Tektronix 310A, Instruction Manual

Page: 37 / 132

Circuit Description-Type 310A
trol grid o f V160B goes negative also. This produces a
positive-going change at the plate o f V160B which is con­
nected to the grid o f cathode follow er V160A. The
positive-going signal at the cathode o f V160A is connected
to the Horizontal A m p lifie r circuit to provide the sweep
signal fo r the CRT. It is also connected to the positive side
of tim ing capacitor C l 75. This feedback to the tim ing
capacitor maintains a constant charging current for C l75 to
produce a linear sawtooth output signal. The actual voltage
change at the grid of V160B is very small compared to the
sawtooth output signal produced by this stage. The output
voltage continues to go positive until the circuit is reset
th ro u g h th e Reset and H o ld o f f Circuit. Divider
R 1 6 0 -R 1 61-R162 in the cathode circuit o f V160A
determines the level at which the sweep is reset. Sweep
Length adjustment R 161 is adjustable to provide the cor­
rect sweep length. Complete operation o f this action is
discussed under Reset and H oldoff Circuit.
Reset and Holdoff Circuit
The positive-going sawtooth from the cathode of V160A
is connected to the grid o f V130B through R162 and R161.
Sweep Length adjustment R161 determines the DC level
and amplitude o f the signal applied to the grid o f V130B to
set the length o f the sweep. The positive-going sawtooth at
the grid o f V130B produces a similar positive-going change
at the cathode o f V130A, which charges holdoff capacitor
C l 35. The level at the cathode of V130B is also connected
to the grid o f V102A through V130A. During sweep time,
V102A in the Sweep Gate M ultivibrator stage is o ff and
V 1 10 is on. The positive going signal from V130A raises the
grid o f V102A positive toward the turn-on point as the
sawtooth produced by this circuit rises positive. When
V102A comes on, its plate goes negative and it resets the
Sweep Gate M ultivibrator to the quiescent state. The level
on the positive-going sawtooth at which this stage is reset
can be adjusted w ith Sweep Length adjustment R 161.
When R161 is turned clockwise (toward —150 volt supply)
a more negative quiescent level is connected to the grid o f
V 1 3 0 B . A l s o , due to th e d iv id e r a c tio n o f
R160-R161-R162, less o f the positive-going sawtooth is
applied to the base o f V130B. Therefore, the sawtooth
must rise farther positive before the Sweep Gate M ultivibra­
to r can be reset through V130B and V130A which pro­
duces a longer sweep length. The action is the opposite
when R 161 is turned counterclockwise to produce a shorter
sweep.
When the Sweep Gate M ultivibrator stage resets, the o u t­
put level at the plate o f V 1 10 quickly rises positive to
forward bias V150A and V150B. Tim ing capacitor C175
discharges rapidly and the control grid o f V160B follow s to
produce the retrace portion o f the sweep. Since the resis
tance o f the discharge path fo r C l 75 is less than the
charging path, the retrace portion o f the sweep has a
steeper slope to quickly return the CRT beam to the left
side o f the display area. The unblanking gate to the CRT
goes negative also, so this retrace is not visible on the dis­
play area. Now the Disconnect Diodes and the Sawtooth
Sweep Generator stage have returned to their quiescent
conditions and are ready to produce another sweep as soon
as the Sweep Gate M ultivibrator stage is ready to receive
the next trigger pulse.
As the cathode o f V130B goes positive along w ith the
positive-going sawtooth, h o ld o ff capacitor C l 35 charges
positive also. However, when the circuit is reset to produce
the retrace portion o f the sawtooth signal, the grid of
V130B drops negative w ith the retrace but the cathode is
clamped by the charge level on C l35. The charge level on
the h oldo ff capacitor clamps the grid o f V102A at a
positive level also, through V130A. This action blocks the
incoming trigger pulses at the grid o f V102A fo r a period of
time to establish a holdoff period and allow all circuits to
return to their quiescent condition before the next sweep is
produced. This holdo ff time is determined by the discharge
rate o f the holdo ff capacitor. As the h o ld o ff capacitor dis­
charges, the grid o f V102A drops negative until it reaches
the level established by S T A B ILIT Y control R125 or PRE­
SET ADJUST R126 as described fo r the quiescent condi­
tion o f the Sweep Gate M ultivibrator stage. Now the Sweep
Gate M ultivibrator stage is ready to accept the next
positive-going trigger pulse. H oldoff capacitor C135 is
changed by the T IM E /D IV switch to provide the correct
holdo ff time fo r the sweep rate selected (see Sweep-Timing
Switch diagram).
When the S T A B ILIT Y control is turned fu lly counter­
clockwise w ith the Trigger Coupling-Mode switch set to AC
or DC, the grid o f V102A is at its most negative level. Now,
when the Sweep Gate M ultivibrator stage is reset and the
holdo ff period ends, the grid o f V102A goes negative
enough to turn V102A o ff again. The result is that the
Sweep Generator circuit is retriggered at the end o f each
holdo ff period to produce a free-running sweep. This sweep
free-runs at the sweep rate selected by the T IM E /D IV
switch to produce a bright reference trace at fast sweep
rates. This is in contrast to the 50-hertz free-running repeti­
tion rate in the AUTO mode.
H O R IZ O N T A L A M P L IF IE R
General
The Horizontal A m p lifie r circuit provides the output
signal to the CRT horizontal deflection plates. In all posi­
tions o f the Horizontal Display switch except EXT HORIZ,
the horizontal deflection signal is a sawtooth from the
Sweep Generator circuit. In the EXT HORIZ position, the
horizontal deflection signal is provided by an external signal
applied to the EXT HORIZ INPUT binding post on the rear
panel. This circuit also contains the horizontal magnifier
circuit and the horizontal positioning network. Fig. 3-6
shows a detailed block diagram of the Horizontal Am plifier
circuit.
Sawtooth Input CF
The input signal fo r the Horizontal A m plifier in the X I
and X5 positions o f the Horizontal Display switch is the
sawtooth produced by the Sweep Generator circuit. The
sawtooth is applied to the grid o f Sawtooth Input CF
3-9