Tektronix 576 - Page 63

i n gs . F or
115-volt op eration t he L I NE S ELE CTO R switch
connects
t
he
two primaries
in
p arallel and for 230-volt op-
eration
connects t h em in series . For 230-volt op eration,
F
703 is connecte d into t he circuit . T he RAN GE S ELE C-
TO R plug d etermines h ow many turns of each p rimary
win
d
i ng
are
utilize
d
to
com p
ensate
for variations in line
voltage
.
-75-volt Supply .
Th e -75-volt sup p ly co n sists of d iode
bri dge D706 Α
,
Β
, C and
D,
filter ca p acitors
C706
and
C707, com
p
a r
ato
r 0716 Α
a
n
d Β , emitter follower 0729,
s hort protection Q725 an d
0727, and series regulato r
Q734 .
9-volt Zener diode D708 sets t h e base voltage of compar-
ator t r ansisto r Q716A w h ile t h e q uiesce n t voltage at t he
base of Q716 B is set by -75 V a d j ustment
R721 .
Any
variatio n in t he -75-volt su pp ly voltage is com p a r ed by
Q716A an d Β . T he resulting rise or fall i n voltage ac r oss
R715 is transmitted by 0729 to t h e base of series regulator
Q734 . Any ch ange in voltage of t he -75-volt su pp ly will be
o pposed by α c
h
ange
i
n c u rrent t hr oug h t he series reg u lator .
T he o
u
t
p ut current
of
t he -75 volt su pp ly is limite d to α
value less t h an n ormal w
h
enever
t h
e
supply is s horted to α
voltage between -75 V an d c
h
assis gro
u nd . T h e s u pp ly
cu r rent of the -75 volt supply
is
co
n t r olle d
by
t he voltage
across R735,
w
h ic h is depen
d
ent on t
h
e base
voltage of
Q734 . T h is voltage is i n turn de p en
d
ent on
t he voltage
across R730 and R731 . As t h e -75
volt sup p
ly
becomes
more p ositive (due to s h orting it to α
mo
r e positive sup p
ly),
t he voltage at t he base of Q734 is raise
d , causing more
3- 22
F
ig . 3-13
.
E xam p le o f operation o f H ori z ontal R eadout decoders .
su pp ly c u rrent to be
con d
ucte
d t h roug h R 735 . As t h e sup-
ply voltage b ecomes mo r e positive, t h
e voltage at t h e j unc-
tion of R730 and R731 rises h ig h enoug h to turn on
Q727 .
Wh e n Q727 turns on, it begins pulling dow
n
on t he base
voltage of Q729 and d ow n on t
h
e base voltage of
Q734,
t h us limiting t he supply c u rrent .
T
he out pu t current of t
h
e
-75-volt su pp ly comes less, t he closer t he su pp ly voltage is
to grou nd .
D732 prevents t he supply from going mo r e t h an 0 .6 volt
above ch assis grou nd if t h e -75 volt su pp ly is s h o r ted to α
p ositive voltage
.
D722
p
rotects
t
h e
-12
.5
volt
s
u
pp
ly
if
it
is
s horted to t he -75 volt su pp ly . If t he -12 .5 volt s u pp ly is
pulled negative,
D722
turns on w
h
en t
he
su pp ly
is
abo u
t
at
-15 volts
w
h ic h d isables com p
arator 0716A and B . The
-75 volt
su pp
ly t hen limits current un til bot h su pp lies are
at about -2 .5 volts . If t he +12 .5 volt supply is s horte d to
t he +100 volt su pp ly, Q725 turns on . Wh en 0725 is on, it
limits current t h roug h R735 i n t he same manner as d is-
cussed p reviously for Q727 . T he result of s horting t he
+12 .5 volt su pp ly to α morep ositive voltage is to turn
off
t h e -75 volts supply . Since t he -75 volt supp ly is t he
reference for t he -12 .5 volt, +12 .5 volt, +100 volt, and
CRT voltage sup p lies, w h en t he -75 volt supply is turned
off, t he other p owe r sup plies are t u rned off .
-12 .5-volt Su p ply . T he -12 .5 volt s up p ly consists of
d io
d
e
bri d
ge D737A, Β , C an d D, filter ca p acitor C738,
com p arator Q744A and Β , emitter follower Q750, s h o r t
protection Q748 an d series regulator 0756 . T h is circuit reg-
ulates t he -12 .5-volt supp ly in essentially t he same manner
as t he -75-volt sup p ly op erates .
Ci
r c u
it Description-Type 576
I npu t
Ρ 950
L am p Co d e to Ho r izo n tal
Co d e R ea d o u t L am p s
1 2 Χ
,2,5,A,V
Η
1
4 15 L
1
G
U951
14 Η
2
Η
L
1
1
5 Χ
5
13
2 .5 Ι
Η
12
L
5
J
11
Η
Η
12 A MP
S
6
V
L
9 L
Α
Κ
10 Η
PER H O R IZ DI V
H O R IZO N TA L Switc h 15
O
FF
m, n Α
Rea d ou t
.5 V CO L L ECTOR Η
2
3 5 15
μ
17
.1 Χ
14
_ Β
500
m V
Η
10 C
13 L
16 , 10 2
12
02
D
L
Η 2
01
10 1
11
L
L
3
U 953
S
NE G
O
FF
L
7