Tektronix 516 series, Instruction Manual

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Circuit Description — Type 5 1 6
at about —2.5 volts with respect to ground. The Disconnect
Diodes are conducting and hold both sides of the Timing
Capacitor at about —2.5 volts. With its cathode grounded
and its grid at —2.5 volts, the Miller Runup tube, V161A,
is conducting heavily and its plate is at about + 30 volts.
A negative trigger pulse, arriving at the grid of V135A
from the Time-Base Trigger, causes the Sweep-Gating Multi­
vibrator to switch rapidly to its other state. That is, V135A
cuts off and V145A conducts. V135A is held in cut off after
the trigger pulse passes by a rise in voltage at the common
cathode connection. As V145A conducts, its plate voltage
goes down, cutting off the Disconnect Diodes. When the
Disconnect Diodes cut off, the plates of the Timing Capaci­
tor are no longer held at —2.5 volts, and the Timing Ca­
pacitor starts to charge toward the instantaneous potential
difference between the —150-volt supply and the potential
at the cathode of V I61B. However, as the lower side of the
Timing Capacitor starts to move in a negative direction, it
takes the grid of V I61A with it. This produces a positive
swing at the plate of V I61A which is coupled, through B167
and V I61B, to the top of the Timing Capacitor. This increases
the voltage to which the Timing Capacitor is trying to charge.
The effect is to “ straighten out” the charging curve by in­
creasing the charging voltage with each increment of charge
on the capacitor. The positive swing at the top of the Timing
Capacitor also tends to prevent the lower side from swing­
ing negatively. Since the gain of V161A is about 150, the
potential at the top of the Timing Capacitor moves about
150 volts with respect to ground while the potential at the
lower side moves about one volt. The result is an extremely
linear sawtooth at the cathode of V I61B, which is applied
through the Horizontal Amplifier to the horizontal deflection
plates of the crt. This sawtooth is also coupled through the
cathode follower V193B to the SAWTOOTH OUT connector
on the front panel.
Sweep Length
The length of the sweep— that is, the distance the spot
moves across the crt— is determined by the setting of the
SWP. LENGTH adjustment, R176. As the sweep voltage rises
linearly at the cathode of V I61B, there will be a proportion­
ate rise in the voltage at the wiper arm of the SWP. LENGTH
adjustment. This rise is coupled through the two Hold-Off
Cathode Followers, V183A and V183B, to the grid of V135A.
When the voltage rises to the point where V135A comes out
of cutoff, the Sweep-Gating Multivibrator will rapidly revert
to its original state with V135A conducting and V145A cut
off. The voltage at the plate of V145A will then rise, carry­
ing with it the voltage at the plates of the Disconnect Diodes.
V152B starts conducting and discharges the Timing Ca­
pacitor. This brings the grid of V I61A quickly back up to its
quiescent level. The rise in voltage at the grid causes the
tube to conduct more, so the plate voltage drops, carrying
with it the grid and cathode of V I61B. When the voltage
at the cathode of V I61B returns to about —2.5 volts, V152A
conducts, clamping the voltage at this point
The time during which the voltage at the plate of V I61A
and the cathode of V I61B drops is the sweep trace in­
terval. The trace is blanked during this time and during the
hold-off time following it.
Hold-Off Circuit
The Hold-Off Circuit prevents the Time-Base Generator
from being triggered following the completion of a sweep
until after the Miller Runup Circuit has stabilized in the quie­
scent condition. It does this by holding the grid of V135A
positive enough to prevent it from being taken into cutoff
by the negative trigger pulses from the Time-Base Trigger.
When no sweep is being generated, the arm of the SWP.
LENGTH potentiometer and, therefore, the grid of V183B are
at about —90 volts. Normal operating bias on V183B is
about 5 volts, so the Hold-Off Capacitor, C l80 and/or C l81,
is charged to about 85 volts. During the sweep, the Hold-
O ff Capacitor discharges to about 45 volts through V183B
as a result of the rise in voltage on the grid. At the end of
the sweep, the voltage at the grid of V183B drops back to
—90 volts. The cathode tries to follow but is held up by the
charge on the Hold-Off Capacitor. The Hold-Off Capacitor
starts to charge again exponentially toward 85 volts, carry­
ing the grid of V183A negative. The cathode of V183A and
the grid of V135A follow the grid of V183A. When the Hold-
O ff Capacitor charges to the point where V183A cuts off
(V183A is normally cut off between sweeps), it loses control
over the grid of V135A, which then returns to the level es­
tablished by the setting of the STABILITY or PRESET STA­
BILITY control.
The amount of hold-off time, then, is determined by the
value of capacitance and/or resistance switched into the
Hold-Off Circuit by the TIME/DIV. control. (See the Timing
Switch circuit diagram.) The amount of hold-off time requir­
ed is determined by the sweep rate. For this reason the
TIME/DIV. control changes the time constant of the Hold-
O ff Circuit as well as that of the Timing Circuit.
Sweep Stability
The STABILITY control, R110, (or the PRESET STABILITY
adjustment, R ill, depending upon the mode of operation)
regulates the dc level at the grid of V135A. This control is
adjusted so that the voltage at the grid of V135A is just high
enough to hold V135A out of cutoff. When the circuit is
adjusted in this manner, a sweep will be produced only when
a negative trigger pulse from the Time-Base Trigger drives
V135A into cutoff. Turning the STABILITY control fully clock­
wise (when SW10B is in the DC, AC, or HF SYNC position)
places a sufficiently negative voltage on the grid of V135A
that V135A cuts off immediately upon decay of the hold-off
voltage and initiates the next sweep without a trigger pulse.
The result is a free-running sweep whose period is the total
of the sweep time plus the hold-off time at any given setting
of the TIME/DIV. control.
Unblanking
The positive rectangular pulse appearing at the cathode of
V135B in the Sweep-Gafing Multivibrator is coupled through
the Unblanking Cathode Follower, V145B, to the grid circuit of
the crt to unblank the crt during sweep time. This unblanking
pulse is also coupled through the cathode follower V193A to
the +GATE OUT connector on the front panel.
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