Tektronix 576 - Page 44

C
Ε
_1C
Device
- τ
1
Ι
_J
350 V & 1500 V
L OO
P I N G
CO MPEN SATIO N
C339
L
OO
P I N G
CO
MPEN SATIO N
C343
F ig . 3-2 . ( Α ) Undesired ca
p
ac
i ta
n ce
ca u sing loop i ng ; ( Β ) L oo p ing com p en satio n .
will go negative . T he result on t he C R T is α loop instea d of
α single line to re p resent t h e curve of
IC
vs V
CE
.
L oo p ing Com p ensation
T he L OO P I N
G
COMPEN SATIO N
ad j ustment, C343 (see
F ig . 3-2 Β and t he Collector
Su
pp ly sc h ematic), H .F .
N OIS E
REJE CTIO N ad j ustment
C341 a
n
d
R 414
t h roug h R 418
(see t h e Dis p lay Sensitivity Switch ing
sc h
ematic)
h ave been
a dded to t h e circuitry as com p ensation for t he stray
an d
guar d box ca p acitance p reviously discusse d .
In
ge n
eral,
t h ese ad j ustments will not com p ensate
for
d evice ca p aci-
tance . T h is a dded capacitance forms α new ca p acitive d i-
vi d er w h ic h transmits AC current to t he vertical am p lifier
in opp osition to t he current transmitte d by t
h
e un
d
esire
d
ca p acitance . T h is o p posing current, t h erefore, nulls t h e
effect of t he und esire d ca p acitance w h ic h causes loo p i ng . In
ad j usting t h ese a dd ed ca p acitors, C343 is ad j uste d
to
com-
pen
sate
for
loo p ing curre n t transmitte d from t he collector
sweep to ground, and C341 is a d j usted to com p ensate for
h ig h fre q uency noise coming in
on t
he line
.
Anot h er source
of loo
p ing current
is
un balance in
t he
sweep
transformer
. As
h as
been d iscusse d i n t he collector
s u pp ly
circuit
d escri p tion,
t h
e swee p
transformer is some-
times use d in α full-wave rectifier ar
r
angement . T h is met h
od
of
transformer op eratio n re q uires t h at t he transformer
be
b alance d about t he center ta p . L
OO
P I NG B A L AN
CE
ad
ju
stment
C301 is
ad j
usted
to
e q ualize
t
h
e ca
p acita n
ce
on
both si d es of
t
he transformer center ta p .
Wh
e
n t h
e
transformer is use d
in bridge o p eration, t he
voltage at one end is h el d esse n tially constant, an
d
t h
e
transfo r mer o p erates unbala n ce d . In t h is case, t he trans-
former ca p acitance is added to t he stray ca p acita n ce found
Ci r c u it Descri p tio n -Ty p e 576
between t h
e
Collector
S
u pp ly an d
grou nd . 350
V
and 1500
V L OO P I N
G
COMP ad j ustment C339 h as been added be-
tween t h e transforme r center ta p a nd t he j un ction of C343
an d C341, fo r bri d ge op eration of t he Collector Su pp ly to
com p ensate for un balance d o p eration of t he transformer .
Interloc k
T h e Ty p e 576 h as an interloc k system d esig ned to p ro-
tect t h e u ser of t h e i nstrumen t from pote n tially d angerous
voltages w h ic h may a pp ear at t h e Collector terminals of t he
Sta ndard Test F ixture . F ig . 3-3 Α s h ows α simp lified
sc h ematic (see Collecto r Su pp ly sc h ematic for com p lete cir-
c u it) of t h is system .
Coil Κ 323 e
n
ables or
d isables
t he Collector Su
pp ly
o
u
t-
p ut t h roug h
Κ
323-
Β ,
e n abling it w h en t he coil is energize d .
T he coil is always e n ergize d wh en t he MAX PEAK VO L TS
switc h is set to 15 . Wh en t h is switc h is set to t he 75, 350 or
1500 p ositions, o
n
e side of t he coil is opened a nd t he
Collector Supp ly is disabled . T he yellow CO LLE CTO R
S UPPL YVO L TAG E DISA BLE D lig h t is turne d on t h roug h
K323-A . In o r der to enable t h e Collector Su pp ly und er
t h ese con d itions, t he P rotective Box must be p ut in p lace
on t he Sta nd ar d Test F ixt u re an d t he lid close d . W it h t he
li d close d , H ig h V oltage Interloc k switc h S W 360 is close d
an
d +12
.5
volts is a pp lie d t h roug h t h
e re
d
DA NG ER OU S
VO L TAG E
lig h t, Β 360,
to coil Κ 323, t h us enabling t he
Collector S upp ly .
W
it h t he coil now activated, t he CO L -
LE CTO R S UPPL YVO L TAG E DISA BLE D lig h t is tur ned
off .
T h is interloc k
may
be
byp asse
d on
t h
e
75 or t h e 75 and
350 p ositions of t h e MAX PEAK
V
O L TS switc h by re-
connecting t he wire conn ected to p in 1 of J 300 to one of
3-3