Tektronix 7704A User Manual

C RT cathode . Q4105 and
Q4115 are connected as α
collector-couple d
complementary amplifier d riven by
U 4110 to
correct the CRT-Cathode Sup p ly for
h
igh-
and
low-frequency
changes respectively .
R egulation occurs as follows : If the
CRT cathode
voltage
b ecomes less negative, α positive-going
change is
couple d to the input of U 4110 at p in
3 and results in α
p ositive-going output at pin 6 .
This
p
ositive-going change is
inverted b y Q4105-Q4115 to α
negative-going change at
their collectors . This causes the voltage
across C4205 to
increase d uring the p ositive
h alf cycle of the input
waveform . During the n egative
h alf cycle, the increased
voltage across C4205 increases
the voltage at the output of
the CRT-Cathode
Supply to correct the original error .
H igh-frequency
correction signals are AC couple d to the
CRT
cathode
through C4208 .
Control-Grid DC R estorer
The
Control-Grid
DC R estorer stage elevates the DC
level of the Ζ
-Axis Amplifier output to α potential more
negative than
the CRT cathode . This action allows the
control grid to control the CRT beam current . The
Control-Grid DC Restorer stage is driven b y
the
square-
wave output of Τ 4201 . C R4217 an d C R4218
are forward
b iased d uring the p ositive and
negative h alf cycles of the
input
square
wave respectively to limit the square-wave
amplitu de at their j unction . Grid Bias ad justment
R
4135
sets the voltage on the cathode of CR4217 to establish t
he
forward-bias level and peak positive level at
the
anode of
CR4217 . The DC
level
of
the Ζ -Axis Amplifier output
d etermines
the voltage on the ano de of CR4218 to establish
t he forward-bias level and peak negative level at the cathode
of CR4218 . The limite d
-amplitude
square
wave at the
j unction of CR4217-CR4218 is
coupled
to
the j unction of
CR4215-CR4216 through C4215 .
During the positive h alf
cycle of the input square
wave, CR4215 is forward b iased
to clamp its anode at
the CRT-cathode voltage level . During
the negative
h alf cycle, C4216 is charged through C R4216
to α voltage
level more negative than the CRT cathode by
an
amount equal to the d ifference between the Gri d Bias
adjustment setting and t he Ζ -Axis
Amplifier
output
level .
H igh-frequency Ζ -Axis Amplifier
signals are coupled to the
control grid t hrough C4219 and
R 4220-C4216 .
F ocus-Grid DC R estorer
The operation of the
F
ocus-Grid DC
Restorer is similar
to the operation of the Control-Grid
DC R estorer . The
limited-amplitude square
wave at the junction of
CR4230-CR4231 is
coupled to the j unction of
CR4233-CR4235 throug
h C4230
.
The amplitu d e of the
p ositive h alf cycle of
the input square wave is clamped at
approximately +150
volts b y CR4230 . The peak negative
amplitude
is
established b y the DC level of the Auto-Focus
Amplifier output through C R4231 .
During the p ositive h alf
cycle, the focus-grid voltage is
clamped
to
the voltage set by
the
F
OCUS
control R 5001 through
Circuit Description- 7704A Service
R4235-CR4235-CR4233 . During the negative h alf cycle,
C4231
charges through C R4233 to establish t he proper
level at the focus-grid electrode .
Ζ -Axis In p ut
The Ζ -Axis Input stage combines the various
signals that
affect the d isplay intensity into one
signal
to
provide the
d rive to the input of
the Ζ -Axis Amplifier stage at the b ase
of
Q4197 . F or normal operation, the Ζ -Axis signal from the
L
ogic
Circuit determines the output of this stage
. Other
signals affecting the output include the front-panel
(Display
Unit) INTENSITY control R5004,
the Intensity L imit
input from the plug-in
compartments, and the HV Sense
signal from the Anode
V oltage M ultiplier . During r eadout
d isplays, the Ζ
-Axis Inhibit signal from the Rea d out System
turns
off Q4177 so the output of this stage is d etermined
by
the ΖR eadout input only .
Ζ -Axis Am p lifier
The Ζ -Axis Amplifier stage is α
current driven, shunt-
fee d back amplifier with voltage output .
The output voltage
provides the d rive signal to control
the CRT intensity level
through t he Control-Grid
DC R estorer stage . The output of
the Ζ -Axis
Amplifier is also connected to the Auto-Focus
Amplifier stage to
control the operation of the Auto-Focus
Amplifier stage .
The base of
Q4197 is the summing point (input) of
the
Ζ -Axis
Amplifier stage . R41109-C41109 provide feedback
from the output of
this stage to the summing p oint .
C41109 is
adjustable to provide optimum step response
with minimum overshoot or ringing .
Otherwise, the CRT-
display intensity would vary
following sudden c hanges in
intensity level . Ζ -Axis
Peaking adjustment R4197 p rovides
a d ditional transient-response
adjustment . The signal at the
collector of
Q4193 is DC-coupled to the base of Q41115
through R41101 to maintain goo d low-frequency response
and to provide α fast falling ed ge on
the output signal . The
fast-changing p
ortions of the input signal are coupled to the
base of
Q41113 through C41107 .
Q41113-Q41114 are connected as
α collector-coupled
complementary amplifier . Q41113 is α
ΡΝΡ -type transistor,
so it responds faster to negative-going
changes
at
its base
than to p ositive-going
changes
.
This action provides α
fast-rising edge on
the output signal (fast-falling edge is
provided b y Q41115,
an ΝΡΝ -type transistor) .
Auto-Focus
Amplifier
The
Auto-Focus Am p lifier stage d evelops control
voltages to
maintain optimum focus of the CRT d isplay .
When the F OCUS control is set for best d efinition of the
CRT d isplay at low to medium settings of the intensity
2- 6 1

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English