Tektronix 576 - Page 65

Circuit Descri p tion-Ty pe 576
0 to +4
.5-volt V ariable Su pp ly .

T he 0 to +4
.5-volt vari-
able su pp
ly consists of dio de bri d ge D758A, Β , C
and D,
filter ca p
acitor C759, com p arator Q767A and
Β , emitter
follower
Q774,
s
h ort p rotection Q772 and series regulator
Q778 . T
h
is
circuit op erates in essentially t he same manner
as t he -75-volt
su pp ly circuit . In t h is circuit, h owever,
t h e
reference voltage at t
he base of Q767A is variable from
0
volts to
+4
.5
volts by t he RE ADO U T I LLUM co
n trol,
R 760, and
d ivi d
er
R 762 an d R 763 . T he outp ut
curre n t of
t he supply
is limite d by Q772 .
+5-volt Su p ply . T h e +5-volt su p
ply consists of error
amplifier Q780, s h ort p rotection
Q784 an d series regulator
Q787 . T h e supp ly s h ares d iode
bri d ge D758A, Β , C and D
and filter ca p acitors
C758 and C759 with the +4 .5-volt su p -
p ly . Any variation
in
the
+5-volt s u pp ly voltage is am p lifie d
by 0780, causi ng
t
he b ase
voltage of Q787 to vary in
opp ositio n to t he variation of
t he su pp ly . T he current con-
ducted t hrough R 788 by t
h
e
supp ly is t hus regulate d ,
w h ic h in turn regulates t
h
e
+5-volt su pp ly . Q784 p rovi d es
s h ort p rotection by
turning on w h enever t he cu rrent
t h roug h R 788 becomes
excessive . Wh en 0784 turns on, t he
base voltage of Q787 is p ulle d d own,
limiti ng t he current
t h roug h 8788 .
+12 .5-volt Supply . T he +12 .5-volt
su pp ly consists of
diode bri d ge D790A, Β , C an d
D, filter capacitor C791,
com p arator 0795 Α and Β , emitter follower
Q803, s h ort
p rotection
Q800, an d series regulator Q808 . T h is circuit
operates in
essentially
t h
e
sa
m
e
manner as t he -75-volt
su pp ly . S h ort protection of t
h e +12 .5-volt su pp ly w hen it is
s h orte d to α more p ositive voltage is p
rovide d by 0725 of
t h e -75-volt su pp ly . If t he +12 .5-volt
su pp ly voltage is
p ulle d up, t he base of 0725
is
also p
u lle d
up,
turning on
Q725 . W it h Q725 t u rne d on, t he base
of Q729 is p ulled
d own turni ng off t he -75-volt su pp
ly, w h ic h will turn off
t h e +12 .5-volt s up ply .
+15-volt Sup p
ly, Camera P owe r . T he +15-volt
su pp ly
con sists of error am p lifier
0810, emitter follower Q817,
s h ort p rotection 0814 and series
regulator Q819 . T h e sup-
ply s h ares d io
d
e
bri d ge D790 an d filter capacitors
C79 Λ
and
C791 with t he +12 .5-volt su pp ly . Any
variation in t he
+15-volt su pp ly voltage is am p lifie d by
0810, causing an
o pp osi
ng variation i n t he voltage at t he base of
0817 . T h is
opp
osing voltage variatio n is transmitted t h roug h t h
e emit-
ter of Q817 to t he base of series regulator
0819 w h ere it
controls t h e current con d ucte d by
R 819 an d t h us regulates
t he supply . Wh en enoug h c u rrent is co
ndu cte d by Q819 to
turn on Q814, t he voltage at
t h
e
base of Q817 is p u lle d
d own, t h us limiting t
h
e current
t h roug h 0819 .
+50-volt Su pp ly .

