Tektronix 2235 - Power Supply; Preregulator; Inverter

t hroug h C854 to reverse b ias C R 854 and to forwa r d b ias

p rotected from large voltage transients
b y supp ressor
C R 855 . T he increased ch arge of C854 is t hen tr ansferred to

VR 901 . Conducted interfere nce origi n ating wit h in
t
h
e p ower
C855 as C854 disc harges towa r d t he Ζ -Axis out p ut level .

s upp ly is attenuated by commo n
-mode
transforme r Τ
901,
Successive cycles of t he ac input to t h e Dc R estorer will

differential-mode tra nsformer Τ 903, line filter FL 9001,
and
cha r ge C855 to α voltage eq ual to t he initial level p l u s t h e

capacitors C900, C901, and C902 .
amplitu de of t he clamp ed sq uare-wave in p ut .
T h e added c h a r ge on C855 d etermi nes t he control-grid
b ias voltage . If more c harge is added to t h at already p r esent
on C855, t h e con trol grid becomes more negative a nd less
c r t w r iting- b eam cu r r en t will flow . Conversely, if less cha r ge
is adde d, t h e co n trol-grid voltage level will b e closer to t h e
cat h od e-voltage level and more crt writing- b eam c urr e n t
flows .
D u ri n g p eriods t h at C854 is c h arging, t h e crt cont r ol-g r i d
voltage is h eld constant by t he long time-co n sta n t d isc harge
p at h of C855 t h rough R 860 .
F ast-rise and fast-fall t r ansitio n s of t h eΖ -Axis ou t pu t sig-
nal are coupled to t h e crt con t r ol g r i d t h ro u g h C855 to start
t he crt writi n g- b eam cu rrent toward t h e new intensity level .
T h e Dc R estorer o u t pu t level t hen follows t h e Ζ -Axis o u tput-
voltage level to set t h e n ew b ias voltage for t h e crt control
g r i
d .
Pr e r egulator
T h
eory of
O p
e
r
atio n -2235 Service
T h e P rereg u lato r pr ovi des α reg u late d do o u t pu t voltage
for u se by t h e Inverter circuitry .
Wh en t h e instrument is tu r ned on, t h e voltage developed
across C906 will c h arge C925 t h ro ugh R 926 . Wh en t he volt-
age h as risen to α level h ig h en oug h t h at U 930 can relia b ly
d rive 09070, U 930 will receive op erati n g su pp ly voltage
t
h
roug
h
Q930 .
T h
is
level
is set by ze
n
er d io d e VR 925 i n t h e
emitter of Q928 and b y t he voltage divide r consisti n g of
R 925 and R 927 .
T
he zen er d io d e will k ee p Q928 off until t h e
b ase voltage reac h es a pp roximately 6 .9 V . T hen Q928 will
b e b iased i n to co n duction a nd t he resulting collecto r c ur rent
will
cause α voltage dr op across R 929 . T h is voltage dro p
will
b ias on Q930, and t he p ositive feed b ac k t hroug hR 930
will rei n force t h e turn-on of Q928 . T hus Q930 a nd Q928 will
drive eac h ot h e r i n to sat u ratio n very qu ic k ly . O nce Q930 is
on, U 930 will begin to fu n ctio n .
P
ulse-widt h modulato r U 930 controls t h e out pu t voltage
N eon lam p s DS858 and DS856 pr otect t he crt f r om ex-

of t h
e
P
reregulator b y regulating t he du ty cycle of t he pu lse
cessive grid-to-cat h ode voltage if t h e potential on eit he r t h e

ap p lie d
to
t h
e gate of Q9070 . It utilizes an oscillator wit h t he
con trol
gri
d or t he cat h ode is lost for any r eason .

fre q uency determined
b y R 919 and C919 (a pp roximately
60kH z) and wit h
α sawtoot h ou t p ut voltage at p i n 5 . T h is
sawtoot h voltage
is com
p
are d internally wit h t he output
P
O
WER
S UPPL Y A ND PR O BE AD JUST

voltage p
roduced b y t he two erro r am p lifiers . Wh e n eve r t he
sawtoot h
voltage is greater t h an t he error amplifie r o u t pu t
T he P ower S up p ly ci r cuit r y converts t he ac p owe r -line

voltage,
Q9070 is b iase d on to sup p ly cu rrent to b ot h C940
voltage
into t h
e voltages needed fo
r i n
str u me
n
t op eratio n . It

a nd t he rest of t h
e circuitry .
T h
e two e r ror am p lifiers main-
co
n
sists ο 1 t he P ower In p ut, P reregulato r
,
a n d I n verter ci r-

tai n α constant ou t p
ut voltage and monito r t heo u t p ut cur-
cuits (w
h
ic h d rive t he p rimary of t h e powe r t r ansfo r me r ) a nd

r
en t of t he P reregulator
. One
i npu
t of eac h am p lifier is
seco
n
dary circuits (w
h
ic
h pr
o du ce t he n ecessary supp ly

connected t h ro ugh
α
divi
d er networ k to t he IC internal +5- V
voltages
for t h
e
inst r
ument) .

refere n ce . T h e
output voltage of t he P re r egu lator is moni-
tored by t h e
voltage
divider
at p in 2 . T he voltage d r op
across R 907, produced
by
t h
e
P
rereg u lator out p ut cu rr e n t,
P ower I npu t

is a pp lied to t he c ur re n t limit amplifie
r at p in 16 .
T he P ower Input circuit converts t he ac p ower-li n e volt-
age to filtered do for use by t he P re r egulato r .
Wh en t he instrument
is
first
t u rned on, t he cu rre n t limit
amplifie r con trols t he co ndu
ctio
n time
of 09070 . Wh ile
P O WER switc h
S901 connects t
h
e ac p ower line t hr ough

Q9070 is co nd ucting, t he ou t pu t
current increases until α
fuse F 9001 to t he b ri d
ge rectifier com
p
ose d of C R 901,

su fficiently la r ge voltage drop is d evelo
p edac
r
oss R 907 to
C R 902, C R 903, an d C R 904 . T h
e b ri dge full-wave r ectifies

i nvoke t he cu rrent-limit mo de . T h e current limit amplifie r
t h e source voltage, an d t h
e output is filtered by C906 . I npu t

h olds t he out p ut current below t h e cu rre n t-limit
t hr es h old of
surge cu rrent at t h e time of instrument
power-up is limite d

a pp roximately 1 Α . Wh en t h e voltage
across C940 r eac hes
by t h ermisto r R T901 . T he t he r mistor
resistance is moder-

app roximately 43 V, t he voltage am p lifie r starts
co
n trolling
ately h ig h w h en t he p ower is fi r st t u r n
ed on, but decreases

t h e duty cycle of 09070 an d t he P reregulato r will n
ot
limit
as t he in pu t cu rrent warms t he d evice
.
T h
e instr u ment is

current un less t h ere is excessive cur r en t dema nd .