Tektronix 2235 User Manual

T heory of O p e
r ation-2235 Service
W
it
h
Q9070
off, C907 c harges to t he output voltage of

p ositive voltage
from t he lead of Τ 944 w h ic h is conn ecte d to
t h
e
P ower Input circuit . Wh en Q9070 t u r ns on, cu r rent

t he b ase of t he cond u
cti ng transistor and reinforce con du c-
t hr oug h t he FE T will come from t he wi nd i ng connected to

tion . Even t u
ally Τ 944 will sat u rate, and as t he voltage
pins 1 and 2 of Τ 906 and from C907 . Cu r rent to C907 is

across
Τ 944
(a
nd Τ 948) b egins to reverse, t he co nd ucting
su pp lie d by t he winding conn ected to pins 4 and 5 of Τ 906 .

transistor will c u t
off
b eca
use of t he drop in b ase d rive . T he
Wh
e
n U 930 sh uts off Q9070, t he collapsing magnetic fiel d ot h er t r ansistor will
not start conductio n un til t he voltage on
will
raise
t he voltage at t he anode of CR 907 . T h is diode t hen

t h e leads of Τ
944 reverse enoug h to b ias it on . T h is process
becomes forward biase d and passes t he curre n ts s upp lied

will continue, a nd t
h
e sat
u ration time of Τ 944 p l us t he tra n -
by C907
and t he windi n g con nected to pins 4a nd 5 of Τ 906 .

sisto r -switc h ing time
will determine t h e f r equency of I n verter
F
or
t h is part of t he cycle, cu rrent to C907 will be s upp lie d by

operatio n (typically
20 kHz) . After t he initial I n verter start up,
t he winding co nn ected to pins 1 a nd 2 of Τ 906 . T h is process

t h e switc h i n g transistors do
not saturate ; t h ey remain i n t he
will contin ue for eac h period of t he oscillator, and t he du ty

active regio n during switc
h ing .
cycle co
n trolli ng t he conductio n pe r io d of Q9070 will be al-
tered
as necessary to maintain 43 V across C940 . To sh ut
off Q9070 duri ng eac h oscillato r p erio d , 0908 is used to

Diodes C R 946 a
n
dC
R 947 serve as α negative- p eak de-
disc h arge t he gate-drain ca p acita nce . P in 10 of U 930 goes

tector to
generate α voltage for controlli ng t he o u t p ut of t he
LO, reverse biasing C R 908 and turning on Q908 to sh ut off

er r o r amplifier
. Capacitor C943 charges to α voltage equ al
t he FE T .

to t h e n egative pea
k voltage at t he collectors of 0946 a
n
d
Q947,
referenced to t he P reregulato r input voltage .
T h
is
voltage
level
is
a pp lied to t he divider composed of
R
937,
Once t h e su pp ly is running, p ower to U 930 will be s up-

R 938, an d
R
939 .
T he error amplifier, com p osed of 0938
p lied from t he wi nd ing connected to p i ns 6 and 7 of Τ 906 .

and 0939, is α
d ifferential amplifier t h at compares t he r efe r-
Dio de C R 920 h alf-wave rectifies t he voltage across p i ns 6

e nce voltage of VR
943 wit h t h e voltage on t h e wi p er of
a
nd 7 to k eep filter ca p acitor C925 charged and to mai n tai n potentiometer R 938 .
Th
e
c u rrent t hr oug h Q939 will set t he
su pp
ly voltage to U 930 t h ro ugh Q930 .

b ase drive of Q944
and t hereby co n trol t h e voltage on
C944 . T h is voltage
will b ias Q946 a nd Q947 to α level t h at
will maintai n t
h
e pea
k -to-pea k inp u t voltage of T948 . The
I n strument p rotection f r om excessive o u t pu t voltage is

am p litude of
t he voltage ac r oss t he tra nsformer p rimary
s upp lied b y silicon-co n trolled rectifier 0935 . S h o u ld t h e

wi n ding, and t hus t
h
at of
t he secon d ary voltages of Τ 948, is
P reregulator out pu t voltage exceed 51 V, zen e r diode

set b y ad j usti ng -8 .6
V Ad j potentiometer R 938 .
VR 935 will co ndu ct, causing Q935 to also con duct . T he
P reregulato r output curre n t will t h en b e s h unted t h ro u g h
Q935, a nd t he output voltage will very q uic k ly go to zero .

At tur n on, Q938
is b iased off a nd Q939 is b iased on . All
W it h t h e supply voltage of U 930 no lo ngerb eing provided b y

t he
current
of
t he error am p lifier will t h e n go t h ro ug h Q939
t he winding co nn ected to pins 6 a nd 7 of Τ 906, t he

to b ias on Q944
. Diode C R 945 allows t he b ase of Q944 to
P reregulator will sh ut down and 0935 will be r eset . T he

go positive en oug
h
to
initially turn on Q946 or Q947 . T he
supply will t h e n attempt to power up, bu t may again sh ut

current t h roug
h Q944 controls t he b ase d rive for Q946 a
n
d
dow n if t he overvoltage co nd ition is again reac h ed . T h is se-

Q947 . B ase
curre n t provi ded b y b ase- d rive tra nsformer
qu e nce

continues

un til

t he

ove r voltage

con d itio n is

Τ 944
will
c
h
arge
C944 negative wit h r es p ect to t he Inverter
corrected . .

ci
r
cuit
floati n g ground (commo n ) level .
I n verte r
Crt
S up p ly
T he Inverter circuit ch anges t he do voltage from t h e

H ig h -voltage multi p lier U 975 u tilizes t he 2- kV win d ing of
Pr e r egulator to ac for use b y t he su p plies t h at are connect-

Τ
948 to
generate 12 kV to drive t he crt an ode . It also u ses
ed to t he secondaries of Τ 948 .

an
inter n al h alf-wave rectifier diode to produce -2 kV for
t h e crt cat hode .
T h
e -2
kV
s
upp ly is filtered by a low-pass
filter com p ose d of C975, C976,
R 976, R 978, and C979 .
T h eo u t p ut of
t h
e
P
rereg
u lator circuit is applied to t he

N eon
lam p
DS870 p
rotects agai n st excessive voltage b e-
ce
n
ter tap of
Τ
948 .
P ower-switc h i n g transistors Q946 and

twee
n
t
h
e crt
h
eater
and crt cat h ode by co nd ucti ng if t h e
Q947 alter n ate conducting cu rrent from t h eP re r egulator

voltage excee
ds
a pp
roximately 75 V .
ou t pu t t h roug h t he primary wind i ngs of Τ 948 . T h e t r ansistor
switc h ing action is co n trolled b y Τ 944, α saturating base-
drive tra nsforme r .

F ocus Circ
u
it
F ocus voltage is
also d evelo ped from t h e -2 kV s upp ly
Wh en t he instr u me n t is first t u rne d on, o n e of t he switc h -

via
α
voltage divider com p ose d of R 894, R 892, F OC US ρο -
ing transistors will sta r t to con du ct and its collector voltage

tentiometer R 893, R 891, R 890, R 889, R 888, and Q885 . T he
will d ro p toward t h ecommon voltage level . T h is will induce α

focus
voltage trac k s t he Α -intensity level t h roug h t he actio n

Tektronix 2235 Specifications

General

Brand	Tektronix
Model	2235
Category	Test Equipment
Language	English