T he +50-volt s u
pp
ly co
n sists of d io de
bri d ge D821A, Β , C and D, and filter
ca p acitors C822 and
C823 . It is α floating unregulate d su pp
ly used to p ower t he
ste p am p lifier outpu t .
+100-volt
Su p ply . T he +100-volt supply consists of
d io d e bri d
ge D828A, Β , C an d D, filter capacitor C829,
3- 24
error amplifier Q834,
emitter follower 0840, sh ort protec-
tion Q837 and series
regulator Q846 . A n y variatio n i n volt-
age by t he +100-volt su p
ply is am p lifie d by 0834 an d tra n s-
mitte d t h roug h 0840 to t h e base of
0846 . Since any varia-
tio n in t he su pp ly is inverte d by
Q834, t he base voltage of
Q846 will always move in opp
osition to α variation of t he
su pp ly . T h e current cond ucte d by
R 846, t h erefore, also is
conducted so as to op pose
any c h ange in su pp ly voltage .
Wh en enoug h current is con d ucte
d by Q846 to turn on
Q837, t he voltage at t he base of Q840 is p
ulle d d own, t h us
limiti n g t he current con d ucte d by Q819 .
C R T V oltage Su pp ly
T h
e
C R T p ower su p ply p roduces two h
ig h voltages, -4
kV a n d +225 volts, for op eration of t h e C
R T and its relate d
co
n trols .
In
a dd ition, t h e +225-volt su pp ly is
used by t he
d is p lay am p lifiers . T he source of p ower for
t h
e
two s u pp lies
is α h ig h freq u e ncy (about 28 kH z) H artley
oscillator w h ic h
consists of
Q851 and t h e two p rimaries of tra n sformer
Τ 850 . T h
e collecto r of Q851 is connected t h roug h t he
col-
lector p
rimary, R 850 an d L 850 to t he +100-volt
supp ly .
Wh
en curre
n t flows t h roug h t he collector p rimary, α
mag-
netic fiel d is built up in t he tra n sformer core .
Due to t h is
fiel d , α reverse base curre n t is
caused to b e co ndu cte d
t hrough Q851 b y t he base p rimary and
Q851 is even tually
t urned off . W it h Q851 off, no current
flows t hrough t he
collector p rimary .
T he resi d ual fiel d i n t he transformer core
now causes forward b ase current
to be con d ucte d t h roug h
0851, tu r ning it on . As
0851 turns
on,
cu rrent again flows
through t he
collector pr imary, t h us begin n i n g α new cycle .
T he fre q
uency
of
t h
e
oscillator and t h us t he out p ut cu r rent
of t he secon d aries is controlle
d by t he voltage on p i n 2 of
the base p rimary .
-4 k ilovolt
Su p ply . T he -4 kV su p p ly
consists of h alf-
wave rectifier D870, filter
ca p acitors C870 an d C871, and
divi d er resistors 8875 t hr oug h R 883
. T h is sup p ly is α h alf-
wave
rectifie d su p p ly with D870 forwar d
biasing on nega-
tive tra n sistions
of t h e voltage on t he -4
kV secon d ary .
T he -4kV su pp ly
voltage after bei ng filtere d by C870 and
C871 is re duced by Zener d iode
D882 to p rovi de t he
-3
Β
g
Π
volt
cat hnλe voltage_ T he gri d voltage is controlle d
by
t he d ivi d er ma d e up of 8882 an
d I N T EN SITY control
R
883 . T he voltage on t he focus screen of
t he CRT is con-
trolle d by
F
OC U
S cont r ol R 880 .
T
h
e
-4kV su pp ly is regulate d from α
reference su pp ly
w h ic h is generated by t he win
d ing between terminals 6 an d
5 of Τ 850 . T h is refere n ce su pp
ly
co
n sists of h alf-wave recti-
fier D866 and D869, and filter
ca p acitor C866 . T he regula-
tor circ u it consists of
error am p lifier Q859 an d emitter fol-
lower Q855 . Any
variation in t he refe r en ce sup p ly voltage
is transmitte d to
t he base of Q859 t hrough d ivi d er
8860-8864
.
T
h
e
va r iatio n is t h en amplifie d an d inverte
d b y
Q859 and
transmitte d t h roug h Q855 to t he base of
Q851,
w
h
ere it
regulates t he d rive of t he oscillator .
Any variation
in
current con d ucte d by t he -4kV su pp
ly
is
conducted by
R 899, w h ic h causes t h e decou p le d su
pp
ly
voltage at t h e
emitter of Q859 to vary, t h us com
p ensati n g for c u rrent
variation in the -4 kV sup p ly .