Tektronix 511A, Instruction Manual

Page: 15 / 74

SECTION III
CIRCUIT DESCRIPTION
CATHODE RAY TUBE CIRCUIT
The Type 511 A uses a type 5CP1A cathode ray tube.
This tube has a five inch screen and utilizes electro­
static focus and deflection. A post-accelerating elec ­
trode (intensifier)
is used to
provide increased bright ­
ness. The total accelerating potential is about 3000 V,
giving a sharp bright image visible in normal room
light.
The various negative voltages necessary to operate
the tube are obtained from a voltage divider
network
across the — 1500 V supply. R142 provides the bias volt­
age which controls the beam current and thereby image
brightness. This control is labeled INTENSITY. C60
serves to bypass the cathode to ground via the filter
capacitor C 77 in the power supply. In order to bring
the electron beam to a sharp focus at the screen, the
potential of the second anode is adjusted by the FOCUS
potentiometer R140. The ASTIGMATISM control is the
potentiometer R135 connected between the +225 V and
— 140 V supplies. This control adjusts the potential of
the second anode so that good focus is obtained on
both horizontal and vertical lines simultaneously. In
order to
transmit
unblanking pulses
to the
grid of
the
cathode ray tube, the .1 mfd 2000 V capacitor C58 is
used. A similar capacitor C59 is used to transmit ex ­
ternal impulses such as timing markers from the CRT
CATHODE binding post.
The twin potentiometers R131A and R131B provide a
means of varying the average potentials of the vertical
deflection plates and therefore vary the position of the
image. These potentiometers are connected between
the +225 V and — 140 V supplies so that as one plate
is made more negative the other is made an equal
amount positive. By this means the average potential
of the vertical plates is not changed by adjustment of
the VERT. POSITION control. This is essential in order
to maintain good focus over a wide area of the screen.
When the DIRECT position of the INPUT CHAN. SEL.
switch is used, the VERT. POSITION control adjusts the
potential of the lower plate only. In this case it may be
found necessary to adjust the ASTIGMATISM control to
obtain the best focus.
SWEEP CHASSIS
The sweep
chassis
provides
the
circuits
necessary to
deflect the electron beam horizontally across the screen
at a uniform rate, starting each sweep in synchronism
with the trigger impulses. Circuits are provided which
cut off the beam between sweeps. Provision is made to
feed various sweep circuit waveforms to binding posts
for external use.
TRIGGER PHASE SPLITTER
The
TRIGGER
SEL. switch,
operating in
conjunction
with V 1, the trigger phase splitter, selects the source of
trigger signal and reverses its phase if necessary. The
trigger amplifier V2 requires a positive impulse on its
grid to provide the correct trigger to the multivibrator.
When the TRIGGER SEL. switch is in the +INT. or
+EXT. positions, signals are taken off the cathode of
V1 and do not change in polarity. Therefore a positive
impulse must be supplied by the external source or
video amplifier as the case may be. With the TRIGGER
SEL. in the — INT. or — EXT. positions, signals are taken
off the plate and reversed in polarity, thus providing the
required positive output from negative trigger sources.
TRIGGER AMPLIFIER
The 6AG7 tube V2 serves as a variable gain ampli­
fier for the positive trigger impulses received from V 1 .
The TRIGGER AMPL. potentiometer R6 varies the bias
and thus determines the gain of V2. R12 serves to pro ­
tect the tube in event of the failure of the — 140 V supply.
TRIGGER COUPLING DIODE
The negative trigger impulses from the plate of V2
are impressed on the plate of V4 by means of the coup­
ling diode V3. The diode is used in preference to a
coupling capacitor because it disconnects the trigger
amplifier from the multivibrator once the multivibrator
is triggered. When the multivibrator
is triggered
the
plate of V4 falls, making the diode plate negative with
respect
to its
cathode
and therefore non-conducting.
This condition is maintained until the multivibrator com ­
pletes its cycle and is ready to be triggered again.
MULTIVIBRATOR
To convert the various shapes of trigger impulses
into square waves of controllable duration, suitable for
operating the sweep generator and unblanking the cath­
ode ray tube, a multivibrator is provided. This consists
of two tubes V4 and V5. During the waiting period V5
is fully conducting since its grid is at cathode potential
and therefore its plate is held at a low value due to the
voltage drop in R23. With V5 in this condition and the
SWEEP STABILITY correctly adjusted, the grid of V4 is
held well below the cut-off by the voltage divider R17
and R18. The circuit remains in this condition until a
trigger impulse is received. The plate of V4 is driven
down by the trigger pulse and also the grid of V5, be­
cause of the coupling capacitor C7. The plate of V5
therefore rises carrying with it the grid of V4 causing it
to conduct. This action continues until V4 is conducting
and V5 cut off. This condition is maintained until the
capacitor C7 discharges, raising the grid voltage of V5
to such a point that it starts to conduct. This makes the
plate of V5 fall, carrying with it the grid of V4 and start­
ing a regenerative cycle which ends with V5 con­
ducting and V4 cut off, the condition which existed be ­
fore the trigger impulse occurred. Thus the plate of V4
produces a square negative pulse and simultaneously
the plate of V5 produces a positive pulse. The negative
pulse is used to control the operation of the sweep gen ­
erator and the positive pulse to unblank the cathode ray
tube. The duration of the portion when V5 is cut off is
determined by the time constant R20A, R21, R22, C7 and
the voltage applied to R20A, and is substantially in ­
dependent of the characteristics of the trigger impulse.
SECTION 3, PAGE 1 TYPE 511-A, 511-AD/