscope intensity when viewing this
waveform). The positive-going pulses produced from the
trigger square wave are shunted by D129 (above SN 15221
only) to prevent them from switching the Sweep Gate
M ultivibrator during a sweep. V102A and V 1 10 are con­
nected as a m ultivibrator w ith V102B providing the
coupling between the plate o f V102A and the control grid
of V 1 10. Quiescently, V102A is conducting and V 1 10 is
off. This produces a positive level at the plate o f V 1 10
which allows V150A and V150B to conduct. The function
o f these diodes is discussed under Disconnect circuit. With
V102A conducting, its plate is negative from the supply
voltage and this produces a negative level at the cathode o f
V102B. This negative level at the cathode o f V102B is con­
nected to the CRT circuit to blank the CRT until a trigger
pulse is received (more details on unblanking given in the
CRT circuit discussion). It is also connected to the control
grid o f V 1 10 through divider R106-R107 to hold V 1 10 o ff
until the trigger pulse is received. S T A B ILIT Y control
R125 determines the quiescent grid level o f V102A so it is
near the forward bias point in the AC and DC positions o f
the Trigger Coupling-Mode switch. In the PRESET (fully
counterclockwise) position of the S T A B ILIT Y control, or
in the AUTO position o f the Trigger Coupling-Mode switch,
the quiescent grid level o f V102A is determined by PRE­
SET ADJUST R 126. The PRESET ADJUST is calibrated to
set the grid o f V102A at the correct level fo r best triggering
operation.
The negative-going trigger pulse applied to the grid of
V102A turns V I 02A o ff and its plate goes positive toward
the supply voltage. This produces a corresponding positive­
going change at the cathode o f V102B. The positive-going
change at the cathode o f V102B unblanks the CRT through
the CRT circuit so a trace can be displayed, and it turns
V 1 10 on through divider R106-R107. When V 1 10 turns on,
its plate goes negative enough to reverse bias diodes V150A
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