Tektronix SC 502, Instruction Manual

Page: 42 / 127

•
Theory
of
Operatlon-se
502
Diodes CR885
and
C
R 884 provide the rectified
negative
control voltage for t ' he crt c o n tro
l
grid. The output level of
this supply is set
by
the Beam adjustment R873. D i o de s
CR879 and
CR882
cli p the crt
grid
bi as voltage from
the
T800 secondary. to determi ne the operati ng level at the
c o n tr
o
l gri d. CR879 lim its the
n
eg a
t
i v
e excursion
of the
bias voltage,
dependi ng
u pon
the
output v
o
lt
a
g
e of the
Z­
Axis amplifier, The positive
c
l i ppi ng level at the
cathode
of
CR882 is
set by the Beam adj ustment.
CR885 acts
as a do
r e st
o
re r and
CR884
as a rectifier. This r e s u l
ts
in a dc
level
across R888
equal to
the
p
ea k
-
ta- p e a k
excursi on
at the
anode
of CR882.
CRT
Control Circuits
In
addition to
the
INTENSI TY control di scussed
previously, front-panel
FOCUS
and
internal asti gmatism
controls
h
av
e been
i ncorporated
for
a
r riv i n g
at
an op­
tim u m crt
display, FOCUS control
R894
provi des
the
correct
voltage
for the se
c
o n d anode in the crt. Proper
voltage for
the thi rd a
n o d
e
is obtained by
a
d
j
u
s
t
i
n
g
Astig
c
o
n t
r
o l R899. I n order
to obtai n
opti m u m
spot
size
and
shape, both the
FOCUS
and Astic controls are
adj
usted to
provide the p r o p e r electrostatic lens configuration in the
crt,
Geom adj ustment
R897
vari es
the positive level on the
horizontal deflection plate
shiel ds
to control t he overal l
geometry o f the display. The trace rotation control,
R895,
permits
adj ustment
of the
dc c
ur r e n t through
beam­
rotation
coi l
La95
to align the display with
the
horizontal
grati cule
l i nes,
D I AG RAM
�
CI RCUIT D ESCR I PTI ON
Low
Voltage Power Supplies and
Cali brator
-20 V
Supply
Power
for
the
-20
V
regulator is
suppl ied
from
the
TM SOO
S
e r
i
es
Power
Module -33.5
V
unregulated
supp­
ly.
The regulator for the
-
2
0 V
supply conSists of
operational
amplifier
U950. and the series pass stage
consist i ng
of
0960 and the N PN transistor
that
is l ocated
i n
the p o
w
e r module. The gai n of the a m p l i fie r is set
by
R949. R947
and
R945. R949
i s the i n p ut resistor, whi le
R947 and
R945
are the feedback resistors for the
operational
amplifier. The
gai n of t
h
e amplifier is
set to
about 2.9 times by R 9 4 7
(-20
V Adj ust). The supply
voltage is
established by
compari ng the voltage at the
negative i nput
(pi n 2)
of
U950
with the v
o l t a g e reference
at
the
positive i nput (pi n 3) . Any differences
between
the
negative
and
positive inputs
to
U950 cause a
change
i n
the
c
on
d
ucti o
n
of
0960
and p o
w
e r module series pass
transistor to
correct
for the
output error.
0960 and
the
power
module NPN
series
p
a s s transistor are
connected
as an o per
a
t
i on al ampl ifier whose o u t p ut is connected
back to
its negative
input.
0955
protects
the supply in the
event of a current overload. The overload (i ncrease in
current t h r
o u
g h
A943)
wi ll cause 0955 to
turn
on,
which
biases
0960
off,
shutt i ng
the -20 V su
p
p l y off.
+20 V
Supply , , :
The
-20
V and + 20
V
supplies are similar i n operati on.
Power
Off Circuit for -20 V end
+20 V
Supplies
PlaCi ng the
MODE
switch ( S3oo)
in
the PWR
OFF
position
applies
+1 1 . 5 V to
the negative i
n p ut
of
U 930 and
3·8
to the base
of
0930.
The
+ 1 1 . S V a pp
l
ie d to
U930
reduces
the output of the +20 V
regul ator,
while the +1 1 . 5 V turns
0930
on, pulling
t
h
e
supply output down to grou nd.
At
the
same time
+1 1 . 5 V is applied to 0930
and
U930,
+1 1 .5 V
is applied to the
emitter
of
common base
amplifier
0950 which i n turn applies
a sl ightly
p
o
s i t i v e voltage to the
positive input of U950. This re d u c e s the output
of
the
-20
V
r
eg
ul a t o
r to
near 0 V.
+5 V
Supply
Power for
the
+5
V regulator is supplied
from
the
TM 500
Series Power Module + 1 1 . 5 V
unregul ated
supp­
ly. The regulator for the +5 V suppl y consi sts of
operational ampl ifier U960 and series pass transistor
0970.
The
amplifier is
con
n e ct e d for unity gai n. R969 is
the feedback resistor for the
operational
amplifi er. The
supply v o l
t
a g e is establi shed by compari ng the voltage at
the negative i nput of
U960
with the voltage reference
at the
positive
i nput.
Any differences
between
the
negative
and
positive
i n puts
of
U960
causes
a change in the conduction
of
Darli ngton
transistor 0970 to correct for the
output
error. 0965 protects the supply in the event of a current
overload. The
o v e rl o a d ( i ncrease i n current throug h R982)
will cause 0965
to turn on,
which bi ases 0970
off.
shutti ng
the +5 V s upp l y off.
NO TE
If
an over-voltage condition causes fuse F970
to
open, an excessive amount of volt age will be applied
to
Zener diode VR969
which may damage the diode.
@
MAY 1 980