Tektronix 7834

Theory of Operation-7834
Collimation Electrode Driver stages . Transistor 02924 is an
emitter follower stage added within the feedback loop to
reduce the amplifier output impedance for ac and transient
signals which are coupled into the output of this amplifier
from the Fast Mesh Driver stage through the inter-mesh
capacitance of the crt . Capacitor C2915 increases the
amplifier response to these injected transients and provides
high-frequency compensation .
Collector and Collimation
Electrode Decoder
This stage consists of
03012, 03016, 02566, 02988,
02904, and associated diode-resistor networks
. This stage
decodes the logic inputs from the Main Timing, Transfer
and Multi, and Storage Mode Switching stages, into discrete
current level signals for input to the collector mesh, colli-
mation electrodes and flood gun anode driver stages . The
stage uses transistor switches and diode current steering
networks
to convert the input logic signals into currents .
Fast Mesh Driver
This
stage consisting of 02862, 02864, 02868, 02873 and
02874
is an expanded version of the simple operational
amplifiers
previously discussed . Transistor 02864 is a volt-
age shifter
stage which permits the output transistors to
operate
from a negative supply . Transistor 02873 provides
current limiting to protect the amplifier from output short
circuits . Transistor 02874 is an emitter follower for low ac
output impedance .
Fast Mesh Decoder
This stage, consisting of 02842, 02852, CR2848, and
CR2849,
decodes inputs from the Storage Mode Switching,
Tsfr and Multi, and
Fast
Prep
stages into discrete current
levels for the Fast Mesh Driver
stage
.
When Fast Prep is LO,
CR2849 is reverse
biased and CR2848 is forward biased . If
VP (+15 V) is LO or 02944 is on, all of the current in
R2902 provides input to the Fast Mesh Driver stage, and
the output voltage to the Fast Mesh is +125 volts . When
VP
(+15 V) is HI and 02904 is off, the current flow through
R2901
offsets some of the current in R2902 causing the
Fast Mesh to drop
to +100 volts . When Fast Prep is H I,
CR2848 is reverse biased and 02852
is off . Resistors R2850
and R2855, in Fast Variable Persistence and
Fast Bistable
respectively, adjust the Fast Prep Level in
each Fast Storage
mode between zero and +20 volts
.
The "Pump Pulses"
which appear on the Fast Mesh when
it
is
at the Fast Prep
Level are the result of the oscillation of the
Fast Prep logic
signal .
During a sweep or a transfer operation, 02842 turns off .
Resistors R2845 and R2846, in Fast Bistable and Fast Var
Persist respectively, adjust the Fast Transfer Level (AV)
in
each Fast Storage mode between zero and
3 volts below the
Fast Prep Level . The STORAGE LEVEL
control, R2720,
provides offset to the Fast Transfer Level in both
Fast
storage modes through R2748 .
3- 76
Storage Mesh Driver
Storage Mesh Decoder
This driver stage consists of two parts : an operational
amplifier 02802, 02804, 02808, and 02814 which is
similar to the Fast Mesh Driver stage, and a 600 volt switch-
ing circuit 02818, 02822, 02826, 02828, 02834, and
02838 .
When Tsfr and Erase are both LO, 02818 and 02833
are
off ;
02826 and 02828 are saturated
. Diode CR2834 con-
ducts current from R2833 and holds the Storage Mesh
potential at approximately 0 .8 volts above the output level
of the operational amplifier .
When the Erase or Tsfr signal go HI, 02818 and 02822 turn
on and 02826 and 02828 turn off . Transistors 02834 and
02838 turn on and drive the Storage Mesh to the +600 volt
supply . CR2839 disconnects the operational amplifier from
the Storage Mesh at this time . Bootstrap capacitor C2833
maintains base drive to 02834 and 02838 throughout the
100 millisecond duration of either the Erase or Tsfr pulse .
Current limiting is provided in both the operational ampli-
fier and 600 volt switch by Light emitting diodes in the
output sections .
This stage, consisting of 02704, 02714, 02730, 02734,
02745, 02752, 02755, and associated diode-resistor net-
works, develops discrete current levels for input to the
Storage Mesh Driver .
Consider operation in the Bistable mode . Initially, the
currents in
R2715, R2717, CR2726,
R2727, and R2736
are all zero . Transistors 02745 and 02755 are saturated
and the net current into the Storage Mesh Driver is the sum
of the currents in R2746, R2747, and R2716 . Resistor
R2745 sets the Bistable Operating Level between
+35
volts
and +120 volts . When Erase goes
HI,
02752 saturates,
turning off 02755
and
02745
and charging C2753 . At
the
end
of the
Erase
pulse, the voltage on the Storage
Mesh
is
at -15 volts set by R2716 . However, C2753 discharges
through
R2753 and R2754, causing the Storage Mesh volt-
age
to
ramp
back
up
to the Bistable
Operating
Level in
approximately 600 milliseconds . In
Fast
Bistable mode
the operation of the
Storage
Mesh Decoder is unchanged .
Consider
operation
in
the Variable Persistence mode
.
Initially, the currents in R2736, 02745, R2727, and R2717
are zero . Diode CR2726 is forward
biased and
02714 is
off so the net current
into
the
Storage
Mesh Driver
is
the
sum of the currents in R2724, R2726, R2715, and R2716 .
Resistor R2725 sets
the Variable
Persistence
Operating
Level between
zero and +15 Volts, while the STORAGE
LEVEL control, R2720,
provides an adjustable offset of