Tektronix 7704A User Manual

T he -50- V Current L imiting stage Q32137
protects t he
-50- V S upp ly if excess curre
n
t is
deman
ded
from t h is
s u pply . Si n ce t he loa d is con n ecte d to t
h
is
supp ly t h roug h
R 32139, all cu rre n t from t h e -50- V olt
Su p ply must flow
t
h
ro
ugh t h is resistor . Transistor Q32137 se
n ses t h e voltage
d ro p
across R 32139 . Und er normal
operation, t h ere is
i n sufficie n t voltage d rop across
R
32139 to
forwar d bias
Q32137 . H owever, w h en excess
cu rrent
is d
eman ded from
t h e -50- V Series R egulato r due to α s
h
ort
circuit or similar
malfunctio n at t he outpu t of
t h is s u pply, t he voltage drop
across
R
32139
increases u ntil it is sufficient to forwar d b ias
Q32137 . T h e collector current of Q32137 results
in
α
re d uction of current t h roug h Q32115 and Q32143 to
limit
t h e conduction of Q32139 .
T h
is c
u
rrent
limiting protects
Q32139 from d amage d
u
e to
excess p ower d issi p ation .
Several p rotection d iodes are also inclu d
e
d i
n
t
h
is
circuit . C R 32139 p revents t he o utpu t of t
h
is su
pp ly from
goi ng more positive t
h
an abo
u
t +0 .6
volt if it is s h orte d to α
positive su pp ly . VR
32121 and C R 32123 s u pply a turn-on
voltage
for Q32130
to
start t he -50- V olt Su p ply w hen t h e
instr
u me n
t is
first t urned on . As soon as t he -50- V olt
Supp ly turns on, VR 32121 a
n
d
CR 32123 turn off to
d isconnect t he turn-on voltage
from Q32130 .
-15- V olt Su pp ly
B asic operation of all
stages in th e -15- V olt Sup p ly is
t h e same as for t
he
-50-
V olt Su pply . R efere n ce level for
t h is supply is establis
h e d by divi d er R 3272-133273 between
grou nd an d t
he -50 V Sense voltage . T he d ivider ratio of
R
3272-
R 3273 sets α level of -15 volts at t h
e
base
of
Q3275A
. T h e level on t h e -50 V Sense li n e is
h el
d sta
b le
b y t
h e -50- V olt Su pp ly . Any c
h ange
at
t h e o u tp u t of t h e
-15- V olt Supply a p pears at t
h e b
ase of
Q3275 B as an error
signal . T h e o u tput voltage is
regulate d in t he same manner
as
d escri b e d for t he -50- V olt
Su p ply . CR 3293 limits t h e
out p ut of t h is supply from goi ng more
positive t han a b o u t
+0 .6 volt if it is s h orte d to o
n e of t he positive
supplies . Diodes C
R
3274
and C R 3275 provide reverse
voltage
protection for transistors Q3275 B and Q3275
Α
respectively .
+5-
V olt Supply
B asic operation
of t h e +5- V olt Su p ply is t h e same as
d escri b
ed for t he p revious s u pplies . T he reference
level for
t
h
is
s upp ly is esta b lis hed by the gro und co nn ection at
t h e
b ase of Q3264A .
F
eedb ac k voltage to
t h e b ase of Q3264 B
is provi d ed by d ivi d
e
r
R
3264-
R
3267
between t h e -50 V
Sen se line and t h e
+5
V Sense
line . T h e d ivi d er ratio of
R
3265-
R
3267 is 10 :1 so
t h e b ase of Q3264 B is at zero volt
w hen t he supply is ope r ati ng prop erly . T he level on t
he
-50 V Se n se line is h el d sta b le by t
h
e -50- V olt
Sup p ly .
T h erefore, any c h ange at t he o
u
tput of t he +5- V
olt Sup p ly
ap pears at t h e b ase of Q3264
B
as an
error signal . T he
output voltage is
regulate d in t h e manner desc r i be d
p revio
u sly
for t
he -50- V olt S upp ly . Diode C R 3261 limits
t
h e
o
utpu t of t h is s upp ly to about -0 .6 volt
if
it
is s h orted
to one of t he negative supplies .
+15- V olt Su p p ly
Circuit Description-7704A Service
T he +15- V olt Sup p
ly operates i n t h e same manner as
d escribe d for t
h
e previo
u s s u pp lies . T h e groun d connection
at t h e base of
Q3235A p rovi d es t he r efere n ce for t
h
is
s u pply .
F
eedb ac
k voltage to t h e b ase of Q3235
B
is
p rovi d e d t h roug
h divi d er R 3236- R 3239 between t he -50 V
Sense line a n d t
he +15 V Sense li ne . T he d ivider r atio of
R 3236-
R
3239 sets
t h e base of Q3235 B at zero volt . Any
ch ange i n t he o
u
tput
level of t h e
+15-
V olt Su pp ly a p pears
at the b ase of
Q3235 B as
an
error sig n al . T h is results in an
opposite ch
ange
at
t he collecto r of Q3235 B and at t he b ase
of Q3238 . T h is c
hange is connecte d to t he +15- V Series
Regu lator stage t
h ro ugh Q3240 to correct t he error
i n
t
h e
ou tput voltage
.
Diode C R 3235 p rotects
Q3235 B against negative
voltages if t h e +15- V olt Supply is s
h
orte
d to ground . Dio d e
CR 3241 limits t h e ou t p ut of t
h
is s
up ply
to a
bou t -0 .6 volt
if it is s h orte d to o n e of t h
e
negative su p
plies
.
+50- V olt Su p p
ly
Operation of t
he +50- V olt Supply is t he same as
d escribe d previo
u sly
for
t he ot h er s up plies . R eference
voltage for t h is s
u
pp ly is esta
b lis h e d by t h e gro un d
con n ection t h roug h
R
3206 at t
he
b
ase
of
Q3209 . F ee d back
voltage to t h e b ase of
Q3209 B is pr ovi d e d b y divi d er
R 3208-
R
3214
between t h e -50 V Sense li ne and t h e
+50
V
Sen se li ne . T h e d ivi d er ratio of R 3208-
R
3214 sets
t he b ase
level of Q3209 B at zero volt w hen t h e o
u
t p ut of t
h
is s
up ply
is cor r ect . T he protectio n dio d es in t
h
is
circuit op erate
si m ilarly to t h e ot h er supplies .
General
RE ADO UT SYST EM
T
he d
efi n ition of several terms follows :
I N T R OD U CTIO
N
TO
RE ADO U T SYST EM
T h e R ea d ou t System
p rovi d es al ph a- nu meric d isplay of
i n formation encoded by t
h e p lug-in u nits . T h is dis p lay is
presented on t
he
C
R T
an
d is written by t he C RT beamon α
time-s h ared b
asis wit h t he analog wavefo rm dis p lay .
C h aracter-
Α ch aracter is α single numb e r, letter, o r symbol
d isplaye d on
t he CR T, eit h er alone or i n com b inatio
n
wit h oth er c
h
aracte
rs .
2- 3 9

Tektronix 7704A Specifications

General

Brand	Tektronix
Model	7704A
Category	Test Equipment
Language	English