Tektronix 5441 Instruction Manual page 42

Theory of Operation-5441lR5441
VR452 provide the negative voltage for the focus grid of the the horizontal deflection-plate shield to control the overall
crt. Approximately 700 V peak to peak from the secondary geometry of the display.
of T410 is connected to the focus grid supply through C450
and R450. The positive clipping level at the anode of CR452
is set by the Focus control setting. This determines the op-
Auto Focus Amplifier
erat~ng level at the focus grid. Under normal operating con-
ditions, the voltage applied to the focus grid is more positive
(less negative) than the control grid or the cathode of the crt.
The signal developed by the Auto Focus amplifier is coupled
to the focus grid by C460. When there is a sudden change in
intensity levels, the focus grid will change to maintain a well-
defined display. Astigmatism adjustment R370, which is
used in conjunction with the Focus control to obtain a well-
defined display, varies the positive level on the astigmatism
grid. Geometry adjustment R365 varies the positive level on
A sample of the unblanking signal is taken from the col-
lector of Q340. This signal is inverted and amplified
nonlinearly by Q300 and Q302 to conform to the require-
ments of the crt focus electrode. As the base of Q300 is
driven negative, CR303 is forward biased, producing a knee
in the amplifier response. The level where the knee occurs is
determined by adjustment R306, auto focus bias. The oper-
ation of the remaining amplifier is identical to that of the Z-
Axis amplifier.
STORAGE CIRCUIT
The cathode-ray tube is a transmission halftone storage
tube. The collector mesh is a coarse mesh, which acceler-
ates electrons toward the target area. The target (storage
mesh) is a fine mesh with a highly insulative dielectric layer
deposited on it. It is in the dielectric layer that storage oc-
curs. The flood guns cover the entire storage target with a
continuous stream of low velocity electrons; these electrons
are prevented from reaching the phosphor screen unless a
display has been written on the storage area.
The collimation bands (electrodes) are used as a lens
that uniformly distributes flood-gun electrons over the stor-
age target area. In the non-store mode, the collimation
bands have no control over the crt.
Switching from the non-store mode to the store mode is
done by pushing in the STORE pushbutton. When changing
modes from non-storage to storage operation, an erase
cycle is generated automatically.
Flood-Gun Filaments
The flood-gun filament supply for the crt consists of
VR690-VR691 and emitter follower Q690. The power to
heat the filaments is derived from the -20 V power supply.
VR690-VR691 sets the base voltage of Q690 and provides
regulation.
Flood-Gun Cathode
The flood-gun cathode is switched either on or off by
switching transistors (2675, (2680, and Q685. Two signals
control whether the flood-gun cathode is turned on; they
are:
a. The STORE switch pushed in signal turns on the
flood-gun cathode.
b. The SAVE pushbutton, S675, when pressed, either
turns the flood-gun cathode completely off (VARIABLE
PERSIST control fully clockwise) or, if the VARIABLE PER-
SIST control is turned away from its fully clockwise position,
allows the flood-gun cathode to be pulsed at a low duty
factor.
When the 5441 power is turned off, in less than a micro-
second Q700 turns off. This eliminates the negative voltage
that would result from the slow decay of the power supply
filter capacitors, leaving only the positive voltage present at
the base of (2675. Q675 is turned on, causing Q680 and
Q685 to be off. This prevents damage to any stored infor-
mation if the save mode is being utilized, since there is not
any flood-gun cathode current available.
Flood-Gun Anode
The flood-gun anode circuit consists of operational ampli-
fier (2670-Q672. The operational amplifier is used as a
current-to-voltage conversion amplifier. A composite erase
pulse is supplied to the base of Q670 to hold the flood-gun
anode high during the erase cycle.
REV MAY 1982